Solar Collectors.
By Emil Bedi, CANCEE and Hakan Falk, "Energy Saving Now".
Using energy from the sun to heat water
is one of the oldest uses of solar energy. Solar collectors are the heart of
most solar energy systems. The collector absorbs the sun’s light energy and
changes it into heat energy. This energy is than transferred to a fluid or air
which are used to warm buildings, heat water, generate electricity, dry crops or
cook food. Solar collectors can be used for nearly any process that requires
heat.
Domestic hot water is the second-highest energy cost in the
typical household in Europe or North America. In fact, for some homes it can be
the highest energy expenditure. Solar water heating can reduce domestic water
heating costs by as much as 70%. Designed to pre-heat the domestic water that is
supplied to conventional water collector, it can result in remarkable savings.
It’s easy to install and almost maintenance free.
Today, solar water
heating systems are being used for single family houses, apartment buildings,
schools, car washes, hospitals, restaurants, agricultural farms and
different industries. This is a diverse list of private, commercial and
industrial buildings, but they all have one thing in common - they all
use hot water. Owners of these buildings have found that solar water
heating systems are cost-effective in meeting their hot water needs all over the
world.
HISTORY
Solar water heating was used long
before fossil fuels dominated our energy system. The principles of solar heat
have been known for thousands of years. A black surface gets hot in the sun,
while a lighter coloured surface remains cooler, with white being the coolest.
This principle is used by solar water collectors which are one of the best known
applications for the direct use of the sun’s energy. They were developed some
two hundred years ago and the first known flat plate collector was made by Swiss
scientist Horace de Saussure in 1767, later used by Sir John Herschel to cook
food during his South Africa expedition in the 1830’s.
Solar
technology advanced to roughly it’s present design in 1908 when William J.
Bailey of the Carnegie Steel Company (USA), invented a collector with an
insulated box and copper coils. This collector was very similar to the
thermosyphon system (described bellow). Bailey sold 4000 units by the end
of World War I and a Florida businessperson who bought the patent rights sold
nearly 60 000 units by 1941. In the U.S. the rationing of copper during World
War II sent the solar water heating market into a sharp decline.
Little interest was shown in such devices until the world-wide oil crisis
of 1973. This crisis promoted new interest in alternative energy sources. As a
result, solar energy has, received increased attention and many countries are
taking a keen interest in new developments. The efficiency of solar heating
systems and collectors has improved from the early 1970s. The efficiencies can
be attributed to the use of low-iron, tempered glass for glazing (low-iron glass
allows the transmission of more solar energy than conventional glass), improved
insulation, and the development of durable selective coatings.
SOLAR COLLECTOR MARKET
 Solar domestic hot-water systems are technically mature and
available practically all over the world. The market for flat-type collectors
has been reported as substantial in Israel, China, Cyprus, Japan, Australia,
Austria, Germany, Greece Turkey and USA. Sales in Europe are mainly for domestic
water heating, which may also include space heating and heating swimming pools.
World production of solar collectors in 1995 was 1,3 million m2 where market in
Europe and Mediterranean countries is reported to be about 40% of the world
market. Total amount of installed solar collectors exceeded 30 million m2 and
the development of sales was very rapid since 1980. Since 1989 there is steady
increase with around 20 % per year.
Among countries in Europe, Greece
has become the leader in production of solar systems and exports 40% of all
collectors produced and comprises 30% of the market in Germany. The
industry‘s goal for the year 2005 represents 1,3 million systems and 5 million
m2 of collectors. A project on Crete will need 20,000 collectors over two
years. The Greek market installs 70,000 solar systems a year, reducing CO2
emissions by 1,5 million tonnes.
Sales in the EU in 1996 were
reported to be over 0,7 million m2 of glazed collectors and about 0,15 million
m2 of unglazed collectors (Renewable energy world, Sept. 1998). All the
indications are that this trend will continue at a rapid pace since measures are
being taken all over the EU for the promotion of solar systems.
Glazed solar collector production in 1994 (Source : Sun in action. The
solar thermal market, a strategic plan for action in Europe. European Solar
Industry Federation. Altener Program).
|
Country |
Production in
1994 |
|
Germany |
170 000 m2 |
|
Greece |
165 000 m2 |
|
Austria |
100 000 m2 |
|
UK |
40 000 m2 |
|
Denmark |
20 000 m2 |
|
Others |
55 000 m2 |
|
EU total |
550 000 m2 |
Installed solar collector area in the world (Source: Sun in action. The
solar thermal market, a strategic plan for action in Europe. European Solar
Industry Federation. Altener Program).
|
Country |
Installed solar
collector area |
|
Mediterranean countries |
8,5 million m2 |
|
USA |
6,5 million m2 |
|
Japan |
6 million m2 |
|
EU |
5,6 million m2 |
|
Australia |
2,5 million m2 |
|
China |
1,5 million
m2 | Installed solar collector
area per head of population was 0,5 m2 in Cyprus in 1992 the largest in Europe
and followed by Greece and Austria. Collector area per head of population
increased in Austria up to 0,2 m2 in 1998 and amounted total area of 1,5 million
m2. Austria is first in sales per capita followed by Greece but both
countries still fall behind the world leaders Israel and Cyprus. Analysis of
statistical figures like collector area per head of population shows that
favourable climatic conditions have less influence than socio-economic boundary
conditions. The success in Cyprus is explained not only by the absence of any
other local source of energy but also by countries regulation. Strong
legislation promoting solar energy utilisation is in force also in Israel.
Israel and Cyprus have imposed statutory requirements for solar heating systems
in all new buildings. These requirements were introduced in stages: thus in
Israel initially all new apartment buildings of up to eight storeys were
required to have a community solar water heating system with appropriate storage
tanks. This was later extended to all new dwellings in the country. Finally in
1983 new regulations required hotels, hospitals and schools to install solar
water heating equipment. These regulations were coupled with financial
incentives. A similar attempt has also been made in Cyprus and it was recently
estimated that 90 % of individual dwellings and 15 % of apartments in Cyprus are
now equipped with solar water heaters.
POTENTIALS
In
Europe the total rapidly exploitable potential for solar collectors production
is estimated to be 360 million m2 , representing a market volume of 50 billion
USD at an annual average growth rate of 23%. In 2005 the area occupied by glazed
solar collector installations in the EU is expected to rise to 28 million m2.
Moreover, unglazed solar collectors for heating swimming pools are
expected to reach 20 million m2.
SOLAR COLLECTORS TYPES
Typical solar collectors collect the sun’s energy usually with rooftop
arrays of piping and net metal sheets, painted black to absorb as much radiation
as possible. They are encased in glass or plastic and angled towards south to
catch maximum sunshine. The collectors act as miniature greenhouses, trapping
heat under their glass plates. Because solar radiation is so diffuse, the
collectors must have a large area.
Solar collectors can be made in various sizes and constructions depending
on requirements. They give enough hot water for washing, showers and cooking.
They can be used also as pre-heaters for existing water heaters. Today there are
several collectors on the market. They can be divided into several categories.
One of them is division according temperature they produce:
 Low-temperature
collectors provide low grade heat, less than 50 degrees Celsius, through either
metallic or non-metallic absorbers for applications such as swimming pool
heating and low-grade water.
Medium-temperature collectors provide
medium to high-grade heat (greater than 50 degrees Celsius, usually 60 to 80
degrees), either through glazed flat-plate collectors using air or liquid as the
heat transfer medium or through concentrator collectors that concentrate the
heat to levels greater than “one sun.” These include evacuated tube collectors,
and are most commonly used for residential hot water heating.
High-temperature
collectors are parabolic dish or trough collectors primarily used by independent
power producers to generate electricity for the electric grid.
Batch Solar Water Collectors
 |
The simplest type of solar water collector is a “batch” collector,
so called because the collector is the storage tank - water is heated and
stored a batch at a time. Batch collectors are used as pre-heaters for
conventional or instantaneous water heaters. When hot water is used in the
household, solar-preheated water is drawn into the conventional water
collector. Since the water has already been heated by the sun, this
reduces energy consumption. A batch solar water collector is a low cost
alternative to an active solar hot water system, offering no moving parts,
low maintenance, and zero operational cost. The acronym for a batch type
solar water collector is ICS, meaning Integrated Collector and Storage.
Batch collectors, also known as “breadbox” , use one or more black tanks
filled with water and placed in an insulated, glazed box. Some boxes
include reflectors to increase the solar radiation. Solar energy passes
through the glazing and heats the water in the tanks. These devices are
inexpensive solar water collectors but must be drained or protected from
freezing when temperatures drop below freezing. |
Flat-Plate Collectors
 Flat-plate collectors are the most common collectors for
residential water heating and space-heating installations. A typical flat-plate
collector is an insulated metal box with a glass or plastic cover called the
glazing and a dark-coloured absorber plate. The glazing can be transparent or
translucent. Translucent (transmitting light only) low-iron glass is a common
glazing material for flat-plate collectors because low-iron glass transmits a
high percentage of the total available solar energy. The glazing allows the
light to strike the absorber plate but reduces the amount of heat that can
escape. The sides and bottom of the collector are usually insulated, further
minimising heat loss.
The absorber plate is usually black because
dark colours absorb more solar energy than light colours. Sunlight passes
through the glazing and strikes the absorber plate, which heats up, changing
solar radiation into heat energy. The heat is transferred to the air or liquid
passing through the flow tubes. Because most black paints still reflect
approximately 10% of the incident radiation some absorber plates are covered
with “selective coatings,” which retain the absorbed sunlight better and are
more durable than ordinary black paint. The selective coating used in the
collector consists of a very precise thin layer of an amorphous semiconductor
plated on to a metal substratum. Selective coatings has both high absorptivity
in the visible region and low emissivity in the long-wave infrared region.
Absorber plates are often made of metal usually copper or aluminium
because they are both good heat conductors. Copper is more expensive, but is a
better conductor and is less prone to corrosion than aluminium. An absorber
plate must have high thermal conductivity, to transfer the collected energy to
the water with minimum temperature loss. Flat-plate collectors fall into two
basic categories: liquid and air. And both types can be either glazed or
unglazed.
Liquid Collectors
 In a liquid
collector, solar energy heats a liquid as it flows through tubes in the absorber
plate. For this type of collector, the flow tubes are attached to the absorber
plate so the heat absorbed by the absorber plate is readily conducted to the
liquid.
The flow tubes can be routed in parallel, using inlet and
outlet headers, or in a serpentine pattern. A serpentine pattern eliminates the
possibility of header leaks and ensures uniform flow. A serpentine pattern can
pose some problems for systems that must drain for freeze protection because the
curved flow passages will not drain completely.
The simplest liquid
systems use potable household water, which is heated as it passes directly
through the collector and then flows to the house to be used for bathing,
laundry, etc. This design is known as an “open-loop” (or “direct”) system. In
areas where freezing temperatures are common, however, liquid collectors must
either drain the water when the temperature drops or use an antifreeze type of
heat-transfer fluid.
In systems with heat-transfer fluids, the
transfer fluid absorbs heat from the collector and then passes through a heat
exchanger. The heat exchanger, which generally is in the water storage tank
inside the house, transfers heat to the water. Such designs are called
“closed-loop” (or “indirect”) systems.
Glazed liquid collectors are
used for heating household water and sometimes for space heating. Unglazed
liquid collectors are commonly used to heat water for swimming pools. Because
these collectors need not withstand high temperatures, they can use less
expensive materials such as plastic or rubber. They also do not require
freeze-proofing because swimming pools are generally used only in warm
weather.
Air Collectors
Air collectors have the advantage of eliminating the freezing and boiling
problems associated with liquid systems. Although leaks are harder to detect and
plug in an air system, they are also less troublesome than leaks in a liquid
system. Air systems can often use less expensive materials, such as plastic
glazing, because their operating temperatures are usually lower than those of
liquid collectors.
Air collectors are simple, flat-plate collectors
used primarily for space heating and drying crops. The absorber plates in air
collectors can be metal sheets, layers of screen, or non-metallic materials. The
air flows through the absorber by natural convection or when forced by a fan.
Because air conducts heat much less readily than liquid does, less heat is
transferred between the air and the absorber than in a liquid collector. In some
solar air-heating systems, fans on the absorber are used to increase air
turbulence and improve heat transfer. The disadvantage of this strategy is that
it can also increase the amount of power needed for fans and, thus, increase the
costs of operating the system. In colder climates, the air is routed between the
absorber plate and the back insulation to reduce heat loss through the glazing.
However, if the air will not be heated more than 17°C above the outdoor
temperature, the air can flow on both sides of the absorber plate without
sacrificing efficiency.
The best features of air collector systems
are simplicity and reliability. The collectors are relatively simple devices. A
well-made blower can be expected to have a 10 to 20 year life span if properly
maintained, and the controls are extremely reliable. Since air will not freeze,
no heat exchanger is required.
However, the use of solar air heating
collectors is still limited to supply hot air for space heating and for drying
of agricultural products mainly in developing countries. The major limitations
for the wide adoption of solar air heaters are the high cost for commercially
produced solar air heaters, the large collector area required due to the low
density and the low specific heat capacity of the air compared to liquid heat
transfer fluids, the extended air duct system required, the high power
requirement for forcing the air through the collector, and the difficulty of
heat storage. In countries with comparatively low insolation and extended
periods of adverse weather, supplementary heat is required which increases
investment costs to a level which limits its competitiveness to conventional
heating systems. Promising ways to reduce the collector cost are the integration
of the collector into the walls or roofs of buildings and the development of
collectors which can be constructed using prefabricated components.
Solar wall.
Heating with the solar wall .
HOW IT WORKS ?
Solar air heaters can be
classified based on the mode of air circulation. In the bare plate collector,
which is the most simple solar air heater, the air passes through the collector
underneath the absorber. This kind of solar air heater is only suitable for
temperature rise between 3 - 5 deg. Celsius due to the high convection and
radiation losses at the surface. The top losses can be reduced significantly by
covering the absorber with a transparent material of low transitivity for
infrared radiation. The air flow occurs in this kind of solar air heater either
underneath the absorber or between absorber and transparent cover. Due to the
transparent cover, the incident radiation on the absorber is reduced slightly,
but due to the reduction of the convective heat losses, temperature rise between
20 and 50 degrees Celsius can be achieved depending on insolation and air flow
rate. A further reduction of the heat losses can be achieved if the air is made
to pass above and underneath the absorber since this doubles the heat transfer
area. The heat losses due to radiation will be reduced by this process due to
lower absorber temperature. However, there is simultaneous reduction in the
absorptivity of the absorber due to dust deposit if air flow is above or on both
sides of the absorber.
Some solar air collectors eliminate the cost
of the glazing, the metal box, and the insulation. Such a collector is made of
black, perforated metal. The best heat transfer can be achieved by using porous
material as absorber. The sun heats the metal, and a fan pulls air through
the holes in the metal, which heats the air. For residential installations,
these collectors are available in different sizes. Typical collector
2,4-meter by 0,8-meter panels are capable of heating 0,002 m3 per second of
outside air. On a sunny winter day, the panel can produce temperatures up to
28°C higher than the outdoor air temperature. Transpired air collectors not only
heat air, but also improve indoor air quality by directly preheating fresh
outdoor air. These collectors have achieved very high efficiencies - more than
70% in some commercial applications. Plus, because the collectors require no
glazing or insulation, they are inexpensive to manufacture.
Evacuated-Tube Collectors
 Conventional simple flat-plate solar
collectors were developed for use in sunny and warm climates. Their benefits are
greatly reduced when conditions become unfavourable during cold, cloudy and
windy days. Furthermore, weathering influences such as condensation and moisture
will cause early deterioration of internal materials resulting in reduced
performance and system failure. These shortcomings are reduced in
evacuated-tube collectors.
Evacuated-tube collectors heat water
in residential applications that require higher temperatures. In an
evacuated-tube collector, sunlight enters through the outer glass tube, strikes
the absorber tube, and changes to heat. The heat is transferred to the liquid
flowing through the absorber tube. The collector consists of rows of parallel
transparent glass tubes, each of which contains an absorber tube (in place of
the absorber plate in a flat-plate collector) covered with a selective coating.
The heated liquid circulates through heat exchanger and gives off its heat to
water that is stored in a solar storage tank.
Evacuated tube
collectors are modular tubes which can be added or removed as hot-water needs
change. When evacuated tubes are manufactured, air is evacuated from the space
between the two tubes, forming a vacuum. Conductive and convective heat losses
are eliminated because there is no air to conduct heat or to circulate and cause
convective losses. There can still be some radiant heat loss (heat energy will
move through space from a warmer to a cooler surface, even across a vacuum).
However, this loss is small and of little importance compared with the amount of
heat transferred to the liquid in the absorber tube. The vacuum in the glass
tube, being the best possible insulation for a solar collector, suppresses heat
losses and also protects the absorber plate and the “heat-pipe” from external
adverse conditions. This results in exceptional performance far superior to any
other type of solar collector.
 Evacuated-tube collectors are available in a number of
designs. Some use a third glass tube inside the absorber tube or other
configurations of heat-transfer fins and fluid tubes. One commercially available
evacuated-tube collector stores 19 litres of water in each tube, eliminating the
need for a separate solar storage tank. Reflectors placed behind the evacuated
tubes can help to focus additional sunlight on the collector.
Due to
the atmospheric pressure and the technical problems related to the sealing of
the collector casing, the construction of an evacuated flat-plate collector is
extremely difficult. To overcome the enormous atmospheric pressure, many
internal supports for the transparent cover pane must be introduced. However,
the problems of an effective high vacuum system with reasonable production costs
remain so far unsolved. It is more feasible to apply and adapt the mature
technology related to the lamp industries with proven mass production. Building
a tubular evacuated solar collector and the maintenance of its high vacuum,
similar to light bulbs and TV tubes, is practical. The ideal vacuum insulation
of the tubular evacuated solar collector, obtained by means of a suitable
exhausting process, has to be maintained during the life of the device to reduce
the thermal losses through the internal gaseous atmosphere (convection
losses).
In high temperature region these collectors are more
efficient than flat-plate collectors for a couple of reasons. First, they
perform well in both direct and diffuse solar radiation. This characteristic,
combined with the fact that the vacuum minimizes heat losses to the outdoors,
makes these collectors particularly useful in areas with cold, cloudy winters.
Second, because of the circular shape of the evacuated tube, sunlight is
perpendicular to the absorber for most of the day. For comparison, in a
flat-plate collector that is in a fixed position, the sun is only perpendicular
to the collector at noon. Evacuated-tube collectors achieve both higher
temperatures and higher efficiencies than flat-plate collectors, but they are
also more expensive.
Concentrating Collectors
Concentrating
collectors use mirrored surfaces to concentrate the sun’s energy on an absorber
called a receiver. They also achieve higher temperatures than flat-plate
collectors, however concentrators can only focus direct solar radiation, with
the result being that their performance is poor on hazy or cloudy days. The
mirrored surface focuses sunlight collected over a large area onto a smaller
absorber area to achieve high temperatures. Some designs concentrate solar
energy onto a focal point, while others concentrate the sun’s rays along a thin
line called the focal line. The receiver is located at the focal point or along
the focal line. A heat-transfer fluid flows through the receiver and absorbs
heat. Concentrators are most practical in areas of high insolation, such as
those close to the equator and in the desert areas.
Concentrators
perform best when pointed directly at the sun. To do this, these systems use
tracking mechanisms to move the collectors during the day to keep them focused
on the sun. Single-axis trackers move east to west; dual-axis trackers move east
and west and north and south (to follow the sun throughout the year).
Concentrators are used mostly in commercial applications because they are
expensive and because the trackers need frequent maintenance. Some residential
solar energy systems use parabolic-trough concentrating systems. These
installations can provide hot water, space heating, and water purification. Most
residential systems use single-axis trackers, which are less expensive and
simpler than dual-axis trackers. For more information about concentrating
collectors see chapter Solar Thermal Power Production.
SOLAR COOKERS AND STILLS
There exists also some other inexpensive,
“low-tech” solar collectors with specific functions like solar box cookers (used
for cooking) and solar stills producing inexpensive distilled water from
virtually any water source.
Solar box cookers (see chapter on Solar
cooking) are inexpensive to buy and easy to build and use. They consist of a
roomy, insulated box lined with reflective material, covered with glazing, and
fitted with an external reflector. Black cooking pots serve as absorbers,
heating up more quickly than aluminium or stainless steel cookware. Box cookers
can also be used to kill bacteria in water if the temperature can reach the
boiling point.
Solar stills (see chapter on Solar water distillation)
provide inexpensive distilled water from even salty or badly contaminated water.
They work on the principle that water in an open container will evaporate. A
solar still uses solar energy to speed up the evaporation process. The stills
consist of an insulated, dark-coloured container covered with glazing that is
tilted so the condensing fresh water can trickle into a collection trough. A
small solar still, which is about the size of kitchen stove, can produce up to
ten litres of distilled water on a sunny day.
Technology Examples
Solar energy has a variety of practical and cost-effective applications
in today’s homes and buildings. The main applications of solar collectors are as
follows :
- hot water preparation in households, commercial buildings and
industry,
- water heating in swimming pools,
- space heating in buildings,
- drying crops and houses,
- space cooling and refrigeration,
- water distillation,
- solar cooking.
The technologies for all applications are considered to be mature and for
the first two, under the appropriate conditions, economically viable. Separate
chapter is devoted to concentrating collectors which are cost effectively used
for power production especially in regions with high insolation (see chapter on
Solar Thermal Power).
Solar Thermal Residential Water Heating
 Today, several million homes and businesses use solar
water heating systems. These systems are providing consumers a cost-effective
and reliable choice for hot water. Taking a shower with solar-heated water, or
heating a house with solar-heated air or water, is a natural and simple method
for both conserving energy and saving fossil fuels. When a solar heating system
has been designed and installed correctly, it can be aesthetically appealing and
also add to the value of the home. On new construction, they can be worked into
the building design to be almost invisible, while on existing construction it
can be a real challenge to make them fit in.
A solar water collector
is saving an owner money but it also help protect the environment. Emissions of
one to two tons of carbon dioxide are saved by a single conventional water
collector every year. Other pollutants, such as sulphur dioxides, carbon
monoxide and nitrous oxides are also displaced when a homeowner decides to tap
into a solar energy.
Hot water production is the most widely distributed utilisation of direct
solar heating. An installation consists of one or more collectors in which a
fluid is heated by the sun, plus a hot-water tank where the water is heated by
the hot liquid. Even in the areas of low insolation like in Northern Europe a
solar heating system can provide 50-70% of the hot water demand. It is not
possible to obtain more, unless there is a seasonal storage (see chapter below).
In Southern Europe a solar collector is able to cover 70-90% of the hot-water
consumption. Heating water with the sun is very practical and cost effective.
While photovoltaics (see chapter on photovoltaics) range from 10-15% efficiency,
thermal water panels range from 50-90% efficiency. In combination with a wood
stove coil/loop, virtually year round domestic hot water can be obtained without
the use of fossil fuels.
HOW IS A SOLAR WATER COLLECTOR COMPETITIVE WITH CONVENTIONAL HEATERS ?
Costs of
complete solar water heating systems differs considerably from country to
country (in Europe and the USA e.g. between 2000 - 4000 USD). They also depend
on hot water requirements and the climate conditions in the area. This is
usually a higher initial investment than required for an electric or gas heater
but when adding all of the costs involved with heating water in home, the
life-cycle cost of a solar water heating system is usually lower than
traditional heating system. It must be noted that simple pay-back time for
investment into solar heating system depends on prices of fossil fuels
substituted by solar energy. In EU countries pay-back times are generally less
than 10 years. The expected life span of the solar heating system is 20-30
years.
Important feature of solar installation is energy pay-back
time - time needed to produce as much energy by solar system as it was needed to
produce this system. In Northern Europe with less solar radiation than in other
parts of the world a solar heating system for hot-water preparation has an
energy pay back period of 3-4 years.
HOW MUCH ENERGY CAN WE GET ?
The amount of energy we can get from solar heating
system depends on available insolation and efficiency of the solar system.
Insolation differs widely in the world and is crucial for solar system. The
amount of solar radiation available in some regions of the world is given in
chapter Solar Radiation. The efficiency of solar system depends on efficiency of
solar collector and losses in the hot water circulation system. As the later
depends on various specific parameters we will focus only on solar collector
efficiency. Efficiency is defined as the ration between the amount of energy
produced and solar energy falling down on collector. Efficiencies are different
for different collector types and depends on solar intensity, thermal and
optical losses - higher losses means lower efficiencies. Thermal losses are
minimal if the temperature of water used for application is the same as ambient
air temperature. Thus simple absorber without glazing used for pool heating
achieve the highest efficiencies up to 90%. But when these collectors are used
for warm domestic hot water preparation (water temperature 40 degrees Celsius
higher than ambient air temperature) their efficiencies are usually lower than
20%. In this case the best results are achieved by flat-plate collectors
(with selective coatings) and evacuated tube collectors which are best
suited for this application. When higher water temperatures are needed (e.g. for
space heating) evacuated -tube collectors are the best but also the most
expensive.
Solar collector efficiencies for insolation typical for Central
Europe at noon
during summer day - 800 W/m2. Efficiency at
temperature difference (*)
|
Collector
Type |
0 deg.
C
pool
heating |
40 deg.
C
domestic hot water |
50 deg. C
(**)
space heating |
|
Absorber without glazing |
90 % |
20 % |
0 % |
|
Flat-plate (non-selective coating) |
75 % |
35 % |
0 % |
|
Flat-plate (selective coating) |
80 % |
55 % |
25 % |
|
Evacuated-tube |
60 % |
55 % |
50 % | *
Difference between ambient temperature and temperature of water inside solar
collector.
** Values are related to lower insolation during early
spring (400 W/m2).
Low efficiency of evacuated tube collector in low temperature region is
caused by high optical losses on curved surface of the glass.
Bearing
in mind that there are huge differences between prices of collectors it is
obvious that the crucial criteria for collector type selection is purpose of its
utilisation. A comparison of different collector types and their economy
features are given in the table below.
Typical characteristics of different types of solar collectors
according German ministry of economy are following.
|
Purpose |
Collector type |
Temp. in deg.C |
Production kWh/m2/year |
Price in DM |
DM/kWh (*) |
|
Pool heating |
Absorber |
20-40 |
250-300 |
100-250 |
0,02-0,04 |
|
Warm water preparation |
Flat-plate
Evacuated-tube |
20-70
20-100 |
250-450
350-450 |
800-1900
1500-2500 |
0,16-0,21
0,21-0,28 |
|
Drying |
Air collector |
20-50 |
300-400 |
400-1000 |
0,06-0,13 |
* per m2 under 20 years collector life expectation.
Guidelines on Solar Water Heating System Sizing
A solar water heating system can be used as the
sole source for hot water or may include a back-up conventional system to meet
heavy or unusual hot water requirements throughout the year. Systems are usually
sized according to the number of rooms, people and household water needs. There
are several different configurations of solar water heating systems. In general,
however, there are two main types: active systems which have pumps and
controls to deliver solar heat to the storage tank, and passive systems like
thermosiphons which utilise natural circulation of hot water.
When
designing a solar water heating system, it is important to decide first how much
hot water will be used per average day. If the amount of hot water is known, the
size of system (collectors, storage tank) have to be calculated. Here are some
general remarks on what should be taken into consideration when designing solar
heating system.
Solar Collector
 The
main part of the solar heating system are the solar collectors. Most frequently
used are flat-plate collectors consisting of an absorber where the solar
radiation is transferred to heat in the solar collector fluid, insulation along
the edge and under the absorber a case that holds everything together, and
allows the necessary ventilation and a glass or plastic cover.
When
glass is used as cover, it is important that the iron content is low or zero, so
at least 95% of the solar radiation pass through the glass. In practice no more
than single layer of glass is used. If a plastic cover is used, it is important
that the plastic can stand up to the UV-rays from the sun. It has been found
that polycarbonate plates are very satisfactory.
 The absorber can be made
of a plate with tubes where the collector fluid flows. Usually the absorber is
made of copper or stainless steel. Experience have shown, that best absorber
tubes are those made from copper. Ordinary steel tubes cause big problems with
corrosion. It is essential that the absorber can stand up to the UV-light from
the sun, and the stagnation temperature (dry-boiling temperature), which is
100-140 deg.C for solar collectors without selective coating, and 150-200 deg.C
with selective coating.
Construction of a flat plate collector
requires soldering and brazing of tubes and physically bonding the tubes to
sheet. The more physical contact between the sheet and the tubes, the more heat
transfer to the fluid moving through the tubes. The absorber is often covered by
a selective black coating, which absorbs the sun rays, but holds back the heat
radiation. The problem with normal black paint is that it will outgas, or boil
off the metal under the extreme heat. Also, under normal cases, black paint will
radiate heat, rather than absorb it for transfer to the fluid.
Many
choices for the framework of solar collectors are reasonably available. Wood,
plastic, steel or aluminium have all been used with varying degrees of success,
but nothing is as good as aluminium. Aluminium weathers the elements with very
low maintenance, and has colour choices baked on, so there is no need to paint
the exterior of solar panel. Over the years, plastics have proven to be a poor
choice for the major parts of a solar panel. For the exterior, plastic has a
nasty habit of degrading from the sun’s ultraviolet rays. Plastic discolours and
eventually becomes brittle and cracks. Plastic also has a high coefficient of
expansion. This means it expands and contracts so much that making the joints
weather tight is difficult. Using steel for framework means also some problems.
One is that the panels need painting regularly and two, they react chemically
with the copper interior.
Solar collectors are usually mounted
directly on top of the roof, or at a frame placed on a flat roof or the ground.
Solar collectors can also be integrated in the roofing. In some cases
problems with sealing between the solar collector and the rest of the roof can
arise.
The size of solar collectors depends on the daily hot water
requirements. In general one person may require approx. up to 50 litres of hot
water at approx. 55° to 60° degrees Celsius per day (for domestic bathing only,
without laundry). It has been shown that in average 1-1,5 m2 solar collector
area is needed per 50 litres daily consumption of hot water. Selection of size
would also depend on availability of standard products. Prizes vary with the
collector size and with the installation charges. Installation is simplest when
the system is incorporated in the initial planning of the construction of a new
house. This allows the architect to incorporate the collectors into the plan,
both esthetically and economically.
SOLAR COLLECTOR ORIENTATION
The orientation of
solar collectors (which way they face and how they are tilted) optimizes their
collection ability. The earth’s atmosphere absorbs and reflects a significant
portion of solar radiation. Thus, the most energy that can be gathered on any
given sunny day is at solar noon, when the direct beam radiation is least
affected by the atmosphere. Solar noon is true south in the northern hemisphere.
Although orienting the collectors to true south will normally maximize
performance, a variation within 20° east or west is acceptable without
additional collector surface area.
A solar collector that traces the
sun, will usually receive about 20% more solar radiation than a south facing
optimum placed collector. This additional output do not compensate the costs
related to a construction, which has to trace the sun. Usually it will be
cheaper to install a 20% larger solar collector.
Local weather
patterns (i.e., morning haze or prevailing afternoon cloudiness) should also be
considered in collector orientation. If local weather is not a factor and
collectors cannot be faced true south, orienting them to the west is generally
preferable due to higher afternoon temperatures (collectors have less heat loss
with higher outside temperatures).
Since elevation of the sun varies
throughout the year depending on local latitude, collectors should be tilted
towards the sun depending upon application. In general, seasonal differences in
irradiation are considerable and must be taken into account for all solar energy
applications. Tilting the collecting surface some 30...50 degrees to the South
in the Northern Hemisphere or to the North in the Southern Hemisphere yields
somewhat better wintertime results for the region in question, but also some
losses in summer. Space heating systems are tilted more to the position of the
winter sun. In the tropics, a nearly horizontal receiving surface is generally
most advantageous because of the sun’s high altitude. The most desired angle of
inclination to mount the solar collector is the local latitude. Positive
difference between latitude and roof angle results better system performance in
winter. Lower solar collector mounting angle than the local latitude will result
in greater system performance in summer. Variations of solar collector tilt
angle for architectural reasons can be compensated with additional collector
size.
Storage Tank
The storage tank shall store the solar heat. This is done by storing hot
water until it is needed. There are several different sizes of tanks available.
All tanks must have connections for cold water inlet and hot water outlet as
well as two connections for circulation pipes. Hot water storage tanks can
easily be fitted to a stand. The most efficient is a vertical tank with good
temperature stratification, so the cold inlet water aren’t mixed with the warmer
water at the top of the tank. A horizontal tank reduces the output by
10-20%.
The heat from the solar collectors is delivered to the water
in a heat exchanger. As heat exchanger is mostly used a coil in the bottom of
the tank, or a cap around the tank with collector fluid. In low-flow and
self-circulating systems a cap are always used. In low-flow systems the solar
collector fluid flows slowly down through the cap of the storage tank, which
gives a stratification of collector fluid in the cap corresponding to the
stratification in the tank. This gives more ideal heat transfer, and thereby a
higher efficiency than in traditional systems.
All hot water storage
tanks must be well insulated to keep the water hot during the night. Heat loss
depends on many factors (ambient temperature, wind, season, etc.) and will be
approximately 0,5 to 1 degree Celsius per hour during the night. The insulation
of the tank must be so good, that hot water from a sunny day still is hot two
days later. Especially the top must be well insulated, and without thermal
bridges. Experience shows that a minimum thickness of insulation of 100 mm
should be maintained.
It must be ensured that piping from the storage
tank do not lead to self-circulation, which can drain the tank for hot water
during periods without hot water consumption. If there is a flow tube pipe for
the hot water, this must not be connected to the cold water; but has to enter at
the upper part of the tank. Usually the outlet of the storage tank is equipped
with a scalding protection, so the water delivered for use never gets warmer
than e.g. 60 deg.C, regardless of the temperature in the tank.
The
solar water collector storage tank should have a size of 80 litres of hot water
storage volume per person with a hot water consumption of 50 litres per day.
These are the average values. If the home have a dishwasher, washing machine,
several children taking daily showers or baths during the day, so all of this
water usage must be figured into the total water needs.
Solar Collector Circuit
The solar collector
circuit connects the solar collector to the storage tank. The components of the
circuit are:
 a
pump that ensures circulation (not needed in self-circulating systems). The pump
is usually controlled by a difference thermostat, so it starts running, when the
solar collector is a bit warmer than the storage tank. If the storage tank has a
heat exchanger coil at the bottom, a more simple control system can be used;
e.g. a light sensor, or a timer that starts the pump during day time.
pipelines
connecting hot water storage tank and collectors. Layout of pipelines should
secure to be of shortest possible distance. Pipes should not be exposed to the
weather if possible. Best is to keep them inside the house where possible. It is
important to have several separate pipes from the collector to the taps to
reduce heat losses (smaller pipes) and to give a fast supply of hot water to the
user, with a maximum delay of about 10 to 20 seconds. Pipelines must be produced
of a non-corroding material. Systems with open expansion are most risky to get
corrosion problems.
a one-way valve which prevents that the solar collector fluid runs
backwards at night, and empties the storage tank for heat (not necessary in all
kinds of installations).
an expansion tank; either an open container at the top of the
installation, or a pressurised expansion tank that contains minimum 5% of the
solar collector fluid.
overpressure protection (only in connection with pressurized
expansion tank); must be a type that manage to let out the solar collector
fluid, if the system is boiling. There must always be an accumulation tank to
the fluid in case of boiling. This is normally a safety valve and a non-return
valve (check), or a non-return valve and a vent pipe which will release
over-pressure due to the increase of volume by heating.
air outlets, automatic or simply
screws; must be used at all height points in the system, as air pockets always
will appear.
filling valve.
dirt filter for the pump, to remove dirt, e.g. from the installation (can be
spared in some installations).
manometers and thermometers according
to need.
the
solar collector fluid must be able to stand frost, and must not be toxic.
Usually is used an approved liquid, consisting of water with 40% propylene
glycol (can stand minus 20 deg.C), and a substance that can be seen and
tasted, if solar collector fluid leak to the tap water. Oil can also be used as
collector fluid, but it is difficult to make a collector circuit with oil
tight.
MAINTENANCE
The
simplicity of solar water heating systems means that maintenance is minimal.
Required maintenance will depend on type of system. Experience shows that once
or twice a year it must be controlled, that there are enough fluid and pressure
on the system. Once a year it should be checked that the solar collector fluid
hasn’t become acid. Acid indicator paper can be used. Acid fluid should be
changed. In case the system is boiling, it is simply needed to fill new fluid on
the system; as the old fluid may be damaged by the boiling.
An
important consideration when designing a system is the freeze-protection
requirements. Some storage tanks must be softened, and the anti-corrosion zinc
block shall be changed after approximately 10 years, it prolongs the life
span significantly.
GUIDELINES FOR SOLAR COLLECTOR SYSTEM SIZING
For a typical solar water
collectors (heating from 8 to 45 deg.C) with selective absorbers, the following
hand rules can be used:
- in
average 50 litres of hot water per person and day is needed.
- 1-1,5 m2 solar collector area is needed per 50
litres daily consumption of hot water.
- the storage tank shall be 40-70 litres per m2 solar collector or 80
litres per head.
- the
heat exchanger in the storage tank shall be able to transfer 40-60 W/deg.C per
m2 solar collector at 50 deg.C.
If these guidelines are followed, a typical solar water collector
installed in Northern Europe will cover 60-70% of the annual hot water
consumption, and be able to produce 350-500 kWh/m2 per year. For larger
buildings (e.g. hotels, hospitals, apartment blocks), the collector areas and
storage volumes required per head are smaller, but good dimensioning needs
detailed analysis of demand and local climate conditions. The experience shows
that solar systems for hot water preparation should be designed to be as simple
as possible and not oversized.
Example
For a family with 4 persons which uses
200 litre of hot water each day solar collector with 6 m2 area are needed.
During the year they can produce up to 3000 kWh of clean energy. When solar
collectors substitute the oil boiler than net saving can achieve at least 300
litres of oil annually.
THERMOSIPHON
Thermosiphons are solar water heating systems with natural circulation
(i.e. by convection) which can be used in non-freezing areas. These systems are
not the highest in overall efficiency but they do offer many advantages to the
home builder. They are simple to make and most of these devices operate without
the assistance of an electric pump. This thermosiphon circulation occurs because
of the variation of water density with its temperature. With the heating of the
water in the collector (usually flat-plate), the warm water rises, and since it
is connected in a riser pipe to the hot water storage tank and a down-comer pipe
again to the collector, it is replaced by the cooler, heavier cold water from
the bottom of the hot water storage tank. It is therefore necessary to place the
collectors below the hot water storage tank and to insulate both connecting
circulation pipes.
Thermosiphon systems have serious problems with
their collectors freezing and bursting, even in areas with only one or two mild
freezes a year. It only takes one frozen night to ruin an unprotected collector.
Some systems are designed to avoid freeze damage by using 10 centimetres or
larger copper tubing in a double glazed, insulated enclosure. Quite simply, the
volume of water in system is too large to freeze and burst in a mild freeze.
This type of installations is popular in sub-tropical and tropical areas.
 The
complete thermosiphon circulation system may be divided into three separate
sections:
The flat plate collector (absorber).
The circulation piping.
The hot
water storage tank (boiler).
Usually solar collector is located on a lower story, porch, or shed roof
so that the top of the panel is at least 50 centimetres below the bottom
of the storage tank. Tank location is usually in a second story, an attic,
sometimes a cupola - somewhere that ensures an 50 cm vertical height difference
between panel and the tank.
Solar Pool Heating
Solar pool heating system is a wise investment. In the USA the Department
of Energy has identified swimming pools as a huge consumer of energy across the
country, and has recognized pool heating as one of the most cost-effective means
of reducing energy consumption. Solar pool heating systems are being used in
virtually every area of the United States or Europe. Over 200 000 pools are
heated by solar in the United States alone. The oldest systems have been in use
for more than 25 years, and are cost-effective, highly reliable and require
minimal maintenance. Important fact is that they function well and are
cost-effective for the swimming season even in northern climates. Systems can
also be designed for indoor pools as well as for larger municipal and commercial
pools.
 Despite
the fact that price of installation varies on the size of the pool and other
site-specific installation conditions if solar systems are installed in order to
reduce or eliminate fuel or electricity consumption, they generally pay for
themselves in energy savings in many countries in two to four years. Moreover
solar pool heating can extend the swimming season by several weeks without
additional cost.
Most homes can accommodate a solar pool heating
system. These systems can be as simple as water running through a black hose.
For outside pools, the only thing which is needed is the absorber portion of the
solar collector. Inside pools need standard solar collectors to provide winter
heating.
Although solar collectors are often installed on a roof,
they can be installed wherever they can be exposed to the sun for a good portion
of the day. The type of roof or roofing material is not important. The
appropriate area of solar collectors required for a given swimming pool is
directly related to the area of the pool itself. The proper ratio of pool area
to solar collector area will vary according to such factors as location, the
orientation of the solar collectors, the amount of shading on the pool or solar
collectors, and the desired swimming season. In general, however, the area of
solar collectors required is usually 50% to 100% of the pool surface area.
HOW DO SOLAR POOL HEATING SYSTEMS WORK?
 Adequate
swimming pool heating can be achieved by having low temperature collectors
directly connected to the filter circulation. In a few cases an additional
“booster pump” or a slightly larger filtration pump may be needed. Today’s most
efficient systems employ the use of an automatically controlled diverting valve.
The pool’s filtration system is set to run during the period of most intense
sunshine. During this period, when the solar control senses that adequate heat
is present in the solar collectors, it causes a motorized diverting valve to
turn, forcing the flow of pool water through the solar collectors, where water
is heated. The heated water then returns to the pool. When heat is no longer
present, the water bypasses the solar collector. Thus, most systems have very
few moving parts which minimizes operation and maintenance requirements.
Additional precautions are required against corrosion in collectors, since the
water is quite aggressive (use of low temperature collectors, possibly made of
plastics).
PLACING THE SYSTEMS
Systems can quite easily be placed out of sight in a remote places, for
example upon a suitable roof; however some basic design rules should be
observed. The chosen site should be level or slightly sloping (less than 30 deg.
to horizontal) with the return manifolds higher than the infeed manifolds and
all hoses rising steadily from one to the other to ensure all air is expelled
during operation.
Both a non-return valve and a vacuum release valve
should be fitted to systems placed at more than 1 meter above pool level to
prevent the reverse flow of water into the pool and the flattening of hoses when
the collector drains at the end of each operating cycle. All connections into
the pool filtration circuit must be made after the filter unit and, if
applicable, before any existing conventional heater to avoid pressurising the
solar system.
OPERATION AND MAINTENANCE
The simplicity of solar pool heating
systems means that operation and maintenance requirements are minimal. In fact,
in most cases no additional maintenance beyond normal filter cleaning and winter
close-up is necessary. The system should be drained in the winter months;
however, in some cases even this may not be necessary because the system drains
itself. In addition, solar pool heating equipment is so reliable that many solar
pool collector manufacturers provide warranty coverage for their products which
far exceeds that of automobiles and household appliances.
SOLAR SPACE HEATING
So far only systems for warm water preparation have been described. An
active solar heating plant can provide hot water, and additional heating via the
central heating system at the same time. To get a reasonable output, the central
heating temperature must be as low as possible (preferably around 50 deg.C), and
there must be a storage for the space heating. A smart solution is to combine
the solar heating installation with under-floor heating, where the floor
function as heat storage.
Solar heating installations for space
heating usually give less profit than hot-water installations, both according
economy and energy, as heating is seldom needed during summer. But if heat is
needed during summer (like in some mountain areas), then space heating
installations is a good idea. In central Europe, some 20% of the total heat load
of a traditional house, and close to 50% low energy house, could be supplied by
an advanced active solar heating system employing water storage only. The
remaining heat need to be drawn from auxiliary energy systems. To increase the
solar fraction, would in practice require larger storage capacities.
For single houses, systems with well-insulated water tanks between 5-30
m³ have been constructed especially in Switzerland (so-called Jenni system) but
the costs are too high and the storage is often unpractical. The solar fraction
of a Jenni-system is >50% and may reach even 100%.
If all of the
load in the above example were supplied by an up-to-date active solar heating
system, a 25 m² collector area and 85 m³ storage water tank with 100 cm
insulation around would be needed. Improving the energy storage capacity of the
storage unit, would dramatically improve the practical possibilities for
storage.
Although individual solar space heating is technically
feasible, it is likely that it would be far more cost effective to invest in
insulation to cut space heating demands.
SEASONAL STORAGE
If a far larger collector together with a much larger storage tank were
fitted, solar energy should be able to supply energy for several houses. Basic
problem with solar energy is related to the fact that most of the energy is
needed during the winter when solar insolation is the lowest and on the other
side much of summer potential output can not be used because the demand is
mostly not there. So capital investment into larger collectors with larger gains
would be wasted.
Despite this fact there are several installations
using summer heat produced by solar collectors and saved through to the winter.
These installations are using large storage tanks (seasonal storage). Problem is
that the volume of hot water storage needed to supply a house is almost the same
size as the house itself. In addition, the tank would need to be better
insulated. A normal domestic hot water cylinder would require insulation of 4
metres thick to retain most of its heat from summer to winter. It therefore pays
to make storage volume really enormous. This reduces the ratio of
surface area to volume.
Large solar heating plants for district
heating are now in use, e.g. in Denmark, Sweden, Switzerland, France or USA.
Solar modules are mostly installed directly at the ground in larger fields.
Without a storage such solar heating installation would cover approximately 5%
of the annual heat demand, as the plant never must produce more than the minimum
heat consumption, including loss in the district heating system (by 20%
transmission loss). If there is a day-to-night storage, then the solar heating
installation can cover 10-12% of the heat demand including transmission loss,
and with a seasonal storage up to 100%. There is also a possibility to combine
district heating with individual solar water collectors. Then the district
heating system can be closed during summer, when the sun provides hot water, and
there is no need for space heating.
PRESENT SOLAR STORAGE SYSTEMS
Large-size seasonal storage systems for
communities have been demonstrated in several countries but are still too
expensive. The size of a central storage system may range from a few thousand m3
up to a few 100 000 m3. The largest storage project in Europe is in Oulu,
Finland where a large rock cavern heat storage of 200 000 m3 will be connected
to a combined heat and power plant burning biomass. This district heating plant
was built under the EU-Thermie programme.
Another successful project
with seasonal storage of hot water has been constructed in Lyckebo, Sweden. This
project is using a rock cavern filled with water (volume of 105 000 m3) and flat
plate solar collectors with area of 28 800 m2 which supply 100% energy (8500
MWh/a) for space and water heating of 550 dwellings. All houses are connected to
communal district heating system. The temperature of supply water is 70 degrees
Celsius and the temperature of return water is 55 degrees.
The
pay-back times of such installations are very long. The important lesson from
space heating systems has been that it is essential to invest in energy
conservation and passive solar design first and then to use solar energy to
help supply the remaining reduced load.
COMBINING SOLAR WITH OTHER RENEWABLE SOURCES
Combining renewable energy sources such as
solar heat with solar storage in form of biomass may be a good solution. Or, if
the remaining load of a low energy house is very low, some liquid or gaseous
biofuels with advanced burners together with solar heating may be used.
Solar heating together with solid biomass boilers may provide interesting
synergy and also solution to the seasonal storage of solar energy. Using biomass
in the summer may be non-optimal, as the boiler efficiencies at partial loads
are low and also relative piping losses may be high - in smaller systems using
wood in the summer may even be uncomfortable. Solar heating may well provide
100% of the summertime loads in such cases. In the winter, when the solar yield
is negligible, the biomass options provides almost all of the heat needed.
Experiences notably from central Europe with solar heating and biomass
together are positive. Some 20-30% of the total load is typically provided by
solar heating and the main load, i.e. 70-80% of the total load, by biomass.
Combined solar heat and biomass may be used for both single-family houses and
for district heating. For central European conditions, around 10 m³ of biomass
(e.g. wood) would be enough for a single-family house with solar heating system
replacing well up to 3 m³ per year in a household.
Solar Thermal Commercial Water Heating
 Many businesses
use solar water heating to preheat the water before using another method to heat
it to boiling or for steam. Being less dependent on fluctuating fuel prices is
another factor that makes solar system a wise investment. In many cases
installation of solar water heating will derive an immediate and significant
savings in energy costs. Depending on the volume of hot water needed and the
local climate a business can realize savings of 40 - 80% on electric or fuel
bills. For example the 24-story Kook Jae office building in Seoul, South Korea
meets over 85% of its daily hot water needs with a solar hot water heating
system. The system has been in operation since 1984 and is so efficient that it
has exceeded it’s design specifications and even provides 10 to 20 percent of
the annual space heating requirement.
Solar heating at Kook Jae building.
There are several different
configurations of solar water heating systems. In general, however, the amount
of hot water that a commercial business demands requires an active system.
Active systems typically consist of solar collectors on a south-facing roof (in
Northern hemisphere), and a storage tank near the existing water collector. When
sufficient heat is present in the solar panel, a “controller” turns on a pump
which begins circulating fluid, either water or antifreeze, through the solar
panel. The fluid picks up the heat from the collector and transfers the heat to
the potable water supply which is stored in a tank until needed. If the
solar-heated water is not at the desired temperature, a back-up energy source
can be used to bring the water temperature up to the desired level. The type and
size of a system is calculated by determining ‘ water-heating load similar to
the way described in chapter on solar collector sizing for households (see
above). Similarly required maintenance for commercial systems will depend on the
type and size of system, but the simplicity of solar water heating systems means
that maintenance is minimal.
While for many businesses the biggest
advantage of a solar water collector is the resulting savings in utility bills,
value must be placed on the substantial environmental benefit. Air pollutants,
such as sulphur dioxides, carbon monoxide and nitrous oxides are also displaced
when a business owner decides to tap into a cleaner source of energy - the
sun.
Industrial Process Heat
Industry requires heat in a variety of temperature
ranges, depending on the process at hand. Many of these processes can be served
by collectors ranging from the flat-plate variety, which are restricted to
temperatures below 100 degrees C, to concentrating collectors which can produce
temperatures of several hundred degrees.
SOLAR COOLING
The world demand of energy for air-conditioning and cooling is still
increasing. This is not only due to an increasing wish for comfort in highly
industrialized countries but also follows the necessity of e.g. food storage and
medical applications in hot climates especially third world countries.
Today there are mainly three techniques available for active cooling.
First of all the compression machine driven by electricity which is today the
standard cooling device in Europe. On the other hand there is the absorption
cooling machine using heat as driving force. Both compression and absorption
machines are able to provide air conditioning, i.e. chilled water at about 5°C,
and refrigeration, i.e. temperatures below 0°C. There is a third possibility
which is desiccant and evaporative cooling used for air conditioning. All
systems can be driven by solar energy and in addition have the advantage of
using absolute harmless working fluids like simple water, solutions of certain
salts in water or ammonia. Possible applications of this technology are not only
air-conditioning but also refrigeration (food storage etc.).
The vast
use of present compression cooling machines is also responsible for an
increasing peak demand of electrical power in summer which reaches already the
capacity limit in some southern countries. Because most of the electrical power
stems from fossil fired power plants this also increases the production of CO2
which is no longer acceptable. A more innovative approach is to use solar energy
from thermal collectors as driving force for air-conditioning systems. This idea
is very promising in the sense that to some extent the demanded cooling power is
correlated with the incident solar radiation intensity which also delivers the
driving force.
In principle compression cooling machine can be driven
by solar energy i.e. by electricity from photovoltaic panels but we will
restrict to sorption cooling machines using heat from a thermal solar collector
due to the advantage of using environmental harmless refrigerants and the higher
market penetration of thermal solar collectors. A higher market penetration is
also found for absorption cooling machines compared to desiccant cooling
systems. Moreover absorption machines can also be used as retrofit in standard
air conditioning systems using chilled water. Solar collectors are used
for vaporization heat in absorption machine.
In Kuwait, where air
conditioning is essential for summer cooling in residential, commercial and
public buildings, the use of solar for air conditioning has received serious
attention during the seventies and eighties. Development has primarily focused
on modifying conventional steam-fired cooling systems for use with solar-heated
water at temperatures below 100°C. Some attention has also been paid to using
photovoltaic systems to generate the electricity needed to operate a
conventional vapour compression air conditioning unit.
SOLAR DRYING
A solar collector that heats air,
can be used as a cheap heat source for drying crops like corn, fruit or
vegetable. Since solar air collectors can efficiently increase the ambient air
temperature by 5 to 10 degrees Celsius (some sophisticated devices by even
more), it can also be used effectively for air conditioning in warehouses.
The use of simple and low cost solar air collectors for heating the
drying air of crop dryers offers a promising alternative to reduce the
tremendous post harvest losses in developing countries. The lack of adequate
storage and preservation facilities in the developing countries result in
considerable food losses. Although reliable estimate of the magnitude of the
post harvest losses in these countries is not possible, some references
indicates estimates of about 50 to 60%. To avoid such losses, growers usually
sell of their produce immediately after harvest at low prices. Reduction in
these losses through the processing of fresh products into dried products would
be of great significant to growers and consumers alike. In several developing
countries, open air sun drying is the widely practiced method of food
preservation. This involves spreading the fresh material on the ground, on
rocks, along the roadside, or on the roofs. The advantage of this method lies in
its simplicity and cheapness. However, the quality of the final product is low
due to long drying time, contamination by dirt and dust, infestation by insects
and degradation by overheating. Furthermore, drying to a low moisture content is
difficult resulting in spoilage during subsequent storage. The introduction of
solar dryers is an appropriate technology that can help to improve the quality
of the dried products and to reduce the wastage.
Various types of
small scale solar dryers were developed for drying small amounts of agricultural
products in developing countries. In the natural convection dryers, the solar
air heater is either incorporated into the dryer, or the air heater is connected
to a cabinet or chamber dryer. The solar air-collector may consist of a black
mat covered by a plastic plate. The air is drawn through the mat, where it is
heated, and thereafter blown through the crops. These dryers can be used both in
arid and humid regions for drying fruits, vegetables and spices. Due to their
enlarged capacity they are mainly used on larger farms or by cooperatives for
producing high quality products. Integrating the solar air heater into the south
oriented roof of the barn is common system used in industrialized countries for
drying hay.
Solar dryers are usually classified according to the mode
of air flow into natural convection and forced convection dryers. Natural
convection dryers do not require a fan to pump the air through the dryer. The
low air flow rate and the long drying time, however, result in low capacity and
product quality. Thus, this system is restricted to the processing of small
quantities agricultural surplus for family consumption. Where large quantities
of fresh produce are to be processed for the commercial market, forced
convection dryers should be used. One fundamental disadvantage of forced
convection dryers lies in their requirement of electrical power to run the fan.
Since the rural or remote areas of many developing countries are not connected
to the national electric grid, the use of these dryers is limited to electrified
urban areas. Even in the urban locales with grid-connected electricity, the
service is unreliable. In view of the prevailing economic difficulties in most
of these countries, this situation is not expected to change in the foreseenable
future. The application of photovoltaic to generate the electricity required by
the fan could boost the dissemination of solar dryers in the developing
countries.
In developed countries the solar air heater usually
consists of a black absorber foil, a transparent plastic foil where the air is
forced by a fan between the space. To enlarge the collector area, the roof is
extended southward to the ground and the whole roof is used as collector. The
solar greenhouse dryer is used for drying medicinal and aromatic plants on large
farms. By using a photovoltaic driven blower, it can be secured that only when
the sun shines, air is blown in. Such installations are commonly used in summer
cottages in Denmark and Sweden, where they keep the houses dry most of the
year.
While solar drying has many advantages over sun drying, lack of
control over the weather is the main problem with both methods. In many regions
weather is not suitable for sun or solar drying because there are few
consecutive days of high temperatures and low humidity. It is likely that the
food will sour or mold before drying is completed.
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