Solar heating
This section is mainly covering active solar heating, where the solar energy is transferred to heat in solar collectors and from there transported by a fluid to its final use. Another important use of solar heat is passive solar heating, where buildings are designed to capture the maximum of the solar energy coming through windows and upon walls to be used for space-heating.

Energy Content
The yearly incoming solar energy varies from 900-1000 kWh/m2 North of the Baltic Sea to e.g. 1077 kWh/m2 in Central Europe (Hradec Kralove in Bohemia) and up to 1600 kWh/m2 in Mediterranean and Black Sea areas on a horizontal surface. On a south sloping surface, the incoming solar energy is about 20% higher.

Resource Estimation
The incoming solar energy on most buildings exceed the energy consumption of the building, e.g. a 5 storey apartment house in Hradec Kralove receives 1077 kWh/m2, while each storey consumes about 150 kWh/m2 for heating and 25-50 kWh/m2 for light and cooking, adding up to 875 - 1000 kWh/m2 for the 5 storeys together (all measured per. m2 horizontal surface).
While the incoming solar energy is sufficient over the year, the practical usable resource is limited by the fluctuations of the solar energy and the storage capacity. Reasonable good estimates of usable solar heat can be made as a fraction of the different heat demands.

For house-integrated systems, the limitations are normally that solar heating can only cover 60-80% of the hot water demand and 25 - 50% of space heating. The variations are depending on location and systems used. In Northern Europe the limitations are respectively 70% and 30% for hot water and space heating coverage.

For central solar heating systems for district heating, analyses and experience show that these systems can cover 5% of consumption without storage, 10% with 12 hour storage and about 80% with seasonal storage. These figures are based on district heating systems which have 20% average energy losses and mainly deliver to dwellings. The energy delivered from solar heating systems without storage is by far the cheapest solution.

For industries that uses heat below 100oC, solar heating can cover about 30% if they have a steady consumption of heat. For drying processes solar energy can cover up to 100% depending on season, temperature, and limitations to drying period.
Solar heating to swimming pools can cover most of the heat demand for indoor pools and up to 100% for outdoor pools used during summer.

To evaluate the potential for solar heating is, thus, most a question of assessing the demand for low-temperature heat.

Barriers
Most applications for solar heating are well developed, and the technical barrier is more lack of local availability of a certain technology than lack of the technology as such. Thus the main barriers, beside economy, are:
lack of information of available technologies and their optimal design and integration in heating systems.
lack of local skills for production and installation.

In some occasions lack of access to solar energy can be a barrier. For active solar heating it is almost always possible to find a place for the solar collectors with enough sunshine. For passive solar energy, where the solar energy is typically coming through normal windows, neighbouring buildings or high trees can give a severe reduction of the solar energy gain.

Effect on economy, environment and employment

Economy
The economy of using solar energy ranges from almost no costs, when simple passive solar energy designs are integrated into building design and land-use planning to very high costs for solar heating systems with seasonal storage. For solar heating systems, some typical prices are for installed systems:

Application	Collector size	Annual production	Invest./area	Invest./annual production
Single family hot water, Northern	4-6 m2	2,000 kWh	1000 ECU/m2	2.5 ECU/kWh
Single family hot water, South EU	4 m2	2,500 kWh	250 ECU/m2	0.4 ECU/kWh
Swimming pool, outdoor	100 m2	10,000 kWh	10 ECU/m2	0.1 ECU/kWh
District heating	1000 m2	440 kWh/m2	181 ECU/m2	0.41 ECU/kWh

Notes:
The application for single family hot water, Northern is a typical system for hot water as used in Nordic countries and Germany with anti-freeze agent, high insolation, and closed circuit. The single family Southern Europe is a single family system as used in Greece. Prices in Central & Eastern Europe can be considerably lower. Self-built systems are also considerably cheaper.
The annual production is given for Northern European conditions, except for the Southern European single family system, where production is given for Southern European conditions.
The savings are net savings, in most applications in Northern Europe, the solar heat replaces an oil or gas boiler that has a very low efficiency (often 30-50%) during summer. The total savings can then be 2-3 times larger than the net savings.

Environment
The heat produced in a solar heater replaces energy produced in more polluting ways, which is the main environmental effect. The energy produced to produce a solar heater is equivalent to 1-4 years of production of the solar heater.
Usually the solar collectors are mounted on top of a roof, in which case there is no local impact of the environment.

Effects of employment
The majority of the employment is in the production and installation of solar heaters. Based on Danish experience, the employment is estimated to 17 man-years to produce and install 1,000 m2 of solar heaters for families. These 1,000 m2 replaces 800 MWh of primary energy (net energy production 400 MWh). With 30 years lifetime of the solar heaters, the constant employment of producing solar heaters to replace 1 TWh will be 700 persons.

Country Estimates
In principle all heat demand can be covered by solar energy with seasonal storage. There is therefore no absolute limit to this resource, only economical limitations. In Denmark it is estimated that without seasonal storage, solar energy can cover 13% of the heat demand, including commercial and industrial use. In more sunny places, this fraction is naturally larger.

Photovoltaics Electricity
Photovoltaic (PV) cells produce direct current electricity with output varying directly with the level of solar radiation. PV cells are integrated in modules which are the basic elements of PV systems. PV modules can be designed to operate at almost any voltage, up to several hundred Volt, by connecting cells and modules in series. For applications requiring alternating current, inverters must be used.

PV cell efficiency is calculated as the percentage difference between the irradiated power (Watt) per area unit (m2), and the power supplied as electric energy from the photovoltaic cell. There is a distinction between theoretical efficiency, laboratory efficiency, and practical efficiency. It is important to know the difference between these terms, and it is of course only the practical efficiency which is of interest to users of photovoltaics.

Practical efficiency of mass produced PV cells:
single crystalline silicon : 16 - 17%
polycrystalline silicon : 14 - 15%
amorphous silicon  : 8 -  9%

PV systems are usually divided in:
1. Stand-alone systems that rely on PV power only. Beside the PV modules they include charge controllers and batteries.
2. Hybrid systems that consists of a combination of PV cells and a complementary means of electricity generation such as wind, diesel or gas. Often smaller batteries and chargers/controllers are also used in these systems.
3. Grid connected systems, which work as small power stations feeding power into the grid.

Tips and Applications
When designing a photovoltaic installation a number of things must be taken into consideration, if an optimum solution is wanted. At first it must be clarified, how much energy is demanded from the photovoltaic installation. After that the total daily consumption in Ampere hours (Ah) must be estimated. From the total daily and weekly consumption the total energy storage capacity can be calculated. It must be considered how many days without sun, the installation shall be capable of functioning. At the end it can be calculated, how many photovoltaic modules are required to produce sufficient energy. The photovoltaic application can also be combined with other energy sources. A combination of small wind generators and photovoltaics is an obvious possibility. The energy can be stored in good lead batteries (solar batteries, traction-batteries) or in nickel/cadmium batteries.

Resource estimation
The solar energy which is available during the day varies because of the relative motion of the sun, and depends strongly on the local sky conditions. At noon in clear sky conditions, the solar irradiation can reach 1000 W/m2 while, in very cloudy weather, it may fall to less than 100 W/m2 even at midday. The availability of solar energy varies both with tilt angle and the orientation of surface, decreasing as the surface is moved away from South.

Commercial cells are sold with rated output power (Watt peak power, Wp). This corresponds to their maximum output in standard test conditions, when the solar irradiation is near to its maximum at 1000 W/m2, and the cell temperature is 25oC. In practice, PV modules seldom work at these conditions. Rough estimate of the output (P) from PV systems can be made according to the equation:

where:
Pp  is rated output power in kW, which is equivalent to efficiency * area in m2
I  is solar irradiation on the surface, in kWh/m2 per day
PR is Performance Ratio determined by the system.

Daily mean solar irradiation (I) in Europe in kWh/m2 per day (sloping south, tilt angle from horizon 30 deg.):

	South Europe	Central Europe	North Europe
January	2,6	1,7	0,8
February	3,9	3,2	1,5
March	4,6	3,6	2,6
April	5,9	4,7	3,4
May	6,3	5,3	4,2
June	6,9	5,9	5,0
July	7,5	6,0	4,4
August	6,6	5,3	4,0
September	5,5	4,4	3,3
October	4,5	3,3	2,1
November	3,0	2,1	1,2
December	2,7	1,7	0,8
YEAR	5,0	3,9	2,8

Typical Performance Ratios:
0.8 for grid connected systems
0.5 - 0.7 for hybrid systems
0.2 - 0.3 for stand alone systems for all year use

Typical System Performance
Stand alone systems have low yields because they operate with an almost constant load throughout the year and their PV modules must be sized to provide enough energy in winter even though they will be oversized during summer.
Typical professional systems in Europe have annual average yields of 200 - 550 kWp.

Hybrid systems have higher performance ratio, because they can be sized to meet the required load in the summer and can be backed up by other systems like wind or diesel in the winter and in bad weather.
Typical annual average yield is 500 - 1250 kWh/kWp depending on the losses caused by the charge controller and the battery.

Grid connected systems have the highest Performance Ratio because all of the energy which they produce can either be used locally or exported to the grid.
Typical annual yield is 800 - 1400 kWh/kWp.

Barriers
Despite a sharp decline in costs, PV cells currently cost 5 US$/Wp (4 ECU/Wp). Electricity generation costs is currently 0.5 - 1 ECU/kWh, which is higher than from other renewable energy sources. In the future, the costs of PV are expected to fall with increasing utilization. Despite its high costs, PV electricity can be cheaper than other sources in remote areas without electric grid and where production of electricity by other means like diesel is difficult or environmentally unacceptable (mountain areas).

Effects on economy, environment and employment
When the only cost-effective applications of PV systems in Europe are remote areas without electric grid, it will have a positive economical effect only for those areas.

There are no environmental effects of using PV systems. Environmental problems can occur in the production of the cells, and in the production and (improper) disposal of the batteries.

The use of PV is not expected to have any measurable employment effect in Europe for the time being.

Hand Rule
In a typical photovoltaic system based on  crystalline Silicon with 12% efficiency each kWp of installed power capacity can produce 1150 kWh of electricity per year for grid connected systems and 300 kWh/year for stand alone systems in Central Europe.

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