Sweden, 35 years experiences of dynamic energy design in buildings. Can it be replicated fast enough?
Falk, Håkan - Isfält, Engelbrekt
Energy Saving Now (http://EnergySavingNow.com)
Abstract
The first obvious measure to reduce the energy use for heating of buildings is to improve the insulation and reduce the air leakage. Only simple steady state calculations are necessary to show this. Today this is reflected in most building codes prescribing
U -values and air flow rates.
When those measures come to a certain limit internal and solar heat gains will cover most of the heat losses and often even cause excess heat and overheating problems. Then there is nothing to gain by going on this way.
Passive technology is the next step in saving energy in well-insulated and airtight buildings. The thermal mass of the building structure is utilized to even out indoor temperature fluctuations and move excess energy from day to night, from one day to the next or from periods of warm weather to colder ones.
This is what always has taken place in buildings before technology made it possible to maintain any desired constant indoor temperature by a prompt supply or extraction of energy.
During the last decades building technology has been dominated by the stationary way of thinking, neglecting the storing - emitting process. One reason for this is the difficulties of handling the process mathematically. However, computers have now made it possible to solve these problems.
A very modern tool has thus helped us to recover old, lost experiences and make advantages to be gained from the building traditions clearly discernible, which leads to the basic philosophy to work with nature instead of against it.
This paper gives examples showing how the building thermal capacity can be utilized to reduce the energy use to a level far beyond what a steady state calculation indicates.
Background.
Many load calculation methods or computer programs cannot be directly employed for thermal storage systems. In Sweden research has been focused on the technique of using building mass since the late fifties. A computer program, BRIS, was developed at the Royal Institute of Technology in Stockholm with support from the Swedish National Board of Building Research. The program was based on fundamental physical relationships and finite difference techniques (Crank - Nicolson) were used to solve the Fourier equations and the boundary conditions were treated in detail. BRIS has been developed continuously with regard to the users and growing computer capacity.
The control strategy is based on a sequence of restrictions on the possible sources of heating, cooling and economiser cycles. The restrictions are relaxed successively within each time step in the building model until a solution is found.
By combining loads and systems minimum energy strategies can be defined and found by the program. When limiting the installed capacities the building dynamics will be more active in the control process which has shown to give a surprisingly high potential to reduce peak power problems and energy use without sacrificing (maybe rather improving) the comfort.
Among serious consultants BRIS is a natural tool in the design process today and we have seen about one thousand results in the shape of real buildings.
Significant energy savings have been realised and in 1990 the originators of BRIS received the Swedish Great Energy Award for "distinguished contributions in the field of energy conservation".
Passive technology is a well-known concept today, but still many buildings with very complicated and oversized HVAC systems are built. Energy costs and peak - power problems now lead to a wakening need to improve the competence and reintroduce the physical laws in the design process.
Now the next generation of BRIS, called IDA, is being developed. This is a modular system for applications on different complicated processes. For description of the mathematical component models a special format, the Neutral Model Format (NMF) has been developed. NMF models are program neutral and can be automatically translated into the formats required by a number of different simulation environments such as IDA, TRNSYS, HVACSIM+ and SPARK. Based on NMF, environment independent application libraries can be established. ASHRAE has assumed the responsibility of maintenance.
Experiences from realised projects.
There is a large potential in utilising the building dynamics together with installed equipment for climatisation. The basic philosophy is to work with nature instead of against it.
Accordingly the experience from buildings where BRIS has been used in the design work (most governmental, official and private business buildings downtown Stockholm) is that high comfort can be provided even with very low installed capacities for cooling (1/2 or 1/3 compared to buildings where more conventional design tools have been used). Also in hot, arid climates considerable savings have been done.
However, the utilisation of building dynamics is poorly understood in practice today. Also modern, advanced control systems seem to have recoiled upon the ambition to maintain constant temperatures or to force the temperature to follow special schedules. If there are large capacities for heating and cooling available this could lead to peak power problems and a tremendous waste of energy. On the other hand, effective climate control often can be achieved with modest capacity and much less energy if passive techniques and the building mass are incorporated in the control policy. This is shown in the following example:
Example.
Office 10 m2, surrounded by similar rooms:
Exterior wall: 12.5-cm brick
12 cm of mineral wool
10 cm of concrete
Partitions: 2*13 mm of plaster board
Floor-
Ceiling slab: 20 cm of concrete,
Window: 1.8 m2 (glass area), three panes of ordinary window glass. Venetian blinds between the outer panes.
Outdoor
Temperature: 19±6 °C max. at 3 PM.
Airflow rate: 5 ACH.
Remark: From the energy efficiency point of view this is a very high value. More common today is 0.5 ACH compensated by fancoils or cooling panels for heat extraction. Due to the sick building problem the supply airflow rate is now being discussed, and will probably increase in the future.
Infiltration: . 2 ACH
Solar (Stockholm July South) and internal heat gains during the office hours are shown in Fig. 1.
Fig. 1. Heat gains. Maximum value = 626 W.
 Fig. 2. Effective temperatures during the office hours using different control strategies.
Curve 1: Here we see a control policy typical in many modern buildings. The cooling system is operating only during occupied periods and the control system is designed to maintain constant, 25 °C, room air temperature. The effective temperature is higher due to radiation. The cooling coil load is large, 472 W. Daily energy for cooling: 3.11 kWh + fanpower 0.50 kWh = 3.61 kWh.
Curve 2: Now we operate the equipment continuously. Space effective temperatures are cooler in the early occupancy hours due to lower surface temperatures, but still well within comfort range. These lower surface temperatures also mean lower capacity and energy required for cooling throughout the day. Also, since the additional hours are mostly during cooler hours, some of the cooling can be provided with outdoor air (economiser cycle). To see if we can go further with this strategy, we have stepwise reduced the installed capacity to 27 % of the original, and use 22 °C set point. We see that we still are well within comfort conditions throughout the occupied period. Daily energy for cooling 1.66 + 1.09 = 2.75 kWh (76 % of the original).
Curve 3: Finally we reduce the cooling coil capacity to 0 and compensate by letting the supply air pass the holes in hollow core concrete slabs (Thermodeck). The stronger thermal coupling between the air and the mass gives a better use of the thermal capacity. The comfort is better without cooling than in the original case. Daily energy use for cooling 1.09 kWh (only fanpower).
Fig. 3. Negative items in the room heat balance when the cooling system operates only at daytime. Set point 25 °C. The major part of the heat gains is extracted by the zone supply air (maximum 442 W). Only 136 W is stored in the structure (coming back at night). Required cooling coil capacity is 472 W. The effective temperature exceeds 25 °C during more than 8 of 9 office hours, see curve 1 in Fig. 2.
Fig. 4. Negative items in the room heat balance when the cooling system operates continuously. Set point 22 °C. A smaller part of the heat gains are now extracted by the supply air (maximum 243 W) and 435 W are stored in the structure. Required cooling coil capacity is reduced to 127 W (27 %). Still the comfort is improved, see curve 2 in Fig. 2.
Fig. 5. Negative items in the room heat balance when using ventilated hollow core concrete slabs and no cooling. The structure is fully utilized and takes care of more than 75 % of the gains in the afternoon. The comfort is better than in the original case, see curve 3 in Fig. 2.
TermoDeck buildings.
There are around 300 to 400 buildings with this system in use in Europe and some in Saudi Arabia today. When the design, control and operation is handled by the originators extremely low energy use and good comfort have been reached.
One example is the Elisabeth Fry Building, University of East Anglia Norwich, England. This building has been carefully investigated by different researchers. Results have been published under the headline "The best building ever?"
Fig. 6 shows that the energy use is only 15 % of a Standard Air-conditioned Office.
Fig. 6.
Conclusions.
It is now time to use simulation programs and knowledge from the use of these tools not only for the design of systems meeting requirements from an uninformed builder, but also to convince him what poorly formulated requirements will cost him.
More cooperation with the control engineers is also necessary. Computerized control systems have a high potential, and could be used not only for prompt compensations, but also for advanced smoothing and forecasting techniques. Energy supply or extraction could then be made using low powers during long periods, for instance during the night hours, to prepare the building for the next morning. Peak periods can be avoided until it is necessary. In between, the building takes care of itself.
A proper use of simulation programs will show how the building dynamics can be utilized and result in much more energy and power efficient buildings in the future.
References.
Brown, G.: 1971. Simulation by Digital Computer Program of the Temperature Variation in a room. Proceedings of the First Symposium on the Use of Computers for Environmental Engineering Related to Buildings. National Bureau of Standards, U.S. Government Printing Office, Washington DC.
Brown, G. & Isfält, E.: 1973. Proper use of the heat capacity of buildings to achieve low cooling loads. Contribution to the CIB W 40 Birmingham meeting. (also published in the IHVE Journal, London).
Andersson, L.O., Bernander, K.G., Isfält, E. & Rosenfeld, A.H.: 1979. Storage of Heat and Coolth in Hollow Core Concrete Slabs. Swedish Experience and Application to Large, American - Style Buildings. Report LBL-8913 (1979). Lawrence Berkeley Laboratory, Berkeley, California.
Bring, A.: 1983. Versatile System Simulation with the BRIS Program. Proceedings of the Fourth Symposium on the Use of Computers for Environmental Engineering related to Buildings. Kenchiku Kaikan, Tokyo, Japan.
Isfält, E.: 1988. Simulation of Building Thermal Behavior - 25 years of Experience from 1000 Swedish Projects. Bidrag till the "IEA Modelling Feasibility Study" i Abingdon, Qxfordshire 7-9 dec 1988.
Isfält, E.: 1989. The Thermal Balance of Buildings. A basis for comparisons of computer programs for calculations of the room climate and the power and energy requirements for climate control of buildings. A detailed presentation of a number of BRIS simulations with comments. IEA Annex 21 Subtask C.
Abel, E., Isfält, E och Ljungkrona, I: Analysis of the dynamic energy balance in an occupied office room using simulations and measurements. TRANS ASHRAE Vol 98,1992 Part 2 p 363.
Sahlin, P., Bring, A., Sowell EF. The Neutral Model Format for Building Simulation. Bulletin No. 32, Building Services Engineering, Royal Institute of Technology, Stockholm 1994.
David, J: Breaking the energy barrier. The Elisabeth Fry Building University of East Anglia, Norwich. Case study. M&E Design Oct 1997, London.
PROBE Team’s verdict on the Elisabeth Fry Building: The best building ever? Building Services Journal. London.
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