SOLAR SPACE HEATING UNDER CENTRAL EUROPEAN CONDITIONS

Transcription

1 SOLAR SPACE HEATING UNDER CENTRAL EUROPEAN CONDITIONS Werner Weiss AEE - Arbeitsgemeinschaft ERNEUERBARE ENERGIE Feldgasse 19, A-82 Gleisdorf, Austria Tel.: , Fax: , w.weiss@aee.at Abstract - In recent years the rate of growth in the use of solar collectors for domestic has shown that thermal solar systems are both mature and technically reliable. The demand for solar combisystems, both for preparation and space, is also increasing rapidly. The combination of thermally well insulated buildings, low temperature heat supply systems (wall or floor systems) and solar systems with short term, allows high fractional energy savings of the domestic-hot water and space requirements to be provided at an acceptable cost. Since 1994, AEE has carried out several projects to develop solar combisystems for single family houses, multiple family dwellings and hotels. Some of the systems have been monitored and evaluated and the results show the possibilities for a huge future market. 1. INTRODUCTION The increase in greenhouse gasses in the atmosphere and the potential global warming and climatic change associated with it, represent one of the greatest environmental dangers of our time. The anthropogenic reasons of this impending change in the climate can for the greater part be put down to the use of energy and the combustion of fossil primary sources of energy and the emission of CO 2 associated with this. Today, the world s energy supply is based on the non-renewable sources of energy: oil, coal, natural gas and uranium, which together cover about 82% of the global primary-energy requirements. The remaining 18% divide approximately 2/3 into biomass and 1/3 into hydro power. The effective protection of the climate for future generations will, according to many experts, demand at least a 5% reduction in the world-wide anthropogenic emission of greenhouse gases in the next 5 to 1 years. With due consideration to common population growth scenarios and assuming a simultaneity criterion for CO 2 emissions from fossil fuels, one arrives at the demand for an average per-capita reduction in the yield in industrial countries of approximately 9%. This means 1/1 of the current per-capita yield of CO 2 [1]. A reduction of CO 2 emissions on the scale shown in figure 1 will require the conversion to a sustained supply of energy that is based on the use of renewable energy with a high share of direct solar energy use. This means: if the direct use of solar energy for purposes via solar collectors is to make a significant contribution to the energy supply, it is necessary that solar- technologies be developed and widely applied that do more than only supply domestic-hot-water. The demand for solar systems for combined domestichot-water preparation and space, so-called solar combisystems is increasing rapidly in several countries (fig.2) In 1997 installed collector area [m²] Share of Solar Combisystems in Selected Countries Total collector area Solar Combisystems 5 Germany Austria Switzerland Denmark Fig. 2: In 1997 installed collector areas and share of collectors for solar combisystems in selected countries [4] Fig. 1: Per-capita emissions of carbon into the atmosphere required to meet climate stabilisation agreements with a doubling of population levels. Research and Development projects from the European Union and the International Energy Agency (IEA) show that international interest has grown in this technology Since December experts from 8 European countries and the USA and 11 solar industries have been working together on Task 26 of the Solar Heating and Cooling Programme of the International Energy Agency (IEA) to further develop and optimise solar combisystems for detached one-family houses, groups of one-family houses and multiple-family houses in the next two years. Furthermore, standardised classification and evaluation processes will be developed for these systems within

2 the framework of this project. These serve as a basis for the elaboration of suggestions for the international standardisation of combisystem test procedures. 2. THE POTENTIAL OF SOLAR SPACE HEATING As a result of climate conferences in the last decade and the discussion about sustainable development, the European Commission has laid down its goals with respect to future development in the field of renewable sources of energy in the White Paper [5]«Energy for the Future: Renewable Sources of Energy». The Commission has as a strategic goal:... to increase the market share of renewable sources of energy to 12% by the year 21. The yearly increase in the installed collector area named in the White Paper in the member states is estimated at 2%. Thus, solar systems in operation in the year 21 would correspond to an overall installed collector area of 1 million square meters. A realistic approach is to assume that in the next ten years, about 2% of the collector area yearly installed in the European Union will be used for solar combisystems. This means that in accordance with the European Commission White Paper, in the countries of the European Union alone around 12 solar combisystems with 1.9 million square meters of collectors can be installed per annum. Million Square Metres Increase in the installed collector area in the EU Total collector area Share of combisystems Fig. 3: Objectives for the installed collector area until 21 in the European Union s member countries. 3. EXPERIENCES AND SYSTEM DESIGNS IN AUSTRIA In recent years in Austria the growth rate of the use of solar energy for domestic-hot-water, has shown that thermal solar systems are both mature and technically reliable. Every day, thousands of installations demonstrate the advantages of this undisputedly ecologically harmless energy source. By the end of 1999, a total collector surface area of 2.1 million square meters had been installed in Austria. This figure puts Austria amongst the leading European countries when it comes to the amount of newly installed collector surface area per head of population per year. The demand for solar combisystems, is increasing rapidly. The combination of thermally well-insulated buildings, lowtemperature heat supply systems (wall or floor systems) and solar systems with a short-term, allows high fractional energy savings of the domestic-hot water and space requirements to be met at an acceptable cost. In comparison to systems with a seasonal, the costs of which are currently not justifiable for single-family houses, this combination provides a more cost-effective system. 3.1 Solar energy availability The solar energy available in central Europe in summer is more than twice that available in winter. Virtually, the opposite applies to the energy demand for space. In comparison to a hot-water supply, the load is dependent upon the outside temperature. Measurements of solar radiation and temperature in the transitional periods (September - October and March - May) clearly show that solar radiation availability is relatively high at the beginning and the end of the space season. Even on winter days, energy demand and solar radiation are partially related. The space demand on cloudy days is lower than on cold, clear, winter days where the solar collectors are able to deliver more energy. Figure 4 shows the solar radiation on a horizontal plane in Graz, Austria. It can be seen that, at this latitude, not only are there strong seasonal variations in radiation, but also that weather patterns cause radiation levels to change quite widely on a daily, or even hourly, basis. In order to make efficient use of the available solar energy supply - it is necessary to even out these fluctuations - by means of either auxiliary or energy systems, to ensure a comfortable room temperature. [kwh/m²] 9, 8,1 7,2 6,3 5,4 4,5 3,6 2,7 1,8,9, Solar-Radiation Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Total: kWh/m² Fig. 4: Solar radiation on a horizontal plane in Graz, Austria A basic requirement for all new buildings is the efficient use of energy through a high insulation standard. Low-energy houses, with an annual space demand of less than 7 kwh per square meter of living space, offer the ideal opportunity for solar space. A low-temperature heat supply system with low inlet temperature is an additional favourable requirement for solar space. 3.2 Heat The wide fluctuations in incident solar radiation shown in figure 4 can be evened out by the use of a heat system. The following opportunities are available for balancing out the variations in energy supply and demand: Hourly, or even overnight, variations can be simply compensated for by the thermal capacitance of the heat supply system (e.g., floor ) or the mass of the building.

3 If insufficient solar radiation is available for several days, a small thermal volume can be used to make up the difference. Seasonal variations in solar energy supply can only be compensated by the use of a seasonal. Several systems in recent years have shown that it is possible to store summer heat in large water reservoirs (6-13 m 3 for a single-family house) for use in winter. In the interest of cost, solar systems with a seasonal will first be used in large systems in conjunction with district, on the basis that specific costs decrease drastically with increasing size. Interesting examples available in Sweden, Denmark and Germany indicate an interesting way forward [6]. For single- and multiple-family dwellings, the concept of partial solar space (solar energy and auxiliary energy source) is more interesting for economic reasons. 4. EXAMPLES WHICH HAVE BEEN CONSTRUCTED Since 1994, AEE carried out several projects supported by the Austrian Federal Ministry for Science and Research and the Federal Ministry for the Environment to develop and optimise solar combisystems for one and two family houses as well as for hotels and multiple-family dwellings. Several thousand solar combisystems have been erected in Austria since the beginning of the 9 s in single-family homes. In the last three years it has, however, also been possible to construct numerous solar combisystems in hotels and multiple family dwellings and for the of production halls. As a result of conquering these new applications, half of the collector area erected between 1998 and 1999 was accounted for by combisystems. This corresponds to an installed collector area of approximately 16, square metres in two years. The section which follows presents some typical solar combisystems. 4.1 Solar combisystems for one and two family houses Typical solar combisystems for single family houses in Austria consist of 15-3 m 2 collector area in combination with volumes of 1 to 3 m 3 dependent upon the conditions under consideration, the heat supply system and the thermal quality of the building. Systems following this concept can readily achieve high fractional energy savings, at reasonable costs. An installation is presented here as an example of a singlefamily housing concept which supplies a single-family home with a heat load of 8 kw to 15 m² of residential area. Basically this comprises a 25 m² collector field and a 2 m³ space store with a built-in stratifier. The preparation is performed via an external plate heat exchanger. The DHW temperature is adjusted to the chosen set-point temperature by controlling the speed of the pump located in the primary loop of the heat exchanger. On the one hand, this concept guarantees low return flow temperatures in the and on the other hand it prevents the growth of legionella since the hot water is heated in the flow. If the solar energy is not sufficient to heat the building, the biomass boiler supplies the auxiliary heat required. This combination of a biomass boiler and a allows high boiler efficiency rates since the boiler can almost always be operated at maximum performance. The solar plant is designed as a low-flow system with a mass flow of 12 kg/ m².h. The is loaded in line with the temperature via the stratifier. In the event of auxiliary heat from the biomass-boiler, the feed is performed via the loading and unloading pipe up in the. Collector Biomass boiler Fig. 5: Hydraulic scheme solar combisystem for a singlefamily house Assembly of collector Flat-plate collectors are used almost exclusively for solar combisystems. Vacuum-tubular collectors have a market share of less than 1%. In principle there are several possibilities for the assembly of the collector: it can be freely assembled, assembled on the roof or integrated in the roof. The collectors are integrated in the roof covering if the orientation and inclination of the roof permit this. Since in this variant all of the components are integrated in the roof, there are fewer outer surfaces and no connecting pipes trailing outside, one encounters the lowest heat losses with this variant. With plants for heat support in particular where larger collector areas are assembled, the collector field forms the rain-proof level in a roof integration. Since the conventional roof covering can be left out, this is the most favourable solution both from an aesthetic as well as an ecological and economic point of view. Large-scale flat collectors with sizes of up to 16 square meters have proved to be particularly effective in this area. Thus it is possible to assemble large-scale collectors in a cost-favourable and fast manner. Fig. 6: Installation of a large-scale collector

4 4.2 Solar combisystems for multiple-family dwellings Since it has been possible to enter the market for thermal solar plants for single family and two family homes as of the beginning of the 9 s, for further dissemination it is extremely important to develop and realise systems adapted for multiple family dwellings. The plants built so far have shown that the average system prices per square meter collecor area can almost be halved in these plants compared to plants for single family homes. Since typical multiple family dwellings in Austria normally comprise 6 to 2 residential units, a plant of this scale is presented here. residential unit - containing 3 l each. In relation to the overall energy requirement of the building (DHW and space ) the solar plant produces a rate of approximately 11 %. The individual flats are supplied by the two 3 l central s via a local network which is operated 22 hours a day with a low temperature level of 4 C (space operation). To prepare, the same local network is operated for two hours (night time) at a higher temperature level (65-7 C). During this period the space is switched off and only the decentralised DHW- s are loaded. On the one hand, this plant concept makes it possible to erect the plant in a cost-favourable manner (two-pipe-net) as well as to reduce network losses to a minimum. 4.3 Solar Combisystems for Hotels Apart from residential buildings hotels also have a high requirement for low-temperature heat. The specific requirement per person is normally higher than in residential buildings. Numerous hotels and places of accommodation were equipped with solar plants supported by grants from the Österreichische Kommunal Kredit AG and the Federal Ministry for the Environment. The system concept and some results from Hotel Sylvretta- Haus, which was measured in detail and evaluated within the framework of the EU project Sunny Resorts by the AEE, will be presented in the following. Fig. 7: Multiple dwelling Kuchl, Salzburg The residential project in Kuchl comprises 3 buildings with a total of 1 residential units. The heat supply to the flats takes place via a central system which comprises a woodchip-boiler and a thermal solar plant. The thermal solar plant which is 85 m² accounts for the of domestic by around 7% on a yearly average. During the period the solar plant also supplies the space in combination with the biomass boiler. co llector area energy T3 T2 boiler Fig. 8: Hydraulic scheme for multiple dwellings The supply concept is based on a central collector plant with two energy s which contain 3 l each and 1 decentral domestic s one in each Fig. 9: Hotel Sylvretta Haus, altitude 22 m The hotel on the Bielerhöhe (22 m above sea level) has 28 beds and a restaurant with 4 seats for day guests. It is open from December to April respectively from July to October. The average consumption of per day equals about 1,25 litres (6 C). Due to the location of the hotel high up in the Alps there is a need for heat for space almost all year round. The annual requirement equals around 11 MWh. The heat requirement for and space is covered by almost 3% by a 6 m² collector plant. As figure 1 shows the heat produced by solar energy can be fed either into the space store or if necessary directly into the supply system. The ( in system) comprises a 14, litre space store with 3 integrated domestic- stores with a capacity of 31 litres each. The auxiliary of the space store is performed via two electrical heaters which are arranged at different levels

5 in the. As the efficiency of a solar space system is also determined by the temperature level of the heat supply and a low return temperature to the space store, the building is equipped with a special low-temperature wall system. The medium flow temperature of the lowtemperature wall system is 3 C. collector area in system circulation pipe 5. CONCLUSIONS Several thousand solar combisystems for single family homes and several hundred plants for multiple family dwellings and hotels demonstrate the high performance of solar space systems in central European conditions. The demonstration plant phase has therefore been overcome. To achieve European wide market penetration in the years to come it will be necessary to optimise the systems and standardise tried and tested system concepts. For the development of efficient technologies it will be particularly important to conduct more research work to open up the way to a solar future. REFERENCES Cold water [1] Lang, R.W., Jud, T., Paula, M.: Impulsprogramm Nachhaltig Wirtschaften, Bundesministerium für Wissenschaft und Verkehr, Wien, 1999 Fig. 1: Hydraulic scheme solar system with in To avoid a covering of snow in the winter as far as possible and to optimise the solar yields for the winter months, the collectors are erected at an angle of 78. Monitoring Reults [2] Nakicenovic, N., u. a.: Global Energy Perspectives to 25 and Beyond, Joint IIASA-WEC Study, Report 1995, International Institute for Applied Systems Analysis, Laxenburg 1995 [3] Langriß, O., Luther, J., Nitsch, J., Wiemken, E.: Strategien für eine nachhaltige Energieversorgung Ein solares Langfristszenario für Deutschland, Bericht des Deutschen Zentrums für Luft- und Raumfahrt e.v. und des Fraunhofer- Instituts für Solare Energiesysteme, Freiburg, Stuttgart, Oktober 1997 [kwh] ,9,8,7,6,5 Qcoll Qaux. SF [4] Suter J.M., Letz T., Weiss W., Inäbnit J. Solar Combisystems in Austria, Denmark, Germany, Sweden, Switzerland, the Netherlands and the USA, Overview 2, Bern ,4,3,2,1 [5] European Commission, White Paper for a Community Strategy and Plan for Action, Brussels 1998 January February March April May June July August September October November December Fig. 11: Monitoring results Hotel Sylvretta Haus The plant has produced excellent operating results for several years. Figure 11 depicts the monitoring results from the year Due to the very good solar radiation conditions (1,33 kwh/m²a) and the optimum operating conditions the collector produces between 65 kwh/m² and 75 kwh/m² per annum on average. [6] Dalenbäck Jan-Olof, Solar Heating with Seasonal Storage, Some Aspects of the Design and Evaluation of Systems with Water Storage, Götebrog 1993 [7] Arbeitsgemeinschaft Erneuerbare Energie, Heizen mit der Sonne, Handbuch zur Planung und Ausführung von solaren Heizungssystemen für Einfamilienhäuser, Gleisdorf 1997 [8] Streicher Wolfgang, Teilsolare Raumheizung, Auslegung und hydraulische Integration, Gleisdorf 1996