1 INTRODUCTION 3 SOLAR DISTRICT HEATING PLANTS

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1 Jan-Olof Dalenbäck and Jochen Dahm Department of Building Services Engineering, Chalmers University of Technology, SE Göteborg, Sweden Phone , Fax , Abstract Kungälv Energi AB has decided to build a solar district heating plant with m² largemodule solar collectors on ground. For the time being this will be the largest plant in Europe. The plant design builds on the design in previous solar district heating plants with the main difference that this is the first large plant with anti-reflex coated glass. The calculated investment cost amounts to 19 million SEK. The annual output will vary from 3.5 to 4 GWh depending on operation and weather conditions. The specific investment, of the order of 5 SEK per annual kwh, is about 40% lower than the specific investment costs in the latest Swedish solar district plants. The plant is built by ARCON Solvarme A/S and partly financed within the EC-THERMIE program. 1 INTRODUCTION Large-scale solar heating for residential building areas has a large technical potential in northern and middle European countries with centralised heating system traditions like Sweden, Denmark, Germany, Austria, Poland, the Baltic countries, Finland, etc. Especially considering the possibilities to use solar heating for existing buildings. Large-scale systems makes it further possible to utilise seasonal storage (store heat from the summer to the autumn and the winter), which in turn would make it possible to achieve appreciable solar fractions in more northern countries (residential heating). Here, a new solar district heating plant in Kungälv (Dalenbäck et al, 1998) is presented and related to three existing plants regarding technology and economy. System design, expected cost and performance are presented together with a discussion about further improvements and possible cost reductions. The experiences from the existing plants, in Falkenberg, Nykvarn and Säter, have previously been presented by Dalenbäck et al (1995). These plants represent systems with and without storage, as well as two different collector technologies and have been evaluated and documented in a similar way enabling a detailed comparison. Solar district heating plants are here defined as plants with large module ground mounted collector arrays applied in existing district heating networks. Dalenbäck (2000) presents a general overview of European largescale solar heating applications. 2 APPLICATIONS The are two major types of large-scale solar heating plants, block heating plants with roof integrated collectors on multifamily apartment blocks, and large district heating plants with large module ground mounted collector arrays. There are further two major large-scale solar heating applications; systems with short-term storage designed to cover % of the annual requirements, i.e. mainly pre-heating (40-50 %) of domestic hot water (DHW), and systems with long-term storage (seasonal) capable of covering % of the annual requirements, for space heating and DHW heating in residential buildings. In both cases the common storage media is water. The design of the solar heating systems is basically similar in the two applications except for the storage itself. The most feasible large-scale solar applications at present are; domestic hot water pre-heating in multifamily apartment blocks, institutions, hospitals, etc. using short-term storage, i.e. insulated water tanks, with a solar heat coverage of %; and block and district heating with short-term storage, i.e. insulated water tanks, in combination with supplementary boilers for existing, as well as new residential building areas, providing a solar heat coverage of about %. The most interesting, from the viewpoint of replacing fossil fuels, are; block and district heating with seasonal storage, e.g. insulated water pit or ground storage, in combination with a supplementary boiler for existing residential building areas, as well as new residential building areas, providing a solar heat coverage of %. 3 SOLAR DISTRICT HEATING PLANTS The development of solar district heating was initiated in Sweden and followed by Denmark. In Sweden, small solar collector systems were tried in a couple of district heating plants already in the late 70's. The first large system using ground-mounted large module collectors was installed in Torvalla, close to Östersund, in See Figure 1.

2 Table 1 shows the most significant solar district heating plants using ground-mounted large module collectors, with or without a short-term storage, briefly described in the following. The sizes vary from 100 to m² of collectors and the total annual loads vary from GWh. The collector array to load ratio is typically m² of collectors per annual GWh resulting in solar fractions from 5 to 15 %. Nykvarn, south of Stockholm, comprises m² of ground mounted collectors connected to an existing small district heating network (20 GWh/a) in combination with m³ water in an insulated steel tank above ground. Nykvarn has been in operation since 1985, from 1985 with m² of collectors to cover about 6 % of the annual load (I) and, from 1991, with an additional m² of collectors to cover about 10 % of the annual load (II). Fig. 1 Large module ground-mounted collector array m². Torvalla, Sweden. Fig. 1 Large module ground-mounted collector array m². Falkenberg, Sweden. The Torvalla plant was more or less built on site and resulted in two development lines, one with prefabricated large module collectors by TeknoTerm applied in e.g. Nykvarn (later ScanCon and Arcon) and one with site-built long modules by Finsun applied in Malung (later Solsam). Falkenberg, south of Göteborg, comprises m² of ground mounted collectors connected to an existing small district heating network (25 GWh/a) in combination with m³ of water in an insulated steel tank above ground. In operation since 1989, this system is intended to cover about 6 % of the annual load. See Figure 2. Tab. 1 Solar district heating plants (with ground-mounted collectors). Built Plant Collector Collector Storage Load [company] [m²] [m²/gwh/a] [m³] [GWh/a] 1982 Torvalla, SE Nykvarn I, SE TeknoTerm Malung, SE Finsun Saltum, DK ScanCon Ry, DK ScanCon Falkenberg, SE TeknoTerm Odensbacken, SE Solsam n.a. n.a. Nykvarn II, SE TeknoTerm exist Säter, SE Solsam Orivesi, FIN Arcon Marstal, DK Arcon Ærøskøping, DK Arcon Torsåker, SE Finsun Kungälv, SE Arcon exist. ~100

3 Säter, northeast of Stockholm, comprises m² of ground mounted collectors (without storage) connected to an existing small district heating network. In operation since 1992, this system is intended to cover a part of the summer load (about 1 % of the annual load in one out of three heating plants). Two systems with similar long collector modules were earlier built in Malung (600 m², 1987) and Odensbacken (1 000 m², 1991). Kungälv, north of Göteborg, will comprise m² of collectors, m² to be installed this summer and another m² next spring, connected to an existing district heating network (~100 GWh/a). Ry, on Jylland, comprises m² of ground mounted collectors connected to an existing small district heating network (32 GWh/a). In operation since 1989, this system is intended to cover about 4 % of the annual load. A similar small system with m² of ground mounted collectors was earlier built in Saltum (1988). Marstal, on Ærø, comprises m² of ground mounted collectors connected to an existing small district heating network (27 GWh/a). In operation since 1996, this system is intended to cover 100 % of the summer load (about 14 % of the total annual load). A similar small system with m² of ground mounted collectors was later built in Ærøskøbing (1998). Furthermore, an existing district heating plant in Orivesi, Finland, is equipped with a small R&D plant, with large module collectors (100 m² without storage, 1993) and Torsåker comprises 500 m² of a new type of compound parabolic collectors optimised for northern latitudes (~60 ) called MaReCo. In the following the plants in Falkenberg, Nykvarn, Säter and Kungälv will be analysed more in detail. 4 SYSTEM DESCRIPTION The plants in Falkenberg and Nykvarn are equipped with short-term storage tanks while the plant in Säter is directly connected to the district heating plant. The Kungälv plant is already equipped with a short-term storage tank, but the collector array will mainly work with a direct connection due to the size of the load. In Nykvarn, district heating is provided by three oil boilers (3x5.8 MW) and one electric boiler (5 MW), while there are two wood-chip boilers (2x3 MW) and two gas boilers (2x6 MW) in Falkenberg. The solar heating plants are designed to cover the major part of the summer load. In Säter, district heating is provided by two wood-chip boilers (2x5 MW) and one flue gas condenser (3 MW) in a main plant operated together with two other plants with electric and oil boilers. The solar heating plant is designed to cover a part of the summer load together with a supplementary electric boiler (600 kw) in the main plant. In Kungälv, district heating is provided by one woodchip boiler with flue gas condenser (9+3 MW) and two oil boilers (2x12 MW). The solar heating plant is designed to cover a part of the summer load together with existing boilers. 4.1 Solar Collectors The presented plants represent two different types of collector array technologies. Figure 3 shows the type of large module ground mounted collector (12.5 m²) that is developed by TeknoTerm, manufactured by ARCON and used in Falkenberg and Nykvarn, as well as in Kungälv. Fig. 3 Large module ground-mounted collectors. Mounting in Falkenberg in The collectors in Säter, built by Solsam Sunergy AB, are built on-site as long modules (70 m), where one module in principle represents one collector row in the Falkenberg and Nykvarn plants. Both supply and return main header pipes (in ground) are located in one end. The basic idea is that site built collectors should be cheaper than manufactured and that the even larger modules will give reduced heat losses. Figure 4 shows the type of large long module collectors applied in Odensbacken (and Säter) by Solsam. Fig. 4 Long module ground-mounted collectors Odensbacken, Sweden.

4 The collector used in Falkenberg and (slightly improved) in Nykvarn (II) represents a new collector design compared to the collector used in Nykvarn (I). Thus, the Nykvarn plant comprises two generations of large module collectors. The collector modules - with absorbers connected in parallel - are connected in series in rows with 10 collectors in each row (design temperature raise of about 30 C). The collector rows are connected in parallel between supply and return main header pipes (in ground) enabling a minimum of connecting pipes and suitable flow distribution. The requirements on the land area are moderate as the collector modules are connected with flexible piping. The collector to be used in Kungälv is improved in the way that it is equipped with a new absorber, NIOX from Sunstrip, and an anti-reflex coated glass cover from SunArc A/S. 4.2 Heat Stores Conventional insulated water storage tanks are used in Nykvarn and Falkenberg, as well as in Kungälv. The tanks, with water volumes of 1 500, and m³, respectively, are made of steel plates on site on a concrete basement. Figure 5 shows the storage tank close to the district heating plant in Falkenberg. 4.3 System Design The overall system designs in Falkenberg and Nykvarn are in principle the same, except for the storage inlets and outlets. In Nykvarn, the tank can be charged (switching valve) and discharged (mixing valve) at two different levels, close to the bottom and at the top, while there is only one inlet and one outlet with diffusers in Falkenberg. Both systems are further designed to enable the tank to be used as buffer storage for the boilers, an option that never has been used. In Kungälv, the solar heating plant is connected to an existing tank already used together with the wood boiler during weekends, etc. Here, the tank can only be charged in the top and discharged from the top. All four plants have pressurised collector systems with a glycol/water mixture and a temperature difference controller controls on/off. The collector systems are operated with constant balanced heat capacity rates, i.e. a variable temperature raise, both in Falkenberg and Nykvarn. In Säter, however, the secondary circuit is operated with variable flow, i.e. the temperature is controlled by a shunt in order to have as high temperatures (around 70 C) as needed in the district heating flow pipe. A similar temperature control strategy is also applied in the Marstal plant using variable speed pumps on both sides of the heat exchanger (Dahm and Heller, 1999). Fig. 6 System layout in Kungälv. Fig. 5 Storage tank adjacent to the heating plant in Falkenberg. Construction in The tanks are also used as expansion vessels for the district heating networks and are in Nykvarn and Falkenberg equipped with steam boilers to generate a steam cushion in the top of the tanks in order to prevent oxygen to penetrate into the water. The Kungälv plant will also be operated with a temperature control, using a variable speed pump in the (primary) collector circuit and a temperature controlled valve in the secondary circuit operating on the system pressure in the heating plant. See Figure 6. The temperature control is generally more suited for the operation of the district heating plant and results in reduced electricity for pumps, but it is here judged to reduce the collector gain by % compared to constant flow operation. Figure 7 shows a comparison between constant and variable flow (constant temperature) control assuming an infinite load (collector inlet temperature not influenced by collector operation mode).

5 Solar Energy [MWh] Fig. 7 Qsol - const. flow Qsol - var. flow (as in Marstal) JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Collector output with constant and variable flow (constant temperature) control. The design in Kungälv will make it possible for the operating staff to adopt the most feasible operation mode from their point of view. 4.4 System Cost The earlier plants are built in different ways in different sizes. The Falkenberg plant is a complete solar plant with storage, Nykvarn (II) comprises a collector array extension, while the Säter plant is smaller and includes a supplementary electric boiler. Furthermore, the main collector distribution pipes are rather long (700 m) in Falkenberg, they had to be replaced in Nykvarn and they are designed for m² of collectors (possible future extension), not as built m², in Säter. The Kungälv plant comprises a collector array connected to the heating plant via a heat exchanger in a small building just outside the existing boiler building. All plants are demonstration plants built with some kind of governmental support. The idea here is to present the real costs in the earlier demonstration plants and expected costs in the Kungälv plant to enable an evaluation of the development. One observation is that although the total investment cost differs a lot, the specific collector cost is about the same in all three existing plants. Tab. 2 Documented investment costs in Falkenberg, Nykvarn (II) and Säter together with expected investment costs in Kungälv (1 000 SEK). Falkenberg Nykvarn II Säter Kungälv incl. storage (extension) incl. el. boiler excl. storage Ground work incl. incl. 170 incl. Collector array Coll. distr. pipes in ground incl. Heating plant incl Storage tank Building Electric boiler Design, etc Costs for the utility incl. Index adjustment Total investment March 1990 July 1991 Oct April EVALUATION 5.1 Thermal Performance The thermal performance is monitored in exactly the same way in all three existing plants and is documented together with more practical experiences in a similar format in three national reports by Schroeder and Isaksson (1993, 1994) and Schroeder (1995). A summary of the evaluations has previously been presented by Dalenbäck et al (1995). The annual collector output shown in Table 3 is the same in all existing plants, while the storage losses reduce the net solar output in Falkenberg and Nykvarn. A detailed evaluation shows, however, that the collector array efficiency in Säter is about 9 % better than in Nykvarn and about 16 % better than in Falkenberg, during typical operating conditions. The improvement in collector performance from Falkenberg to Säter is thus compensated by differences in system performance and local weather conditions.

6 Tab. 3 Annual measured performance in Falkenberg, Nykvarn (I+II) and Säter together with estimations for the new plant in Kungälv. The abbreviation e.w. means that the temperature is energy weighted. Falkenberg Nykvarn Säter Kungälv Monitored year Collector area [m²] Storage volume [m³] Latitude [ ] Collector tilt [ ] Solar rad. on collector (1) [GWh/m²] Solar rad. during operation (2) [GWh/m²] Collector array output (3) [GWh/m²] Electr. for coll. pumps [GWh/m²] Electr. for steam boiler [GWh/m²] Tank heat loss [GWh/m²] Net solar heat (4) [GWh/m²] Net solar heat [GWh/a] Total load [GWh/a] ~100 Solar fraction [%] ~3.5 Coll. pump operation [h] Collector array efficiency (3/2) [%] Plant efficiency (4/1) [%] Amb. temperature [ C] Amb. temp. dur. operation [ C] Coll. temp. dur. operation [ C] ~63 Coll.-amb. temperature [ C] Temp. before coll. hx (e.w.) [ C] Temp. after coll. hx (e.w.) [ C] Temp. after tank (e.w.) [ C] Temp. district heat (e.w.) [ C] The plants have different sizes and the Nykvarn shows the largest solar fraction, close to 10 % (1992). The corresponding solar fractions were 5.7 % in Falkenberg (1992) and 1.2 % in Säter (1993). The collectors are on average operating K above the ambient temperature. The average temperature drop over the collector system heat exchanger is twice as large in Säter as in Nykvarn. Especially in Falkenberg, large efforts to lower the return temperatures have been carried out. As an example, the average return temperature was reduced from C in 1990 to C in 1992 (May to August), mainly by rebuilding sub-units from threeway valve to two-way valve control. The same return temperatures are much lower, between C, in Nykvarn and Säter. The efforts to establish lowest possible temperature levels in the district heating systems, is generally motivated by reduced distribution heat losses. Another reason is that heat sources, such as large heat pumps, industrial waste heat, heat from flue gas heat recovery systems, etc. are widely used. Here any decrease in the temperature levels will add substantially to the overall efficiency, as in solar heating plants. The Kungälv plant will be operated at relatively high temperatures, as the return temperature is similar to the one in Falkenberg and due to the temperature control. However, with improved collectors (absorber and cover) it is expected that it anyhow will gain slightly more than the existing plants. The thermal performance of the collector system in Kungälv will be evaluated in reference to a guaranteed collector system power according to Figure 7. If a lower power than guaranteed is detected, it will be compensated by a corresponding enlargement of the collector array. This approach was also applied in Nykvarn and Falkenberg. A further observation in relation to thermal performance is that there is a higher degree of stratification in the Nykvarn tank (fixed inlets/outlets) compared to the Falkenberg tank (diffuser). One reason is that the diffuser is not properly designed. The Kungälv plant is already designed for a high degree of stratification as a rather high and more or less constant temperature charges the storage tank.

7 Fig Collector power (G=800 W/m²) Coll. Average temp. - Ambient temp. (Tc-Ta) [ C] Guaranteed power output in Kungälv. 5.2 Operation All existing plants have been in operation for several years, the Nykvarn plant in 15 years, without any major problems, and the annual maintenance and operational (M&O) costs are around 1 % of the investment in all three plants. The main problems that have occurred are related to unsatisfactory collector design in Falkenberg (broken convection barriers) and initiated corrosion in storage tanks (not properly working steam cushions). The problem with the convection barriers seems to have been solved in Nykvarn and Säter. In order to solve the problem with corrosion, future plants should either include larger steam boilers or be operated to have a constant tank top temperature. Another possibility is to consider the use of a vapour barrier. 5.3 Cost/performance ratio Combining the investment costs, the average annual performance and the operational costs makes it possible to determine the actual heat costs in Falkenberg, Nykvarn (II) and Säter. The costs shown in Table 4 are specific costs from specific applications and contain other costs than those directly related to the solar heat generation. However, a comparison between the cost/performance ratio for the original plant in Nykvarn (I) with the cost/performance ration for Falkenberg shows a rather interesting development. The ratio has dropped from ~12 SEK per annual kwh in Nykvarn to ~9 SEK per annual kwh in Falkenberg (both adjusted according to contracting price index H84 to be valid in Oct. 1992). Tab. 4 Total investment cost, net solar, cost/performance ratio and operational costs in Falkenberg, Nykvarn (II) Säter and Kungälv (calculated). Falkenberg Nykvarn II Säter Kungälv 5500 m² 3500 m² 1250 m² m² incl. storage (extension) incl. el. boiler - Total inv. cost [1000 SEK] Aver. net solar [MWh/a] Ratio [SEK/kWh/a] Prices valid March 1990 July 1991 Oct April 2000 M&O [1000 SEK/a] This positive development is further confirmed by the fact that the expected cost/performance ratio in Kungälv is down below 6 SEK per annual kwh at the same time as H84 has increased with about 25% from 1992 to CONCLUSIONS Dalenbäck and Åsblad (1994) describe an interesting potential for solar heating to replace oil burning in small district heating plants in Sweden, probably in combination with domestic fuel boilers (e.g. as in Falkenberg and Kungälv). The total use of oil in plants with annual loads from 7 to 100 GWh corresponds still to several TWh/a. A pre-requisite, however, is that the cost/performance ratio can be reduced down below 4 SEK per annual kwh (incl. storage). Solar district heating is still in an early development phase with only a few full-scale demonstration plants. There are three major issues that can bring the costs down to an interesting level: lower (return) temperature levels in district heating networks, further collector and system development and industrial collector production. The first issue is of general interest and the Kungälv plant is an important step regarding collector and system development. A pre-requisite for industrial production of solar collectors is of the order of m² per year in one industry. The present European market for solar collectors amounts to m² per year and is increasing with % per year, and still small companies are dominating.

8 ACKNOWLEDGEMENT The support by BFR (Swedish Council for Building Research), which has financed the early plants as well as the research work related to the evaluations and this paper, is gratefully acknowledged. The support by EC (REB266/98) and the Swedish government for the Kungälv plant is also acknowledged. Our thanks are also due to: Telge Energi, Falkenberg Energi, Säter Energiverk, and all contractors involved, that provided three well performing solar district heating plants, as well as Kungälv Energi, that has decided to take on the development. REFERENCES Dahm, J. and A. Heller. (1999) The Marstal Central Solar Heating Plant: Design and Evaluation. Proceedings ISES World Congress 1999, Israel. Dalenbäck, J-O. and A. Åsblad (1994). Förutsättningar för solvärme i gruppcentraler och mindre fjärrvärmesystem (Pre-requisites for Solar Block and District Heating). Report to NUTEK by CIT Energiteknisk Analys, Göteborg. (In Swedish) Dalenbäck, J-O, K. Schroeder and P. Isaksson. (1995). Solar District Heating. Seminar documentation Fjärrvärmeproduktion (District heating production), Nordic Minister Board, Januari, Helsinki University of Technology, Helsingfors. Dalenbäck, J-O. et al (1998). Solar Heating Plant kw. THERMIE proposal no REB/266/99. CIT Energy Management AB, Göteborg. Dalenbäck, J-O. (2000). European Large-scale Solar Heating Network. Paper submitted to Eurosun Schroeder, K. and P. Isaksson (1993). Solfjärrvärmeanläggning - Utvärdering Falkenberg (Solar District Heating: Evaluation Falkenberg). BFR Report R23:1993. Swedish Council for Building Research, Stockholm. (In Swedish) Schroeder, K. and P. Isaksson (1994). Utbyggnad av solfjärrvärmeanläggning i Nykvarn - Utvärdering (Extension of the Solar District Heating Plant in Nykvarn: Evaluation). BFR Report R34:1994, Swedish Council for Building Research, Stockholm. (In Swedish) Schroeder, K. (1995). Solfjärrvärme i Säter - Utvärdering (Solar District Heating in Säter: Evaluation). Document D1:1995, Monitoring Centre, Chalmers University of Technology, Göteborg. (In Swedish)