A NEW CONCEPT OF A HYBRID STORAGE SYSTEM FOR SEASONAL THERMAL ENERGY STORAGE IN SOLAR DISTRICT HEATING

Transcription

1 A NEW CONCEPT OF A HYBRID STORAGE SYSTEM FOR SEASONAL THERMAL ENERGY STORAGE IN SOLAR DISTRICT HEATING M. Reuß*, J.P. Mueller*, B. Roehle**, M. Weckler**, W. Schoelkopf** * Institut of Agric. Engineering, TU-Munich, Voettinger Str. 36, D Freising **ZAE Bayern e.v., Abt. IV, p/o box 44254, D-8751 Munich 1. INTRODUCTION Seasonal thermal energy storage in a temperature range up to 9 ºC is a basic requirement for implementation of solar district heating in Mid Europe and for more rational use of conventional energy sources. For this longterm storage of high amounts of thermal energy underground heat storage is favourable from the technical and economical point of view. These storage techniques are classified in three categories /1/: - storage medium water (convective): rock cavern storage, pit or underground tank storage - storage medium soil (conductive): duct storage - storage medium ground (mixed, convective, conductive): aquifer storage, gravel water pit. Convective storages, especially pits or tanks are typically used for shorter periods. They have the big advantage that the storage medium can also be used as heat transport fluid. Thus high heat transfer power at low temperature differences is gained which is of importance e.g. in combination with solar energy with extreme variations in charging power. The major disadvantage of this storage type are the relatively high construction costs. Conductive storages (ducts) in soil or rock have an additional heat exchange process from the fluid to the underground. This makes the system less flexible. The big advantage of this storage type are much lower construction costs compared to water tanks or pits. The new storage under consideration is a combination of both types. It consists of a central cylindric water tank surrounded by a ring of ducts. This combination of the two systems promisses to combine the operational advantages of water tanks or pits with the economical ones of the duct storage. This storage type will be investigated as an important component in a solar district heating system. The major objectives are to optimize the construction with respect to function and costs as well as the development of a simulation model which is implemented in TRNSYS /2/ to carry out system simulation. With the simulation model a solar district heating system with seasonal underground storage will

2 be designed and analized for 11 low energy buildings. 2. COMPONENTS OF SOLAR DISTRICT HEATING The general layout of the solar district heating is shown in fig. 1. It consists of the solar collector array, the storage, the piping of the district heating system, a gas boiler or an absorption heat pump as backup and consumers. The solar collector delivers the energy directly to the district heating or to the storage. In case of additional demand, energy is taken from the store or the backup. Fig. 1: Layout of the solar district heating system solar collector consumer collector piping district heating T duct storage seasonal pit storage absorption heat pump The objective of the research work is the development of a system which allows to cover at least 5 % of the demand for space heating and domestic hot water by solar, the rest is taken from the backup sytem. Thus the fluid temperature in the district heating has to be higher than 6 C. The solar collectors are roof integrated which yields much lower system costs. The central component is the underground storage. 3. HYBRID STORAGE In several projects a variety of underground storages of different types for different applications were investigated. Pit stores or water tanks have the big advantage of high heat capacity and good heat transfer properties. An analysis of several projects /3/ shows a significant degression in construction costs with size of the storage. A 6 m; pit in Rottweil costs about 7.-- DM/m; (39.-- US$/m;), the 45 m; storage in Hamburg amounts to about 4.-- DM/m; (22.-- US$/m;) and the largest one with 125 m; in Friedrichhafen costs DM/m; (14.-- US$/m;). A detailled analysis of the cost structure shows a significant contribution of the liner to tighten the pit and the insulation material to reduce thermal losses. Duct storage systems have

3 construction costs of DM/m; water equivalent ( US$/m;) with some potential for degression. The hybrid storage should combine the technical advantages of both systems with much lower costs compared to pits. In the center of the storage the pit built out of concrete is located surrounded by rings of ducts. They heat up the underground and thus the thermal losses of the water pit which has only a thermal insulation layer on the top are reduced. The duct storage will have a significant radial temperature gradient. It is also an active thermal insulation for the pit. The thermal losses of the duct storage can partly be covered by solar energy which could not be delivered to the pit because of too high temperatures in the water. The operation period of the collector array is enlarged and thermal losses are covered by otherwise not usable solar energy. As the pit is located in a warm environment the temperature gradient in the concrete wall is rather moderate which results in less mechanical stress for the construction material. For a typical system layout with 25 m5 solar collector array, 5 m; storage (cylinder 11.5 m radius, 12 m high) for 11 buildings, monthly and daily water temperatures in pit were calculated. A finite differences model was developed and used to analize the temperature profile in the concrete walls. The highest temperature gradients in the upper corner found are about 14 K, in the middle of the wall 6 K and in the lower corner about 9.5 K. The discretization is shown in fig. 2.,3 2 x,2 7 x,1,3 depth z in [m] 19 x,6 distance r in [m] 5,2 3,2 1,6,8 water 6 x,1,4,4,2,2,8 1,6 3,2 insulation layer concrete wall of the pit grid for calculation steps,3 5 x,1,2,4,8 underground (e.g. soil) 1,6 Fig. 2: Discretization of the storage and the surrounding area The temperature distribution in- and outside the pit is shown in fig. 3. It has to be recognized that the distances are not equidistant but have the size shown in fig. 2.

4 Fig. 3: Temperature distribution in the pit storage with T max = 14,2 K in the upper corner This result shows that it is possible to build such a storage without thermal insulation on the side and the bottom walls. The thermal induced stress on the material will be rather low and thus no additional cracks in the concrete are expected. The water losses estimated for a 5 m; tank will be in the order of 5 - m;/a. In case of drinking water these water losses are no severe problem. The costs for refill over the life expectancy are several orders of magnitude smaller than the costs for a liner. 4. SYSTEM ANALYSIS OF SOLAR DISTRICT HEATING In parallel to the model and component developement the a whole system is studied. The demand for the case study is specified by 11 town houses of low energy building standard, 8 m5 each, with a specific heat demand of 4 kwh/m5a and about 45 kwh/m5a for domestic hot water (DHW). Due to the high insulation standard the demand for DHW calculated for an average occupancy of 3.2 persons per house is specific higher than that for space heating. Fig. 4 shows monthly energy flows (solar, backup) of a 25 m5 solar collector field with a 5 m; pit storage and a gas boiler as backup. Additionally the mean storage temperature in each month is shown. The solar fraction of the system is 74 %, the solar system delivers about 29 kwh/m5a. This plant covers the total heat demand in the period May to November by solar, backup energy is neccessary from end of December to April.

5 useful energy [MWh] Jan Mar May Jul Sep Nov mean storage temperature [ C] useful solar heat mean storage temperature backup energy Fig. 4: Monthly energy balance and storage temperature (25 m5 collector, 5 m; pit, gas boiler) [kwh/m²a] [%] performance ratio pit storage [%] solar fraction [%] performance ratio duct storage [%] pit: 5m³ pit: 2m³ duct: 84m³ pit: m³ duct: 112m³ spec. collector gain [kwh/m²a] spec. system gain [kwh/m²a] pit: 25m³ duct:133m³ 2 1 Fig. 5: Collector and system yield for a solar district heating with hybrid storage (uncoupled model) and gas absorption heat pump as backup

6 Furthermore various configurations of the hybrid storage were analized starting with 5 m; pit without duct storage, the volume of the pit was reduced (2 m;, m;, 25 m;) while the volume of the duct store was enlarged (84 m;, 112 m;, 133 m;). As the combined model of the hybrid storage is not yet working satisfactory the calculations were carried out with two separate models. This will underestimate the performance of the whole system. The program development is already finished but requires validation. The results of this system analysis are shown in fig. 5. The storage performance ratio of the pit is.83 for the >pit only= version and is increasing to.93 % by reduction of the size of the pit. For the duct storage the performance ration is.5 (84 m;) increasing to.56 with increasing volume (133 m;). The solar fraction is almost constant and varies from.76 to CONCLUSIONS Seasonal thermal energy storage is an important issue in solar district heating. A new storage concept, a combination of pit and duct storage, is investigated because of its technical and economical advantages. In a first step technical problems of the pit were analized which can be caused by thermal stress in the walls. The most endangered parts, the upper and lower corners of the walls, will see a maximum temperature gradient of about 14 K in the concrete. Thus expensive thermal insulation can be avoided. Additionally no cracks due to thermal stress are expected which means only a small leakage and so it is planned to do without liner. System simulations were carried out for 11 semi-detached houses build in low energy standards with solar district heating. A solar system of 25 m5 and a storage capacity of 5 m; water equivalent give a solar fraction of 75 % for space heating and domestic hot water. Variation of pit and duct storage size show only very little dependancy for the solar fraction. Reduction of the pit storage size and increase of duct storage volume gives an increase of the storage performance ratio. The results of this project will be used for a feasibility study for a new building area of 25 units in Freising. 6. References G. Bakema, A.L. Snijders, B. Nordell: Underground thermal energy storage, state of the art ISBN x, Arnhem, The Netherlands, TRNSYS - A Transient System Simulation Program. Solar Energy Laboratory, University of Wisconsin - Madison, WI 5376 USA. V. Lottner, E. Hahne: Status of seasonal thermal energy storage in Germany. In Proceedings of Megastock >97, Vol. 2, pp. 931, Sapporo, Japan 1997.