Proceedings of the ASME 2010 International Mechanical Engineering Congress & Exposition IMECE2010 November 12-18, 2010, Vancouver, British Columbia, Canada Proceedings of the ASME International Mechanical Engineering Congress & Exposition IMECE 2010 November 12-18, 2010, Vancouver, British Columbia, Canada IMECE2010-40549 IMECE2010-40549 THERMOECONOMIC COST ANALYSIS OF CENTRAL SOLAR HEATING PLANTS COMBINED WITH SEASONAL STORAGE Miguel A. Lozano Antonio Anastasia GITSE - Aragon Institute of Engineering Research Department of Energetics, Politecnico di Torino, (I3A), Universidad de Zaragoza, Zaragoza, Spain Torino, Italy Luis M. Serra Vittorio Verda GITSE - Aragon Institute of Engineering Research Department of Energetics, Politecnico di Torino, (I3A), Universidad de Zaragoza, Zaragoza, Spain Torino, Italy ABSTRACT INTRODUCTION The European Union and its Member States have With the new legislation on buildings construction [1], the committed themselves to achieving a 20% share of renewable Spanish Government has started a lukewarm promotion of the energy by 2020. If the focus remains solely on solar thermal installation of thermal solar systems in buildings. Specifically, systems for domestic hot water (DHW) preparation, as in the new Spanish legislation on buildings construction imposes Spain, then the solar contribution will be very limited. Central the coverage with thermal solar energy of the 30% - 70% Solar Heating Plants combined with Seasonal Storage (depending on the climatic area in Spain) of energy demand (CSHPSS) systems enable high solar fractions of 50% and corresponding to the domestic hot water. In other countries of more. Most CSHPSS demonstration plants in Europe have been Central and North Europe an important experience on thermal built in Central and North Europe, mainly in Denmark, solar energy has already been gained and several high scale Germany and Sweden. South Europe has little experience. Central Solar Heating Plants (CSHP) have been designed to This article presents a thermoeconomic cost analysis of cover the thermal energy demand of urban districts and even CSHPSS systems. The objective of thermoeconomics is to small cities [2-4]. Some of these plants have proven the explain the cost formation process of internal flows and appropriate operation of seasonal thermal energy storage [5-7]. products of energy systems. The costs obtained with The experience gained in Central and North Europe and the thermoeconomics can be used to optimize the design of new better conditions of solar radiation in Spain suggest the plants and to control the production of existing plants. A feasibility of installing Central Solar Heating Plants combined simulation study on solar assisted district heating systems with with Seasonal Storage in those Spanish areas in which there is a high solar fractions and seasonal thermal energy storage was significant demand of thermal energy for heating in winter. carried out with TRNSYS taking into consideration the In this work, is presented a technical and economic meteorological conditions in Zaragoza (Spain). A CSHPSS evaluation of a CSHPSS plant that could be able of covering plant was designed for a district of 500 dwellings with an close of 70% of the thermal energy demand for DHW and annual thermal energy demand of 2,905 MWh/year. The heating of a residential area with 500 dwellings in Zaragoza, a process of cost formation has been analyzed considering the city located in the north of Spain. In this way it is coupled the very specific features of the CSHPSS designed system: free offer of thermal energy in periods of high solar radiation solar energy, seasonal and DHW thermal energy storage, (summer) with high thermal energy demand for heating continuous variation of the operation due to highly variations of (winter), obtaining energy independence, energy saving and solar radiation and energy demands (hourly and seasonal). reduction of pollutant emissions. After the analysis of These features impose important difficulties in the calculation preliminary studies [8-9] it is proposed a system that will be of the costs of internal flows and products in this type of used as a reference case for the selection and sizing of the systems. required pieces of equipment as well as for the evaluation of its economic feasibility. The dynamic behavior of the system has been analyzed using the TRNSYS software [10] 1 Copyright © 2010 by ASME Figure 1. Diagram of the base case of the analyzed Central Solar Heating Plant combined with Seasonal Storage SYSTEM DESCRIPTION thermal energy demand is 2,905 MWh/year, being 507.5 The proposed system is depicted in Figure 1. MWh/year for the domestic hot water and 2,397.5 MWh/year The energy harnessed by the solar collectors is transferred for the heating. either to the seasonal thermal energy storage or to the DHW DHW is produced at 60ºC. The proposed CSHPSS plant storage (preferably to this one). An independent DHW storage, produces water at 50ºC for a heating network of low which is smaller than the seasonal thermal energy storage, temperature, which is favorable for maximizing the efficiency provides the required temperature of the water in a few hours in thermal solar plants. A typical low temperature heating and allows, as it is explained below, to cover a high fraction of system is for instance the radiant floor heating system. the DHW demand with solar energy. The dynamics of loading The selection of pieces of equipment has been made for and unloading of the seasonal thermal energy storage is solar collectors, thermal energy storages, auxiliary boilers, heat significantly slower, because it is oriented to cover partially the exchangers and pumps, based either on the information heating demand in winter using the thermal energy stored appearing in catalogs (boilers, heat exchangers and pumps) or during the summer period. The auxiliary boilers (BH and on the information published in the scientific literature BDHW) will support and guarantee the coverage of the thermal (collector field and seasonal thermal energy storage). demands when the temperature of the water in thermal energy Flat plate collectors are used to collect solar radiation. storages is insufficient. These collectors have surfaces higher than 10 m2 and they can The CSHPSS plant is designed to serve 500 dwellings in be installed either on the roof of a building and/or on the the residential area called Parque Goya, located in Zaragoza. ground. The proposed surface to be installed, in the analyzed The thermal energy demand for DHW and heating is expressed case, is 2,760 m2 on the ground, corresponding to a ratio of considering 12 representative days (one for each month of the 0.95 m2/(MWh/year). year) divided each in 24 periods of 1 hour [11]. The annual Figure 2. TRANSYS model of the Central Solar Heating Plant 2 Copyright © 2010 by ASME The sizing of the DHW storage was based on the medium Table 1. TRNSYS model of the different plant components demand corresponding to a daily coverage of 23 m3. The Component Type Parameter Value volume of the selected DHW was 47 m3, sufficient for two 2 days. Collector area 13.575 m The volume of the seasonal thermal energy storage Collector number 204 required for reaching a solar fraction close to 2/3 of the annual Slope 50º 3 heating demand was estimated about 15,810 m . Due to the big Azimuth 0º Collectors 1a size, this thermal energy storage should be built in the place a0 0.738 where it will be located. Solar a1 1.63 W/(m2K) Auxiliary boilers with a thermal power of 208 kW for the field 2 2 production of sanitary hot water and 1,800 kW for heating are (sf) a2 0.0299 W/(m K ) 2 able of covering all the thermal demand (100%) without usage Specifc flow 20 kg/(h·m ) of the solar thermal system. Total length 1000 m The heat exchangers were designed establishing a overall Diameter 0.1 m Pipes 709 UA coefficient that could guarantee a heat exchanger Ins. thickness 0.06 m effectiveness of 0.95, even in the worst operation conditions. Ins. conductivity 0.144 kJ/(h·m·K) The sizing of the feed pumps was established considering Volume 15810 m3 the maximum volume of flow and the pressure loss in the ducts. 2 Seasonal storage Thermal loss 0.45 kJ/(h·m ·K) This pressure loss is the addition of the pressure loss in the 4c different pieces of equipment connected in series to the pump (a1) Height/Diameter 0.6 and the pressure loss corresponding to the length of the ducts Number of nodes 12 and auxiliary components. Volume 47 m3 2 DHW storage Thermal loss 1.6 kJ/(h·m ·K) TRNSYS MODEL 4a TRNSYS is a software tool providing a complete (a2) Height/Diameter 1.5 environment for the dynamic simulation of energy systems, Number of nodes 6 including buildings. Figure 2 and Table 1 shows the TRNSYS Nominal power 1800 kW model of the analyzed CSHPSS plant depicted in Figure 1. Heating Boiler 6 Efficiency 0.93 Climatic data (solar radiation and ambient temperature) for (BH) the city of Zaragoza (Height: 247 m; Latitude: 41.39º N; Service Temp 50⁰C Longitude: 1.00º W) were obtained from EnergyPlus [12]. Cold Nominal power 208 kW temperature from the domestic water supply network is taken DHW Boiler Efficiency 0.96 from Standard UNE 94002 [13]. 6 (BD) Hourly thermal demands, both for domestic hot water and Service Temp 60⁰C for heating, are registered in a text file and are provided to the Area 282 m2 model. All the demand values were obtained from direct Heat exchanger 1 5b measurements taken in several dwellings located in the (ex1) Overall U 3942 W/m2·K residential area or Parque Goya, located in the city of Zaragoza. 2 In Table 1 are shown the different TRNSYS types that have Heat exchanger 2 Area 282 m 5b been used for the simulation of the components, as well as the (ex2) Overall U 3942 W/m2·K most relevant technical parameters of the designed system.