Optimisations of a seasonal solar thermal energy storage system for space heating in cold climate

Abstract

© 2019 Sheikh Khaleduzzaman ShahA number of seasonal solar thermal energy storage (SSTES) systems have been investigated for heating in cold climate locations due to the utilisation of solar energy. The system overcomes the drawback on the intermittency of solar energy and contributes to storing heat from summer to be used in winter. Heat pump and solar collectors with lower temperature thermal storage are the influencing factors to improve the system performance. A double U-tube borehole thermal energy storage (BTES) system integrated with ground coupled heat pump (GCHP) and evacuated tube solar collectors (ETSCs) is proposed for residential space heating in selected cold climate locations. Experimental data were collected from a double U-tube borehole heat exchanger (BHE) test rig; the data were used to validate the TRNSYS Type 257, which models double U-tube BHE. The performance of the proposed double U-tube BTES-GCHP-ETSC system was evaluated by computer simulations. A cluster of 30 residential houses in six selected cold climate locations: Lukla (Nepal), Dras (India), Sivas (Turkey), Harbin (China), Ulaanbaatar (Mongolia) and Verkhoyansk (Russia) were investigated. The design variables of the proposed system were optimised to minimise total life cycle cost (TLCC) and total life cycle greenhouse gas emissions (TLCG). The initial investigation was to provide a detailed review of various parameters (options) of SSTES systems. The BTES system has better energy performance with relatively low cost compare with other thermal storage systems. The lower temperature SSTES system may be more suitable for the cold climate conditions. However, the lower temperature stored heat cannot be directly used for space heating, and a heat pump needs to be coupled to upgrade the temperature of the delivered heat. The SSTES system is a promising technique for cold climate locations with adequate solar radiations. It was found that the annual average heating coefficient of performance (COP) of the heat pump, COP of the system, and the ground temperature are increased by adding solar collectors to a conventional heat pump (HP) system. In contrast, the required borehole depth, heat pump energy consumption and extracted energy from the ground are decreased due to the inclusion of solar collectors in the system. The highest COP of a heat pump is found at a system with ETSC compared to other solar collectors. Furthermore, double U-tube BHE has lower LCC and higher heat transfer rate than single U-tube BHE configuration. An experimental study was conducted to validate the TRNSYS Type 257. All system components, pipe network, data acquisition systems, operation procedures and schedules, weather station and measuring instrument were described. The experiments were conducted for heat charging and space heating operations. The measured undisturbed ground temperature (UGT) at the experiment site was presented. The UGT were found to be 17.64 degree celsius and 17.68 degree celsius at 21 m and 40 m depth respectively. The validation of the double U-tube BHE model was presented. The simulated and measured temperatures were compared. Statistical parameters: mean bias error (MBE), root mean square error (RMSE) and correlation coefficient (CC) were used to quantify the agreements between the simulated and measured data. Both heat charging mode and space heating mode models were validated. The duration for the comparison was one-week for both operation modes. The CC of double U-tube BHE model was found to be 0.99 and 0.91 for heat charging mode and space heating mode respectively. The MBE were found to be 0.15 and -0.07 for heat charging mode and space heating mode respectively. The RMSE were 0.37 and 0.51 for heat charging mode and space heating mode respectively. A TRNSYS model was developed to simulate the proposed BTES-GCHP-ETSC system for a cluster of residential houses in selected six cold climate locations. The residential house model for each location was developed based on the typical local dwelling. It was found that the annual heating loads per unit floor area (in GJ m-2) are: 1.71, 1.79, 0.71, 1.19, 1.45 and 2.59 for, Lukla, Sivas, Dras, Harbin, Ulaanbaatar and Verkhoyansk respectively. The system was investigated for heat charging and space heating (heat discharging) modes where the ground temperature and heat losses were analysed. The results of the simulation over 20 years period showed that the average ground temperatures were stable in each location with solar charging option. The energy balance of the system at the 20th year was analysed. The highest seasonal compressor heating coefficient of performance (SHCOP) was found to be 6.65 at Lukla and lowest was 6.03 at Sivas. On the other hand, the highest COPsys (4.39) was found at Verkhoyansk and lowest (2.68) at the Sivas. It was found that the proposed system fulfils the 95%, 92%, 93%, 93%, 96% and 100% of space heating demand for Lukla, Sivas, Dras, Harbin, Ulaanbaatar and Verkhoyansk respectively. The TRNSYS simulation model coupled with multi-objective building optimisation (MOBO) software was used to optimise system variables. There are three separate optimisation investigations: two single objective and one multi objectives for the system. The first investigation is to minimise total life cycle greenhouse gas emissions (TLCG, t), and the second investigation is to minimise total life cycle cost (TLCC, $) included cost of GHGE. The third investigation is to minimise both life cycle cost (LCCRsysR) of system and cost of greenhouse gas emissions (Cghge) (multi objectives). The optimum proposed system configuration (total solar collector area, number of boreholes and total borehole length were determined. The annual life cycle cost (ALCC), the unit heating cost (UHC), net energy ratio (NER), simple payback time (SPBT), carbon payback time (CPBT), and energy payback time (EPBT) were determined to analyse the proposed SSTES system. The proposed SSTES system was optimised for minimising TLCG, where total amount of embodied and operational GHGE were considered for 20 years project life. The maximum GHGE was found at Dras and the minimum at Lukla. The maximum ALCC was found at Verkhoyansk and the minimum at Sivas. The maximum UHC was found at Sivas and the minimum at Verkhoyansk because the size of the heating system is larger at Verkhoyansk than other location. The maximum CPBT and SPBT were found at Sivas and the minimum at Lukla. The EPBT was found highest at Sivas where NER was found lowest. On the other hand, the EPBT was found lowest at Ulaanbaatar where NER was found highest. The maximum SHCOP and COPsys were found at Lukla and Dras respectively. In addition, the proposed BTES-GCHP-ETSC system was optimised for minimising TLCC for 20 years project life, where the system cost and cost of GHGE were considered. The maximum total life cycle cost (TLCC) (cost of system and GHGE) was found at Verkhoyansk and the minimum at Sivas. The operational cost was found highest at Verkhoyansk than other locations because the pumps operate longer to meet the heating demand. The cost of GHGE was the height at Dras and the lowest at Lukla. The ALCC was maximum at Verkhoyansk and minimum at Sivas. The maximum UHC was found at Sivas and the minimum at Verkhoyansk. The NER was found to be the height at Verkhoyansk and the lowest at Sivas. The maximum SPBT was found at Sivas and the minimum at Lukla. The minimum EPBT was found at Verkhoyansk and maximum at Sivas. The SHCOP were found to be 7.00, 6.03, 6.72, 6.22, 6.12 and 6.45 for Lukla, Sivas, Dras, Harbin, Ulaanbaatar and Verkhoyansk respectively. The COPsys were found to be 3.23 (Lukla), 3.57 (Dras), 2.68 (Sivas), 3.53 (Harbin), 3.52 (Ulaanbaatar) and 4.39 (Verkhoyansk). Further, multi-objectives optimisation of SSTES system for space heating in cold climate locations was investigated. Multi-objective functions were applied to determine the Pareto front of variables. The LCCsys and the Cghge were the two objective functions. The minimum LCCsys and the minimum Cghge points as the Pareto fronts were determined. However, minimum TLCC points have 53% (Lukla), 37% (Dras) and 42% (Ulaanbaatar) shorter borehole length than minimum TLCG points