| Centralized dual-tank solar water heating systems have been widely applied in buildings,owning to huge heat storage capacity and water supply stability compared with single-tank solar water heating systems.To ensure the system stability and safe operation,particularly in scenarios where a large amount of water is required in a short period of time(e.g.student dormitories and factory dormitories),both the water supply tanks and heat storage tanks are commonly equipped with auxiliary heat sources in practical applications,and the water temperature of the heat storage tanks is usually controlled to be close to the supply water temperature.However,the solar energy utilization efficiency is limited for such systems since they are unable to store the solar energy absorbed by the collector in the heat storage tank when the solar irradiation is insufficient to heat the water in the collector to reach the startup temperature of solar loop pumps.This will cause the solar energy to be lost in the environment and thus lead to energy waste.To address this issue,in this study a novel triple-tank solar water heating system which integrated with a heat storage tank,a water supply tank and a makeup water tank,as well as the corresponding control strategy,was proposed based on the traditional dual-tank solar water heating system.The proposed system can store the above-mentioned wasted solar energy in the makeup water tank and heat makeup water.Moreover,phase change materials were added in the makeup water tank of the proposed system to enhance its heat storage capacity,thereby further improving its solar fraction and meeting users’ requirements.To evaluate the performance of the proposed system,an experimental setup was established and TRNSYS simulation models were established and validated.Based on this model,the annual solar fraction of the proposed system and their influencing parameters were analyzed.The main conclusions in this paper are as follows:(1)Compared with the traditional system,the proposed system can take advantage of more solar energy and reduce the operation time of auxiliary heat sources,thereby significantly improving the solar fraction of the system.Regarding the annual solar fraction,it is 39.79% and 48.71% for the traditional system and the proposed system respectively,with an increase ratio of 22.42%.(2)Simply increasing the volume of the make-up water tank would not certainly result in the increase of the annual solar fraction.The results indicated that as the volume of the makeup water tank increases from 10 L to 150 L,the solar fraction of the system increased first and then remained stable,and reached the maximum when the volume of the makeup water tank reached 100 L.Correspondingly,the annual solar fraction was 48.71%.Considering that a larger tank volume will bring about the increase of initial costs,a tradeoff between the annual solar fraction and initial investment of the makeup water tank should be made to select its volume in practical applications.(3)Compared with the traditional system,the proposed system significantly improves the solar fraction under different water usage modes.In particular,the annual solar fraction of the proposed system increased by 22.42% and 33.81% in the household water usage mode and the school bathroom water usage mode,respectively.Compared with the nighttime water usage mode,a higher solar fraction was obtained for daytimenighttime water usage mode.(4)An economic analysis was conducted for applying the proposed system in a residential building in Changsha.The results showed that when the volume of the makeup water tank is 11m3,the annual solar fraction reached the largest value of51.21%。The annual operation cost was reduced from 66043.58 Yuan to 53948.89 Yuan,with a decrease ratio of 18.31%.As a result,the payback period was 0.66 years.(5)The makeup tank coupled with phase change material not only stored more solar energy but also increased the water temperature of the makeup tank,and consequently improved the solar fraction of the solar water heating system.Specifically,compared to the system without phase change material,its annual average water temperature of the makeup tank increased from 30.37°C to 34.95°C,with an increase ratio of 15%.The annual electricity consumption of the solar water heating system decreased from 961.14 k Wh to 754.95 k Wh,with a decrease ratio of 21.7%.The annual solar fraction increased from 48.71% to 59.93%,with an increase ratio of 23.03%.(6)The selection of phase change temperature of phase change material showed a marked influence on the solar fraction of the system.Both reducing the phase change temperature to increase the heat storage when the solar irradiation is low and increasing the phase change temperature to increase the average make-up water temperature when the solar irradiation is high do not necessarily result in the improvement of the annual solar fraction.Therefore,the phase change temperature should be identified in term of the local meteorological conditions and the water temperature of makeup water tanks in real applications.(7)The influence of the thermal conductivity of PCM on the solar fraction of the system is impacted by water usage modes.Increasing the thermal conductivity of phase change material does not necessarily result in the significant rise of the system solar fraction.Therefore,the thermal conductivity of phase change material should be determined with considering users water usage modes in practical applications. |
