Energy efficiency optimization of the waste heat recovery system with embedded phase change materials in greenhouses: A thermo-economic-environmental study

The continuous increase of greenhouse gas emission and the growing cost of fossil fuels are two motive forces to utilize energy sources more effectively. The agricultural greenhouses are developed to supply the food resources safely and with the higher quality, especially in off-season periods of year. In developing countries like Iran, the fuel usage of the greenhouse is very high, and there is a large margin for improving heating systems. The waste heat recovery system (HRS) in two forms of with and without embedded phase change material (PCM) added to the existing heating system, and their effects studied experimentally. The HRS is a cross-flow gas flow heat exchanger with dimensions of 193cm*75cm*85 cm, which is located between the exhaust gas and the circulating air streams. The study case is a greenhouse of 20 x 66 m(2) in city of Isfahan located nearly in central region of Iran. The mean energy efficiency improved by 40% and 33% and the 2nd law efficiency enhanced by 263% and 127% for heating systems equipped with HRSs and PCM HRS, respectively. Using PCM HRS, the inside air and exhaust gas temperatures changed by + 17% and -8.2%, respectively. The fuel savings gained by using HRS with and without embedded PCM are 19% and 48% with the investment return back periods of 3 and 4 months, respectively.