Research on rationality of energy system in buildings based on energy level theory- A case study

Abstract

Exergy plays an important role in evaluating and optimizing the performance of energy using equipment and improving the efficiency of energy use. However, the optimization usually requires the deconstruction of equipment to analyze the exergy flow through each subsystem and find out the links of exergy destruction. For decision makers, it is difficult to achieve, and some clear indicators are needed to assist the quick identification of the unreasonable energy use, and help make optimal decisions. In order to make the decision-makers find the main link of unreasonable energy use more conveniently and quickly, this study establishes the energy level balance model of buildings based on the energy level theory, and evaluation are made based upon the exergy degradation of equipment and the exergy matching degree of energy supply and demand. This method is applied in two energy-efficient retrofit cases to verify the effectiveness of the methods. The results show that: (1) compared with gas boiler and direct electric heating, the exergy matching degree between supply and demand for hot water produced by heat pump can be improved by 42.16 ∼ 82.7%; (2) the energy level balance coefficient is inversely proportional to the exergy efficiency, while the exergy matching coefficient is directly proportional to the exergy efficiency; (3) reference temperature has a great impact on the index parameters of hot water production and cooling in summer; by contrast, air humidity ratio only has limited impact on the index parameter of cooling in summer, and has no impact on that of hot water production; (4) only when COP>ηtngηe∙ηnet, using heat pump to produce hot water is more rational than using gas boiler, and the critical value of COP is 2.96 in this study; similarly, only when the COP of heat pump reach 2.83, will the heat pump be better than the air conditioner. This paper provides a theoretical basis and decision indicators for decision makers to select energy using equipment and optimize energy using modes.