Low-cost optimization of geothermal heating system with thermal energy storage for an office building

In recent years building energy consumption has attracted increased attention around the world. As for China, the building sector accounts for about 30% of the primary energy consumption [1], in which the energy used for space heating in Northern China occupies one-quarter or so [2]. Among all types of buildings, the number of office buildings has increased rapidly with economic development, which leads to office buildings becoming the largest energy consumption buildings [3].

Using renewable energy to meet building heating is an effective and wise alternative. As one of the main renewable energy sources, geothermal energy can play an important role in building space heating, which is generally unaffected by geography, climate and season [4]. The geothermal energy for building heating can be generally classified into two types, i.e. the shallow and medium-deep geothermal systems. Although the shallow one is popular, besides the cooling and heat loads are not easy to be balanced, the average temperature of soil is relatively low, which restricts its application for building heating; Besides, a large area is needed for the shallow to meet the heat load of building due to the weak performance of borehole heat exchangers [5]. Comparatively, the medium-deep geothermal heating system can overcome the aforementioned shortcomings and has an outstanding advantage in a heating supply stability and carbon reduction [6]. Also, it is believed that with the progress of exploration technology and the reduction of drilling costs, the medium-deep geothermal heating system has a promising prospect. Currently, in view of engineering applications, the medium-deep geothermal systems are mainly dominated by hydrothermal heating technology. By the end of 2020, the heating area supported by the hydrothermal geothermal source in China has exceeded 580 million square meters, including over 7.0 million square meters in Xiongan, Hebei province, China [7], which meets approximately 95% of the heating demand in the county. Considering its rich resources equivalent to 853 × 10³ million tons of standard coal [8], the development of hydrothermal geothermal for building heating has become a significant measure for China to achieve its energy conservation and emission reduction goals.

The hydrothermal geothermal heating system commonly consists of a submersible pump, a plate heat exchanger or an addition of water source heat pump, which in general provides the baseload heat supply while a backup system covers any excess demand [9]. To date, the heat production only from the geothermal heating system to satisfy the heating demand has not been studied extensively. What is more, with the advancement of technology, more and more geothermal heating systems use variable frequency pumps, but some investigation assumes the geothermal water flow rate is constant, see for example [10]. It is well known that the heat demand of an office building features a significant intermittent profile; however, the submersible pump of geothermal well should not be started and stopped frequently within a heating season [11], which makes it difficult to match the continuity of heat source with the discontinuity of heat load and thus resulting in a serious waste of geothermal resources. Therefore, some researchers proposed to employ thermal energy storage (TES) devices to bridge the discrepancy between the heat supply and demand [10]. For instance, Daniilidis et al. [9] designed a model predictive control strategy for the heat production of a geothermal system including a storage device in relation to the demand from a district-heating network; Guo and Li [11] taking a public building in Xianyang, China as an example, suggested a method to improve the efficiency and benefit of geothermal heating by using heat storage technology; Lee et al. [12] compared the energy consumption of a single well-circulating heat pump system before and after optimization, and showed that the system energy consumption decreased by 14% after adding a thermal storage tank; Manente et al.[13] used short-term thermal energy storage to decouple the heat production sector from the distribution network of Ferrara in Italy, and showed that adding a thermal storage system combined with time-of-use (TOU) optimization control can save 11% of the system energy consumption; Kyriakis and Younger [10] investigated a geothermal heating system including two storage tanks, one on the supply line and one on the return line of the system that will store hot and cold water respectively, which showed that adding a thermal storage tank could reduce both heating cost by 4.2% to 14.9% and carbon emissions by 54.2%. These researches prove the effectiveness of thermal energy storage in geothermal heating.

However, it is well known that the introduction of TES will increase the capital investment of the geothermal system. Besides, concerning the electric supply side, a smart grid with TOU tariffs is conducive to the system’s economic operation [14], but to the best of the authors’ knowledge, this has not previously been addressed well in the geothermal heating system. Aiming at obtaining a low-cost system while promoting the energy supply and demand balance, the present paper introduces the TOU tariffs into the geothermal heating system with TES for an office building and two-stage optimization is performed. With these analyses, some suggestions are given as resources for the design of the geothermal heating system with TES for office buildings.