Geothermal Heat Pump Systems

This research work aims at the multinational study, in Europe, of the emissions and energy costs generated by the operation of low enthalpy geothermal systems, with heat pumps fed by different energy sources. From the economic point of view, gas natural and biogas prizes are, usually, lower than electricity ones. So, it may be advantageous to use these energy sources to feed the heat pumps instead of electricity. From the environmental point of view, it’s intended to highlight the fact that under certain conditions of electricity production (electricity mix.), more CO₂ emissions are produced by electricity consumption than using other a priori less “clean” energy sources such as natural gas. In order to establish the countries where each of the different heat pumps may be more cost-efficient and environmentally friendly, multi-source data have been collected and analyzed. The results show that in the whole majority of cases, the electric heat pump is the most recommendable solution. However, there are some countries (such as Poland and Estonia), where the gas engine heat pump may be a better alternative.

This chapter discusses two low-enthalpy geothermal applications in Perth, Western Australia. The first application pertains to using tepid groundwater for the municipal heating of Olympic-size outdoor swimming pools. The second application examines the viability of ground source heat pumps (GSHP) against air source heat pumps (ASHP). In the first application, the objective is to develop an accurate sizing methodology to improve the capital effectiveness for geothermal swimming pools. The predicted pool-water temperature and heating demands are compared against on-site measurements at a Leisure Centre. This model can replicate 71 and 73% of the measured heating capacity data within ±25 kW for the 30-m pool and ±35 kW for the 50-m pool, respectively. In the second application, we assess the feasibility of implementing a GSHP vis-à-vis an ASHP for domestic applications. For the second application, the GSHP has a constant coefficient of performance (COP) of 3.8 ± 6.7%, while that of ASHP ranges from 2.2 to 2.7 ± 6.5%. For cooling, the GSHP has a constant COP of 3.1 ± 13%, while that of ASHP varied between 1.4 and 2.4 ± 11.5%. When a GSHP is considered with a planned installation of a borehole for irrigation, the payback period ranges from near-immediate to four years.

Energy geostructures are a special type of closed Ground Source Heat Pump (GSHP) system in which heat exchange pipes are installed in the foundation elements (e.g., piles, walls) to extract or inject thermal energy from/to the ground. Due to its dual function and high-energy efficiency, this technology is an alternative to reduce the environmental impact of the growing energy demand for space conditioning while avoiding the high initial cost of traditional GSHP systems. This chapter summarizes the basic concepts of energy geostructures, with emphasis on energy piles, including heat transfer mechanism, site investigations procedures, thermal and mechanical analysis approaches. Additionally, the chapter discusses the most recent design considerations and some construction recommendations.

The use of low-impact energy sources is gradually growing with the aim of reducing greenhouse gases emission and air pollution. The alternatives offered by geothermal systems are one of the key solutions for a future renewable development, enabling the electrification of heating systems and the use of biofuels. This research addresses an overview of geothermal heating systems using ground source heat pumps in different European countries. Besides the traditional electrical heat pumps, gas engine heat pumps aided by natural gas or biogas are analysed in three areas. From a previous research, the technical parameters defining the geothermal system are used here to evaluate the most appropriate system in each scenario. The evaluation of different influential factors (operational costs, initial investment, environmental impact, and availability) allows defining the most recommendable systems for each area. Results of this multi-parametric study show that gas engine heat pumps aided by biogas could mean an excellent solution in all countries, also contributing to the management of waste and polluting substances. If biogas systems were not available, the electrical heat pump would be the first option for areas 1 and 3 (Italy and Sweden) but not for area 2 (United Kingdom), where natural gas is preferred.

In a GSHP system, one of the most important components is the ground-coupled heat exchanger through which the thermal energy is exchanged between heat carrier fluid (i.e., water or water-antifreeze fluid) and soil. Since the ground heat exchanger is responsible for a major part of the initial cost of GSHP system and the efficiency of this system depends on the performance of ground heat exchanger, a careful design of ground heat exchanger is crucial for a successful application of GSHP system. Although the simplified analytical model or 3D numerical model for single U-tube ground heat exchanger has been proposed in the past several decades, the transient thermal-resistance-capacitance model of ground heat exchanger has become more popular due to the high numerical precision and low computational demand. This chapter will discuss the above-mentioned transient thermal-resistance-capacitance model for the typical single U-tube ground heat exchanger, the corresponding thermal resistance network within a borehole, and how to determine the specific thermal resistances for different thermal resistance networks with a borehole.

There is not a widespread use of the very low enthalpy geothermal energy in a large number of countries. The reason is frequently the high initial investment these systems require. In this regard, geothermal heat pump is largely the responsible of this important outlay. This research focuses on substituting this device by the integration of a set of renewable alternatives. The ground contribution is complemented with a thermal and a power renewable module that provide the remaining energy. The model was evaluated on a real case by technical calculations of the common ground source heat pump (GSHP) system and the one considered here. An economic feasibility study showed notable savings in the initial investment as well as in the annual costs on the part of the suggested system. CO₂ emissions were reduced by half in comparison to the traditional geothermal plant. The model represents a general improvement in relation to the conventional GSHP system; the heat pump is removed and the total drilling length reduced. For a single family home placed the substitution of the heat pump by renewable modules achieves a reduction of 22.84% in the initial investment, 77.54% in the operational costs and CO₂ emissions are reduced 49.50%.

Unplanned rapid urbanization is considered to be one of the major drivers of change in cities across the world. It leads to an inadequate transformation of urban environments, affecting strategic energy and water management infrastructure, resulting as well in an escalation in energy demand and a greater pressure on stormwater facilities. It is estimated that one third of the total energy demand in the European Union (EU) is associated to air-conditioning in buildings, whilst conventional drainage systems have become unsustainable under the current scenario of climate change. In this context of uncontrolled challenges, the EU is encouraging the incorporation of Nature-Based Solutions (NBS) in order to promote resilient infrastructure and to reduce instability. Sustainable Drainage Systems (SuDS) have been selected as key Stormwater Control Measures (SCM), contributing to a paradigm shift in urban water management. As the need for multifunctional spaces evolves due to the lack of urban land, SuDS are increasingly becoming a potential asset to house renewable energy structures, helping to develop the water–energy nexus. Thus, this chapter deals with the opportunities arising in this new research line combining surface geothermal energy systems and SuDS. Both laboratory and field experiences have been analyzed, compiling the lessons learned, identifying the present knowledge gaps, and proposing the future prospects for development. Therefore, paving the way for the effective combination of both technologies.

Thermal imbalance is usually encountered in the application of ground-source heat pumps (GSHP) in cooling or heating load-dominated areas, which results in the performance degradation of the GSHP system. Hybrid ground-source heat pump (HGSHP) combining cooling tower, domestic hot water (DHW), or other auxiliary technologies has been developed to attemper the underground heat or cold accumulation in cooling or heating load-dominated zones. This chapter will discuss several typical hybrid ground-source heat pump systems, how to build and validate the numerical model for the HGSHP system based on the TRNSYS code, and prediction and analysis of the long-term performances of the HGSHP systems considering the ground thermal balance via the optimization of the system operation strategies. This chapter focuses on the research of auxiliary heat dissipation that is suitable for heat accumulation systems. Several schemes for alleviating the underground heat accumulation of GSHP and keeping higher system efficiency are proposed and validated by the simulation work. For a typical GSHP system equipped in an office building, the optimum design and operation parameters are given for each scheme, respectively.

Renewable heating and cooling energy can be distributed with low temperature district heating and cooling networks connected to several renewable heat sources. These fifths generation district heating and cooling networks (5GDHC) deliver energy to settlements up to whole city districts. Because of the low temperature levels, the energy is distributed with almost no distribution losses and made usable to the consumer with decentralized heat pumps. Several renewable heat sources like large-scale geothermal collector systems, frozen soil storages or external dry cooler can be used. Moreover, sewage water and waste heat at low temperature levels can serve energy for 5GDHC. Especially, the use of geothermal heat sources like the large-scale geothermal collector system or the frozen soil storage enable the possibility to shift heating and cooling loads seasonally, contributing to flexibility in the heating network. In addition, the soil can be cooled below freezing point due to the strong regeneration potential caused by the solar irradiation. Geothermal collector systems with more than one layer can be used to generate excessive cooling by freezing individual areas within the collector system to provide cooling energy for residential buildings, office complexes or industrial applications. Planning these systems requires expertise and understanding regarding the interaction of these technologies in the overall system.

Shallow geothermal heat pump system has been extensively applied to commercial and residential buildings owing to its high-energy efficiency and low operating cost. The book chapter aims to investigate the energy performance of the energy piles for a shallow geothermal heat pump system, and analyse the influence of “thermal short-circulating” between two pipes of a U-tube based on the finite volume method. Additionally, the economic feasibility analysis is performed by using the Monto Carlo simulation approach. It is found that the heat pump (heating and cooling) could achieve annual average COP of 3.63 and EER of 4.62. Meanwhile, when the shank spacing could be controlled between 0.06 and 0.10 m and the fluid flow rate is in a range of 0.5−0.7m³/h, this contributes to avoiding the low outlet fluid temperature and decreasing the influence of “thermal short-circuiting”. Furthermore, the life cycle cost evaluation demonstrates that the system net present value is approximately £26,095 with 4.31 years of payback period at the market discount rate of 8.75% for a 20-year operating period, which is sensitive to the main parameters including electricity price, capital investment and energy production. And also, it is indicated that the low discount rate and high energy production are beneficial to the system with the high NPV and cash flows.