Geothermal heat pump technology and building energy conservation

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Geothermal heat pump technology and building energy conservation Abstract: A heat pump (refrigerator) is a device that uses work to transfer heat from a medium at a lower temperature to a medium at a higher temperature. The air conditioning system of a building should generally meet two opposing requirements: heating in winter and cooling in summer. Traditional air conditioning systems typically require separate cooling sources (refrigerators) and heating sources (boilers). Since building air conditioning systems require a cooling source (chiller), operating them in heat-pump mode during winter allows for the elimination of boilers and boiler rooms. This not only reduces initial investment costs but also relies solely on clean energy in the form of electricity throughout the year, **reducing air pollution caused by heating.** Keywords: air-source heat pump, surface water heat pump, groundwater heat pump. I. Heat pumps and building heating and cooling. With economic development and the improvement of people’s living standards, heating and cooling for public buildings and residences have become common needs. In developed countries, the energy consumption for heating and air conditioning can account for 25-30% of the total energy consumption in society. Our country’s energy structure relies primarily on fossil fuels, especially coal. The combustion of fossil fuels produces a large amount of pollutants, including various harmful gases such as SO2 and NOX, as well as greenhouse gases like CO2. The environmental problems caused by the large-scale combustion of fossil fuels have increasingly become a focus of concern for governments and the public around the world. Heating in our country has evolved from small coal stoves for individual households to coal-fired boilers. The further prohibition on building small and medium-sized coal-fired boiler rooms in towns reflects a **high level of emphasis on protecting the atmospheric environment. Therefore, in addition to centralized heating systems, there is an urgent need to develop other alternative heating methods. A heat pump is a new heating and cooling technology that can effectively save energy and reduce air pollution as well as CO2 emissions. A heat pump (refrigerator) is a device that uses work to transfer heat from a medium at a lower temperature to a medium at a higher temperature. The air conditioning system of a building should generally meet the two opposing requirements of heating in winter and cooling in summer. Traditional air conditioning systems usually require separate cooling sources (refrigerators) and heating sources (boilers). Since building air conditioning systems require a cooling source (chiller), operating them in heat pump mode during winter allows for the elimination of boilers and boiler rooms. This not only reduces initial investment costs but also relies solely on clean energy in the form of electricity throughout the year, **reducing air pollution caused by heating.** Using heat pumps to heat buildings can **reduce the consumption of primary energy. Typically, we generate heat by directly burning fossil fuels (coal, oil, natural gas), and it is ultimately used to heat buildings through various heat transfer processes. Under the ideal condition of no heat loss in the boiler and heating pipelines, the utilization rate of primary energy (that is, the ratio of the heat supplied to the building to the calorific value of the fuel) can reach up to 100%. However, burning fossil fuels typically generates high temperatures of 1500–1800°C, which represents high-quality thermal energy, whereas what is needed for building heating is low-quality thermal energy at 20–25°C ； Direct combustion of fossil fuels for heating buildings results in a significant loss of available energy. If high-temperature heat energy generated by burning fuel is first used to generate electricity, and then that electrical energy is used to drive a heat pump to absorb low-grade heat energy from the surrounding environment, raise its temperature appropriately, and supply it to buildings, the high-grade energy contained in the fuel can be fully utilized, **reducing the consumption of primary energy for heating. The performance coefficient of heat pumps used for heating, that is, the ratio of heat supplied to the electrical energy consumed, can now reach 3-4 ； The efficiency of thermal power plants can reach 35–58% (with higher values for gas combined cycle plants). By using fuel for power generation and heat pumps for heating, the utilization rate of primary energy can exceed 200% under existing advanced technological conditions. Therefore, using heat pump technology to supply heat to buildings can **reduce the fuel consumption for heating; this not only saves energy but also **lowers the emissions of CO2 and other pollutants resulting from the combustion of fossil fuels. The low-temperature heat source used by heat pumps can usually be the environment (atmosphere, surface water, and ground) or various waste heats. It should be noted that the heat absorbed by the heat pump from these heat sources is a renewable form of energy. II. Classification of air-conditioning heat pumps and their advantages and disadvantages. There are many types of heat pump systems designed for air conditioning in buildings (including heating and cooling). Examples include heat-recovery heat pumps that utilize the heat (or coolness) from the building’s ventilation system, as well as water-loop heat pump systems used between different areas within large buildings. Here, we mainly discuss heat pump systems that utilize the surrounding environment as the heat source and cold source for air conditioning. By their nature, foreign literature generally classifies them into two main categories: air source heat pumps (ASHP) and ground source heat pumps (GSHP). Geothermal heat pumps can be further divided into surface-water heat pumps (SWHP), groundwater heat pumps (GWHP), and ground-coupled heat pumps (GCHP). China has not yet established a standardized usage for terms related to heat pump systems. For example, ground-source heat pump systems have the commercial name of “ground temperature air conditioning” ； Ground-coupled heat pumps are referred to as “soil-source heat pumps” in some literature, or simply as “geothermal heat pumps”. An air-source heat pump uses outdoor air as a heat source. In heating mode, outdoor air is used as a low-temperature heat source; heat is absorbed from this outdoor air, elevated in temperature by a heat pump, and then supplied indoors for heating. Air-source heat pump systems are simple and have a low initial investment. The main drawback of air-source heat pumps is that their efficiency **decreases** during hot summer weather and cold winter conditions. Furthermore, its heating capacity decreases as the outdoor air temperature drops, which is exactly the opposite of the trend in building heat load requirements. Therefore, when the outdoor air temperature is below the equilibrium temperature at which the heat pump operates, it is necessary to use electricity or other auxiliary heat sources to heat the air. Furthermore, in heating mode, frost forms on the evaporator of air-source heat pumps, requiring regular defrosting, which also consumes a large amount of energy. Frosting on heat pump evaporators in cold and high-humidity areas can pose a significant technical challenge. In hot summer weather, the cooling capacity decreases as the outdoor air temperature rises, which can also lead to the system not functioning properly. Air-source heat pumps are not suitable for cold regions; they have been widely used in areas with milder winter climates, such as the middle and lower reaches of the Yangtze River in China. Another type of heat pump uses the earth (soil, strata, groundwater) as a heat source, and can be called a “geothermal heat pump”. Since the deeper soil layers maintain a constant temperature throughout the year without being disturbed, and this temperature is much higher than the outdoor temperature in winter but lower than it is in summer, ground-source heat pumps can overcome the technical limitations of air-source heat pumps, resulting in **improved efficiency**. Furthermore, in winter, heat pumps are used to raise the temperature of the heat stored in the ground, thereby heating buildings; at the same time, this process lowers the temperature of the ground, storing cold energy that can be utilized in summer ； In summer, heat pumps are used to transfer heat from buildings to the ground, thereby cooling the buildings, while storing that heat in the ground for use in winter. In this way, the ground acts as an energy storage device in geothermal heat pump systems, further improving the annual energy efficiency of the air conditioning system. The heat source for underground water-source heat pump systems is groundwater extracted from wells or abandoned mines. The groundwater that has undergone heat exchange can be discharged into surface water systems, but for larger applications it is usually required to reinject the groundwater back into its original aquifer through recharging wells. In recent years, ground-source heat pump systems have seen rapid development in China. However, the application of such groundwater heat pump systems is also subject to many limitations. Firstly, such a system requires abundant and stable groundwater resources as a prerequisite. Therefore, before deciding to adopt a groundwater heat pump system, it is essential to conduct a thorough hydrogeological survey and drill exploration wells first in order to obtain data on underground temperatures, groundwater depth, water quality, and flow rates. The economics of groundwater heat pump systems are highly dependent on the depth of the groundwater layer. If the groundwater level is low, not only do the costs of drilling the well increase, but the electricity consumption of the pumps during operation also **reduces the efficiency of the system. Furthermore, although in theory the extracted groundwater is supposed to be re-injected back into the groundwater layer, the technology for groundwater re-injection is not yet mature in China. Under many geological conditions, the rate of re-injection is **lower than the rate of extraction; as a result, the water drawn from underground cannot be fully re-injected back into the aquifer after passing through heat exchangers, leading to a loss of groundwater resources. Furthermore, even if it is possible to reinject all the extracted groundwater, ensuring that the groundwater layer remains unpolluted is also a challenging issue. Water resources are currently the most scarce and valuable resources, and any waste or pollution of water resources is absolutely unacceptable. Abroad, due to increasingly strict regulations and laws regarding environmental protection and the use of groundwater, the application of ground-source heat pumps has gradually declined. One heat source for surface water heat pump systems is surface water in ponds, lakes, or streams. Using large natural water bodies such as rivers, lakes, and seas as the low-temperature heat source for heat pumps in areas near them is a type of air-conditioning heat pump worth considering. Of course, this type of surface water heat pump system is also subject to natural constraints. Furthermore, since the temperature of surface water is greatly influenced by climate, similar to air-source heat pumps, the heating capacity of the heat pump decreases as the ambient temperature drops, and its coefficient of performance also declines. The heat and cold load that a given body of surface water can handle depends on various factors such as its area, depth, and temperature, and calculations need to be carried out based on specific conditions. The impact of the heat pump’s heat exchange on the ecological environment in water bodies sometimes also needs to be considered in advance. An underground coupled heat pump system is a geothermal heat pump system that utilizes a closed-loop circulation of heat in the underground rock and soil. “The name “underground coupled heat pump” is a literal translation from English and is not common usage. It is also commonly referred to as a \"closed-loop ground source heat pump\" to distinguish it from groundwater heat pump systems, or simply as a \"ground source heat pump\". It achieves heat transfer between the system and the ground through the flow of circulating fluid (water or antifreeze with water as its main component) in enclosed underground pipes. During winter heating, the fluid absorbs heat from underground and then uses the system to deliver that heat indoors. During summer cooling, the system operates in reverse mode, that is, it removes heat from the interior and then transfers that heat to the underground rock and soil through the system. Therefore, the underground coupled heat pump system retains the advantage of groundwater heat pumps in using the earth as a heat source and sink, without the need to extract groundwater as a heat transfer medium. It is a new building energy-saving technology for sustainable development. In 1998, the U.S. Department of Energy issued regulations requiring the widespread use of underground coupled heat pump heating and cooling systems in the buildings of federal agencies across the country. To show his support for this new energy-saving and environmentally friendly technology, U.S. President Bush also installed such a geothermal heat pump air conditioning system in his home in Texas (see Xinhua News Agency, May 18, 2001). III. Economic analysis of ground-source heat pump heating and cooling systems. Ground-source heat pump systems can provide heating and cooling for buildings, as well as hot water for domestic use, offering multiple functions in one unit. A single system can replace the original two systems of boiler and chiller. The system is compact, eliminating the need for a boiler room and cooling towers, which saves building space and also contributes to the aesthetic appeal of the building. Another significant feature of ground-source heat pump systems is that they **improve the efficiency of primary energy utilization, thus offering the advantage of high energy savings.** Geothermal heat pumps are about 40-60% more efficient than traditional air conditioning systems. Furthermore, the relatively constant temperature of geothermal sources enables more reliable and stable operation of heat pump units, and the maintenance costs for the entire system are also **lower than those of boiler-chiller systems, ensuring the system’s efficiency and cost-effectiveness. To date, the main obstacles to the application of underground coupled heat pump systems in China are the high initial investment for burying pipes underground, as well as a lack of awareness regarding this technology among architects, building designers, and the general public. The economic efficiency of ground-source heat pump air conditioning systems depends on various factors. Different regions, geological conditions, energy structures, and prices will all directly affect their economic viability. Based on international experience, due to the low operating costs of ground-source heat pumps, the increased initial investment can be recovered within 3–7 years, and the average cost of ground-source heat pump systems over their entire service life is lower than that of conventional air conditioning systems. IV. Conclusion The use of heat pump technology in building heating and air conditioning can effectively improve the utilization rate of primary energy, reduce emissions of the greenhouse gas CO2 and other pollutants generated by combustion, representing a new energy-saving technology for sustainable building development. Among the several main heat pump systems discussed in this article, air-source heat pumps have the lowest initial investment, but their efficiency is lower and their application conditions are somewhat limited; nevertheless, they will continue to be widely used in some regions with mild winter climates. Groundwater heat pump and surface water heat pump systems are constrained by water resource conditions, which limits their scope of application. Geothermal heat pumps (underground coupled heat pump systems) have a wide range of applications, low operating costs, and significant energy-saving and environmental benefits. Geothermal heat pumps have been widely used in North America and Europe – they are a mature technology ； However, the application of ground-source heat pumps in our country is still in its infancy. Promoting geothermal heat pump technology requires **policy guidance, training for designers and constructors, the availability of necessary equipment and materials, as well as raising public awareness of this technology. The main constraints to the application of heat pump technology in heating and air conditioning used to be insufficient power supply and low consumption levels among the population, resulting in a lack of demand for heat pump air conditioning systems. With the economic development of our country and the improvement in people’s living standards since the reform and opening up, these two constraints no longer exist. Air conditioning and heating have become necessities for ordinary people, and their use is gradually spreading to rural areas and the southern regions, offering excellent market prospects. Through the joint efforts of **departments, research institutions, and engineering technicians, and by drawing on successful foreign experiences, the application of ground-source heat pumps in China will see rapid promotion and development.