**1. Introduction**

458 Heat Exchangers – Basics Design Applications

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The global warming itself and its consequences cause considerable problems. It results in extreme climate events such as droughts, floods, or hurricanes, which are expected to become more frequent. This puts extra strain on people and has great impact on public health and life quality especially in poor countries.

Internationally, there is a political understanding that global warming (or climate change) is the main challenge of the world for decades to come. Thus, all states must work together in order to overcome climatic change consequences.

Although, studies suggest that there is indeed relationship between solar variability and global warming (Lean and Rind, 2001), two causes of the warming have been suggested:


This implies that current warming is anthropogenic and caused by human activities, i.e. global use of non-renewable energy. So far, the total global energy consumption has already exceeded 15. 1010 MWh/year and it is projected to have an annual growth rate about 1.4 % until 2020 (EIA, 2010).

Much of the energy used worldwide is mainly supplied by fossil fuels (~85 % of the global energy demand while renewable energy sources supply only about 6 %) (Moomaw et al., 2011, Jabder et al., 2011, Jaber et al., 2011). Owing to global dependence on oil fuels has resulted in a daily oil consumption of 87.7 million barrels (Mbbl), Fig. 1 (IEA, 2010, EIA, 2007). Consequently, about 3.1010 ton of carbon dioxide emissions are annually emitted into the atmosphere. In other word, for each consumed kWh about 205 kg of carbon dioxide is being emitted into the atmosphere.

Environmental reasons urge us to find more efficient ways in converting and utilizing the energy resources. From the environment point of view, there is now almost universal scientific acceptance that profligate energy use is causing rapid and dangerous changes in the global climate. There is mounting evidence that the mean global temperature has increased over the period 1880 to 1985 by 0.5 to 0.7 oC (Hansen and Lebedeff, 1987). While surface air temperature (SAT) compilations shows that SAT has increased 1.2 oC last century. If a current climatic change trend continues, climate models predict that the

Ground-Source Heat Pumps and Energy Saving 461

Current work emphasizes the importance of using ground source heat pumps in reaching towards the renewable energy goals of climate change mitigation, and reduced

The ground source heat pump (GSHP) system are also known as ground coupled heat pump (GCHP), borehole systems or borehole thermal energy storage (BTES), and shallow geothermal system. Due to its high thermal performance, the ground source heat pump (GSHP) have increasingly replaced conventional heating and cooling systems around the world (IEA, 2007, Hepbasli, 2005, De Swardt and Meyer, 2001). Essentially GSHP systems refer to a combination of a heat pump and a system for exchanging heat from the ground. The GSHPs move heat from the ground to heat homes in the winter or alternatively, move heat from the homes to the ground to cool them in the summer. This heat transfer process is achieved by circulating a heat carrier (water or a water–antifreeze mixture) between a ground heat exchanger (GHE) and heat pump. The GHE is a pipe (usually of plastic) buried vertically or horizontally under the ground surface, Fig. 2 (Sanner et al., 2003). At the beginning of 2010 the totally installed GSHP capacity in the world was 50,583 MW producing 121,696 GWh/year with capacity factor and annual grow rate of 0.27 and 12.3%,

**Heating mode**: In this case, the GHE and the heat pump evaporator are connected together and the heat pump moves the heat from the ground into the conditioned space. The liquid of relatively low temperature is pumped through the GHE, collecting heat from the surrounding ground, and into the heat pump. Since the temperature of extracted liquid, which is around mean annual air temperature, is not suitable to be used directly for heating purpose, heat pump elevates the temperature to a suitable level (30-45 oC) before it is

**Cooling mode**: In this case, the GHE and the heat pump condenser are connected together and heat pump moves the heat from the conditioned space into the ground. The liquid of relatively high temperature is pumped through the GHE, dispersing heat into the

As known, heat transfers from a warmer object to a colder one. Heat, as stated by the second law of thermodynamics, cannot spontaneously flow from a colder location to a hotter area unless work is done. The heat pump is simply a device for absorbing heat from one place and transporting it to another of relatively lower temperature. So, such device can be used to maintain a space temperature at desired level by removing unwanted heat (e.g. a fridge or air conditioning unit) or to transport heat to where it is wanted (space or water heating). In space conditioning application, heat pump system is composed of an indoor unite and an outdoor unite and the task of the heat pump is to transfer heat from one unite to the other. In order to keep inside temperature at comfort level in the winter, for example, the heat pump absorbs heat from outdoor and expels it into building. In the summer the reversed

process occurs, i.e. the heat pump moves heat from indoor and expels it to outside.

The temperature difference between the indoor unite and outdoor unite is referred to as temperature lift. This temperature plays a major role in determining the coefficient of performance of heat pump (COP= delivered energy/driving energy). A smaller temperature

environmental impacts.

**2. Principle of GSHP systems** 

respectively (Lund et al., 2010).

submitted to a distribution system.

surrounding ground, and into the heat pump.

average global temperature are likely to have risen by 4 to 6 oC by the end of 21st century (Gaterell, 2005). As climate change progresses, all the other environmental problems are becoming worse and harder to solve. Therefore, a sustainable future requires worldwide efforts to prepare for new energy sources and a more efficient use of energy.

Fig. 1. World oil consumption.

Thanks to the awareness of the impact of global warming and its relationship with human activities, there has been a growing interest in reducing fossil energy consumptions. Specifically, more efficient use of energy and increased use of renewable energy seem to be our main weapon against the ongoing global warming.

Heating and cooling in the industrial, commercial, and domestic sectors accounts for about 40-50 % of the world's total delivered energy consumption (IEA, 2007, Seyboth et al., 2008). Although, buildings regulations aim to reduce the thermal loads of buildings, as the economic growth improves standards of living, the energy demand for heating and cooling is projected to increase. For example, in non-OECD nations, as developing nations mature, the amount of energy used in buildings sector is rapidly increasing. Consequently, the implementation of more efficient heating/cooling systems is of clear potential to save energy and environment. However, the use of renewable energy systems for heating and cooling applications has received relatively little attention compared with other applications such as renewable electricity or biofuels for transportation. Yet, renewable energy sources supply only around 2-3% of annual global heating and cooling (EIA, 2010). It is worth mentioning that a century or more ago renewable energy accounted for almost 100%. In other word, all current researches aim to approach what was the case in the past.

Nowadays, and due to its high thermal performance, the ground source heat pump (GSHP) has increasingly replaced conventional heating and cooling systems around the world. Such system extracts energy from a relatively cold source to be injected into the conditioned space in winter or alternatively, extracts energy from conditioned spaces to be injected into a relatively warm sink in summer.
