Advances in Ground-Source Heat Pump Systems

Simon Rees, Simon Rees

• Verlag: Woodhead Publishing
• Jahr: 2016
• ISBN:
  9780081003220
• Thema: Technology & Engineering
• Thema: Renewable Power Resources

Advances in Ground-Source Heat Pump Systems relates the latest information on source heat pumps (GSHPs), the types of heating and/or cooling systems that transfer heat from, or to, the ground, or, less commonly, a body of water.

As one of the fastest growing renewable energy technologies, they are amongst the most energy efficient systems for space heating, cooling, and hot water production, with significant potential for a reduction in building carbon emissions.

The book provides an authoritative overview of developments in closed loop GSHP systems, surface water, open loop systems, and related thermal energy storage systems, addressing the different technologies and component methods of analysis and optimization, among other subjects. Chapters on building integration and hybrid systems complete the volume.

• Provides the geological aspects and building integration covered together in one convenient volume
• Includes chapters on hybrid systems
• Presents carefully selected chapters that cover areas in which there is significant ongoing research
• Addresses geothermal heat pumps in both heating and cooling modes

1

An introduction to ground-source heat pump technology

S.J. Rees University of Leeds, Leeds, United Kingdom

Abstract

Ground-source heat pumps (GSHPs, or geothermal heat pumps) have great appeal in offering levels of efficiency for building heat and cooling that are – both theoretically and practically – higher than other technologies. This chapter introduces the technology and reviews its historical development and current state of exploitation around the world. Current challenges and prospects are discussed.

Keywords

Carbon emissions; Development; Ground-source heat pump technology; History; Outlook; Policy

1.1. Introduction to the technology

1.1.1. Heat pump principles

Heat pumps are a form of heat engine that uses mechanical work to transfer heat from a low temperature source to a higher temperature sink. There are a wide range of applications of heat pumps but in this context we are concerned with transferring heat between buildings and the external environment – either rejecting heat to the environment and cooling the building or extracting heat and heating the building. Although various forms of thermodynamic cycle can be used to move heat between source and sink, the predominant form is based on the vapour–compression cycle in which a refrigerant gas is evaporated, compressed and condensed in turn to transfer heat. The principle components in the cycle are shown in Fig. 1.1. The state of the refrigerant throughout the cycle is well illustrated in an enthalpy–pressure diagram such as that on the right of this figure. The enthalpy changes in the condenser and evaporator indicate the heat transfer rate per unit mass of refrigerant flowing. Chapter ‘New trends and developments in ground-source heat pumps’ provides an in-depth review of refrigeration technology applied in heat pumps adapted for ground-coupled applications.

The prime reason for the interest in using heat pumps to provide heating and cooling is that it takes less work to move heat from source to sink than it does to convert primary energy into heat. In other words, the power required is noticeably less than the heating or cooling delivered. This effect is quantified in classical thermodynamics by the coefficient of performance (COP). If QH

is the quantity of heat delivered and W is the work required, then COP=QH/W

. Classical thermodynamics also tells us that this has a theoretical maximum value (Carnot efficiency) limited by the absolute temperature of the source (TC)

and sink (TH)

so that, COPmax=TH/(TH−TC)

in heating mode. This implies that higher efficiencies can be achieved if the source and sink temperature are close together – as is generally the case when considering buildings and their surrounding environment. It will be shown later (Section 1.1.3) that conditions are particularly favourable where the heat source takes the form of a geothermal heat exchanger.

Figure 1.1 A conceptual model of a heat pump (left) and an idealized cycle represented on an enthalpy–pressure (right) (Naicker, 2016).

The relatively high COP values that are achievable with GSHPs mean that the energy consumed is effectively leveraged so that much higher quantities of heating and cooling are delivered. This effect means that even with electricity derived from thermal power generation, where a lot of primary energy becomes waste heat at the power station, the heat exchanged with the building is greater than the primary energy consumed. This is illustrated in the upper Sankey diagram in Fig. 1.2. If one considers renewable energy sources (RES) of electricity (eg, wind power), where there is no wastage of primary energy at the source, the heat pump effect is even more significant and the carbon emissions associated with the heating and cooling demand can approach zero. RES are a limited resource and so when one considers provision of heating demands, a heat pump can be seen to be a valuable mechanism to leverage the available generation capacity (resistance heating being a poor use of renewable electricity in comparison). This is indicated in the lower Sankey diagram in Fig. 1.2.

1.1.2. Performance metrics

Strictly speaking, the heat-pump COP has to be quantified for a particular state or set of steady operating conditions. This metric is consequently useful when it comes to product testing and specification and is also sometimes termed the energy efficiency ratio (EER). When it comes to whole-system performance, we would like to consider a range of operating conditions and use a metric that represents performance when the heat pump is installed and forms part of a heating and cooling system. Choice of a suitable metric has long been a concern amongst researchers in the field. When Miriam Griffith published some of the first work on GSHPs (Griffith, 1957), she used the term performance energy ratio, and one of the audience members at the presentation of her paper commented, ‘It is unfortunate that, having become accustomed to the term “coefficient of performance”, we now have to think of “performance energy ratio”’. This term did not persist, but common agreement on a suitable metric was not found for some time.

Figure 1.2 Sankey diagram representations of the energy flowing from source to delivery by a heat pump. A typical thermal power generation process is illustrated above, and power drawn from renewable sources below (an SPF of 3 is assumed).

One approach to deriving a seasonal metric is to use a weighted mean of COP values measured at a few different operating conditions. This gives rise to metrics such as seasonal energy efficiency ratio (SEER) for air conditioners and heating seasonal performance factor (HSPF) in the case of heat pumps. However, this can also be a source of confusion. The definition of SEER in the ARI standard 210/240 is not a simple ratio but has units BTU/Wh (ARI, 2008) and similarly for HSPF. This definition of SEER consequently differs from the European ESEER metric, which is a simple ratio of energies and is also weighted slightly differently. North American SEER values have values that are 3.41214 greater than the ratio as a consequence of the choice of units. Accordingly, some caution is required when interpreting manufacturers' data.

Another approach is to derive a seasonal metric from measurements of complete systems over one or more seasons. This is not useful for rating equipment but is more useful when making comparisons with other technologies or making realistic estimates of running costs or carbon emission savings. This type of metric is termed a seasonal performance factor (SPF) and can be applied to heating or cooling performance. Defining the exact meaning of this sort of metric is not as straightforward as it first seems, however (Gleeson and Lowe, 2013). In real systems there are a number of other sources of power demand that could be constituted to be part of the whole-system demand. For example, some heat pumps include a supplementary electric resistance heater to boost output at times of peak demand. Some packaged heat pump equipment incorporates the ground loop and possibly heating circulation pumps; in other systems these pumps are installed elsewhere. The complexities of agreeing on definitions of SPF was addressed by a European project SEPEMO that published definitions of SPF with defined scope (Zottl et al., 2012). The metrics are SPF_H1 through SPF_H4 depending on which supplementary electrical demands are included.

Figure 1.3 Seasonal performance factor scope according to SEPEMO definitions.

The definition of SPF_H1 includes only the heat pump equipment (ie, compressor and controls) and no supplementary heaters or circulation pumps. SPF_H2 additionally includes the ground loop circulating pump energy. SPF_H3 furthermore includes any electric heater packaged with the heat pump and SPF_H4 includes all circulating pumps and supplementary heaters. Consequently, values of SPF_H1 are closest to the manufacturers' EER values and SPF_H4 values are somewhat lower. The different system ...