Solar Assisted Ground Source Heat Pump
Abstract This chapter touches the system configurations of solar assisted ground source heat pumps, giving an overview of the main system components at first. Series and parallel/dual sources configurations have been discussed for a technical point of view, identifying main operating modes in accordance with the use of the produced solar heat. Indicators for assessing energy performance of main system components singularly and overall system have been also presented, paying attention also on the final energy supply chain from the used energy fuels.
Keywords Solar assisted ground source heat pump • System layouts • Operating modes Energy performance indicators
Overview of the System Components
As mentioned within the introduction chapter, solar assisted ground source heat pump (SAGSHP) systems use solar collectors also for assisting the heat pump, being somehow connected to the heat pump evaporator. The operating principle behind using solar heat for increasing the evaporator temperature is to have lower Q1/T1 ratio (Eq. 1.3, Sect. 1.2) by increasing T1 and, consequently, reducing the heat pump work. Another important operating mode is the ground regeneration. To avoid confusion, the regenerative process of the heat pump cold thermal source (ground) is intended when solar thermal energy is used to heat up the soil when the heat pump is off.
As good practice suggests, storage tank is often used when solar collectors are adopted in order to collect solar heat surplus, as shown in Sect. 2.2. Therefore, solar heat can be used in a different time than when produced. The tank selection is based © The Author(s) 2017
F. Reda, Solar Assisted Ground Source Heat Pump Solutions, SpringerBriefs in Applied Sciences and Technology,
Fig. 3.1 Pressure drop per meter V-GHX. Note lp pipe length; di inner pipe diameter (Jalaluddin and Miyara 2015)
on the solar collector size, usually the volume of the tank is around 50 l/m2(solar collectors surface) for residential application (Li et al. 2015). However, both climate and specific application affect the tank size.
Circulation pumps are a key component of SAGSHP systems, being responsible of pushing the fluid through solar collector array and ground heat exchanger. Circulator pumps have to overcome the pressure drops in ducts, ground heat exchanger, solar collector array and, if any, heat exchangers. Usually, solar collector array and ground heat exchanger are connected to two different pumps mainly because of the specific pressure requirements and for enabling multiple system operating modes; this aspect will be discussed in the next chapters. Anyhow, pressure drop magnitude in ducts and the borehole heat exchanger depends upon the specific hydronic layout and fluid flow rate. Particularly, pressure drops related to piping (due to length, diameter restrictions, turns and so on) are calculated in accordance with the conventional method (Bernoulli and Darcy-Weisbach equations). Instead as regard V-GHXs, pressure drop varies with the borehole configuration as shown in Fig. 3.1 in case of a spiral or straight pipe (Fig. 3.2). Moreover, in case of multiple boreholes, the number of boreholes connected in series and in parallel affects the pressure drop magnitude.
As regard solar collectors, solar array configuration affects the pressure drops: number of panels connected in series and in parallel and working fluid flow rate. Pressure drop per single collector, as a function of the working fluid flow rate, is a data given by the manufacturer. As a remark, pressure drops have to be carefully estimated in order to properly size the circulator pumps. Oversizing the pumps can impair SAGSHP systems performance; a more powerful pump would increase both system energy consumption and investment costs in vain.
Many configurations of SAGSHP exist. These mainly vary with the connection type between the solar collectors and between them and the system. The collectors
Fig. 3.2 Schematic diagrams of the U-tube and spiral-tube within a V-GHX (Jalaluddin and Miyara 2015)
can be connected in series or in parallel to the heat pump evaporator for assisting the heat pump. It has to be mentioned that the author refers to “solar assisted” denoting the use of solar thermal collector technology only.