Elsevier

Energy and Buildings

Volume 39, Issue 12, December 2007, Pages 1211-1217
Energy and Buildings

Exergetic modeling and assessment of solar assisted domestic hot water tank integrated ground-source heat pump systems for residences

https://doi.org/10.1016/j.enbuild.2007.01.007Get rights and content

Abstract

The present study deals with the exergetic modeling and performance evaluation of solar assisted domestic hot water tank integrated ground-source heat pump (GSHP) systems for residences for the first time to the best of the author's knowledge. The model is applied to a system, which mainly consists of (i) a water-to-water heat pump unit (ii) a ground heat exchanger system having two U-boreholes with an individual depth of 90 m, (iii) a solar collector system composing of rooftop thermal solar collectors with a total surface area of 12 m2, (iv) a domestic hot water tank with a electrical supplementary heater, and (v) a floor heating system with a surface of 154 m2, and (vi) circulating pumps. Exergy relations for each component of the system and the whole system are derived for performance assessment purposes, while the experimental and assumed values are utilized in the analysis. Exergy efficiency values on a product/fuel basis are found to be 72.33% for the GSHP unit, 14.53% for the solar domestic hot water system and 44.06% for the whole system at dead (reference) state values for 19 °C and 101.325 kPa. Exergetic COP values are obtained to be 0.245 and 0.201 for the GSHP unit and the whole system, respectively. The greatest irreversibility (exergy destruction) on the GSHP unit basis occurs in the condenser, followed by the compressor, expansion valve and evaporator.

Introduction

Ground source heat pumps (GSHP), or also referred to as geothermal heat pumps, are essentially a combination of a heat pump and a system for exchanging heat with the ground. Basically, they consist of either a ground heat exchanger (closed loop system), or a system fed by ground water from a well (open loop system). The heat can be extracted from the ground through: (i) groundwater wells (“open” systems), (ii) borehole heat exchangers, (iii) horizontal heat exchanger pipes (including compact systems with trenches, spirals etc.), and (iv) “geostructures” (foundation piles equipped with heat exchangers) [1].

The growing awareness and popularity of GSHPs have had the most significant impact on direct use of geothermal energy. The annual energy use for these pumps grew at a compound annual rate of 30.3, compared to 2000, and 19.6%, compared to 1995. The installed capacity grew 23.8 and 23.6%, respectively. This is due, in part, to the ability of geothermal heat pumps to utilize groundwater or ground-coupled temperatures anywhere in the world. GSHPs have the largest energy use and installed capacity, accounting for 54.4 and 32.0% of the worldwide capacity and use. In 2005, the installed capacity was 15,384 MWt and the annual energy use was 87,503 TJ/year, with a capacity factor of 0.18 (in the heating mode). Almost all of the installations occurred in North America and Europe, increasing from 26 countries in 2000 to the present 33 countries. The equivalent number of installed 12-kW units (typical of US and Western European homes) was approximately 1.3 million, over double the number of units reported for 2000. The size of individual units, however, ranged from 5.5 kW for residential use to large units of over 150 kW for commercial and institutional installations [2].

Exergy, which is a way to a sustainable development, has become a very effective tool in evaluating system effectiveness and in designing the system to maximize energy savings. In this regard, during the last decade, various investigations have been conducted by some researchers in the performance assessment of GSHPs using exergy analysis method. Hepbasli [3] has reviewed comprehensively exergetic analysis and performance evaluation of a wide range of renewable energy resources for the first time to the best of his knowledge. He summarized studies conducted on exergetic evaluation of GSHP systems. Ozgener and Hepbasli [4] dealt with the energetic and exergetic modeling of GSHP systems for the system analysis and performance assessment. The analysis covered two various GSHPs, namely a solar assisted vertical GSHP and horizontal GSHP. The performances of both GSHP systems were evaluated using energy and exergy analysis method based on the experimental data. The exergy efficiency peak values for both whole systems on a product/fuel basis were found to be in the range of 80.7 and 86.13%. Akpinar and Hepbasli [5] modeled and evaluated exergetically two different GSHP systems. The fist one was a GSHP system designed and constructed for investigating geothermal resources with low temperatures, while the second one was a GSHP system with a vertical ground heat exchanger. Hepbasli and Balta [6] studied on the modeling and performance evaluation of a heat pump system utilizing a low temperature geothermal resource, which is approximated to a geothermal reservoir. Energy and exergy efficiency values on a product/fuel basis were found to range from 73.9 to 73.3% and 63.3 to 51.7% at dead (reference) state temperatures varying from 0 to 25 °C for the heat pump unit and entire system, respectively.

In the present study, a solar assisted domestic hot water tank integrated GSHP system, which is introduced in Section 2, is modeled and assessed in terms of exergetic aspects for the first time to the best of the author's knowledge. This has provided the motivation for this study.

Section snippets

System description

Fig. 1 illustrates a schematic of the solar assisted domestic hot water tank integrated GSHP system used in a 180 m2 private residence, to which the model given in Section 3 is applied in the heating mode since the experimental data were available in this mode only. This system was designed for heating/cooling the residence and domestic hot water needs. The detailed description of the system is given elsewhere [7]. Solar heat is used in priority to heat domestic hot water (DHW) and is injected

Modeling

General mass, energy, entropy and exergy balance equations are given in more detail elsewhere [3], while the following section covers the relations on the system component basis illustrated in Fig. 1.

Mass and energy balances as well as exergy destructions obtained from exergy balances for each of the solar assisted domestic hot water tank integrated GSHP system components illustrated in Fig. 1 are derived as follows:

  • Compressor (I):m˙1=m˙2,s=m˙act,s=m˙rW˙comp=m˙r(h2,acth1)E˙xdest,comp=m˙r(ψ1ψ

An Illustrative example

The system described in Section 2 is evaluated from the exergetic point of view, while the model presented in Section 3 is applied to this system.

The following several assumptions are made for the exergy analysis of the system given as an illustrative example.

  • (a)

    All processes are steady state and steady flow with negligible potential and kinetic energy effects and no chemical or nuclear reactions.

  • (b)

    The directions of heat transfer to the system and work transfer from the system are positive.

  • (c)

    The

Conclusions

Solar assisted domestic hot water tank integrated GSHP systems for residences are exergetically modeled in this study, while the performance of a GSHP system along with their essential system components (i.e., GSHP unit, ground heat exchanger, solar collector, circulating pumps, solar domestic hot water tank and floor heating system) is assessed through a comprehensive exergy analysis in the heating mode. The experimental and assumed values are utilized in the analysis. The exergy destructions

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