Elsevier

Energy Conversion and Management

Volume 77, January 2014, Pages 550-557
Energy Conversion and Management

Solar-assisted heat pump – A sustainable system for low-temperature water heating applications

https://doi.org/10.1016/j.enconman.2013.09.050Get rights and content

Highlights

  • DX-SAHP water heaters systems are economical as well as energy conserving.

  • The economic analysis is performed using the life cycle cost (LCC) analysis.

  • LCC can be optimized with respect to the collector area at a specific temperature.

  • For high load temperature range a two stage heat pump system is more appropriate.

Abstract

Direct expansion solar assisted heat pump systems (DX-SAHP) have been widely used in many applications including water heating. In the DX-SAHP systems the solar collector and the heat pump evaporator are integrated into a single unit in order to transfer the solar energy to the refrigerant. The present work is aimed at studying the use of the DX-SAHP for low temperature water heating applications. The novel aspect of this paper involves a detailed long-term thermo-economic analysis of the energy conservation potential and economic viability of these systems. The thermal performance is simulated using a computer program that incorporates location dependent radiation, collector, economic, heat pump and load data. The economic analysis is performed using the life cycle cost (LCC) method. Results indicate that the DX-SAHP water heaters systems when compared to the conventional electrical water heaters are both economical as well as energy conserving. The analysis also reveals that the minimum value of the system life cycle cost is achieved at optimal values of the solar collector area as well as the compressor displacement capacity. Since the cost of SAHP system presents a barrier to mass scale commercialization, the results of the present study indicating that the SAHP life cycle cost can be minimized by optimizing the collector area would certainly be helpful in lowering, if not eliminating, the economic barrier to these systems. Also, at load temperatures higher than 70 °C, the performance of the single stage heat pump degrades to the extent that its cost and efficiency advantages over the electric only system are lost.

Introduction

In recent decades, global energy consumption has increased substantially. A major part of the global energy consumption comes from conventional energy sources such as fossil fuels. In 2004, the total worldwide energy consumption was 15 TW (=1.5 × 1013 W) with 86.5% from fossil fuels [1]. However, combustion of fossil fuels involves production of carbon dioxide and other gases that cause the greenhouse effect, leading to global warming. The atmospheric concentration of CO2 has increased by 31% above pre-industrial levels since 1750 [2]. Growing demand for petroleum-based fossil fuels has led to concerns that they may be depleted in the next two decades to a level that would cause a major disruption in the energy supply chain. As a result, finding alternative energy sources that are cleaner as well as economical has become a critical societal need which led to the development of renewable energy sources, such as solar and wind, in recent years.

The United States is the world’s largest energy user, consuming 100 quadrillion BTU (29,000 TW h). About twenty-one percent of this total energy is used in the residential sector [3]. Water heating alone accounts for about 20% of the energy consumed in a typical American household and about 2.6% of the total energy consumed in commercial building in the United States [4]. It is estimated that nearly 7% of the total US energy consumption is in the form of low temperature (<80 °C) water heating applications in which energy demand is met primarily through either natural gas or electrical resistance heaters. For instance, in the residential sector nearly 70% of water heating needs are met through natural gas heaters and about 20% through electrical resistance heaters [5]. However, the use of these conventional systems involves carbon emission, either directly or indirectly, into the earth’s atmosphere. For instance, a natural gas water heater typically contributes about 2 tons of CO2 annually to the atmosphere. Electric hot water systems are particularly deleterious to the environment since they are indirectly responsible for about three times the emission of CO2 for each kW h of electrical energy, compared to natural gas. So why are electric hot water heaters still in use? According to the US Energy Information Agency there are nearly 40 million households with electric hot water heaters [1]. In the United States the reason for their presence in the market place has to do with their very low capital cost compared to other alternatives. Even though their operating cost is higher compared to alternatives such as natural gas water heaters, some consumers still prefer them due to their low initial cost. However, due to implicit environmental cost for remediation or sequestering of CO2 emission from fossil fuel burners it is worthwhile to look at alternatives such as renewable energy sources that operate with reduced or very little CO2 emission. Ideally a sustainability consideration of these renewable energy systems should entail two broad elements: (a) reduction or elimination of primary energy (coal, natural gas, etc.) consumption through substitution of renewable energy sources to achieve reduced CO2 emission and (b) economic competitiveness demonstrated through a life cycle cost analysis. These aspects will be discussed in more detail in later sections of this paper.

Section snippets

The proposed system and scope of study

In the present study, a direct expansion solar-assisted SAHP system is used for water heating applications as an alternative to electric or natural gas water heater. The theory and operational aspects of this system have been described in the literature and the readers can find details in Refs. [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40]

Long-term thermal analysis

Thermal performance of the SAHP system is primarily indicated by parameters such as heat pump coefficient of performance (COPH), the system effective coefficient of performance of (COPeffective), and primary energy ratio (PER). All these parameters are evaluated for specified geographical location based on chosen condensing temperature (T3). The thermal performance of the proposed system is analyzed for the city of Norfolk VA, USA. In order to predict the long-term averaged thermal performance

The compressor model

The compressor work, Wcomp, or the compressor power Ẇcomp(=ṁref.Wcomp) for a given pressure ratio P2/P1 is determined from the expression,Wcomp=P1v1ηcompkk-1P2P1k-1k-1where k is the ratio of specific heats, and for R-134a it has a value of 1.106. The assumption of ideal gas behavior during the compression process turns out to be fairly reasonable since the compressor work calculated in this manner is somewhat overestimated compared to the compressor work calculated directly from the

The collector model

The collector model is used to determine the collector temperature T1 for given values of ambient temperature Ta and I¯coll, collector parameters (τα, F′, Ac and UL), refrigerant properties h1 and h2 and the heat pump parameters (the displacement volume rate VD). The steady state energy balance on the collector, expressed by Eq. (2), states that the net energy absorbed by the refrigerant circulating through the collector equals incident solar radiation minus the heat loss from the collector.VDv1

Economic analysis

In the present study, economic analysis is performed using the life cycle cost (LCC) method. The life cycle cost is the sum of all costs associated with the SAHP system over its’ entire lifetime, in present cost, that takes into account the time value of money. Several factors such as initial cost, operating cost, and maintenance cost need to be taken into account. The method for calculating LCC is well established and readers are referred to Ref. [44]. Taking into account the utility inflation

Primary energy resource utilization

For analysis of primary energy utilization in competing systems for domestic hot water (DHW) applications, one can use the primary energy ratio (PER), a parameter that characterizes the overall efficiency of a number of interconnected energy transformation processes. For instance, in the DHW applications, in order to determine the PER one must trace the energy flow all the way back to the primary energy source (coal, natural gas, oil, etc.) to determine how many kilo Joules of thermal energy

Discussion of results-annualized system performance

In this section, the effects of changes in the collector area, compressor displacement volume (VD) and the load temperature on the long-term system performance are discussed. Fig. 6 shows the variation of the system life cycle cost (LCC) with the collector area for a specified load temperature and compressor displacement volume. The presented load temperature is 60 °C and the volume displacement of the compressor is 0.0003518 m3/s. From Fig. 6, one observes that a minimum value of LCC occurs at a

Conclusions

The present study employed modeling and long-term averaged monthly solar data to simulate the long-term averaged thermal performance of a direct-expansion solar assisted heat pump. Results were obtained for a range of load temperature and for a number of collector areas and compressor displacement volumes. The thermo-economic analysis results of the SAHP system indicates that the system is an appropriate match for the temperature water heating applications. Since a large amount of primary

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