Compressed-liquid energy storage with an adsorption-based vapor accumulator for solar-driven vapor compression systems in residential cooling

doi:10.1016/j.ijrefrig.2015.11.015

International Journal of Refrigeration

Volume 64, April 2016, Pages 176-186

https://doi.org/10.1016/j.ijrefrig.2015.11.015 Get rights and content

Highlights

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A cycle-integrated energy storage strategy for vapor compression refrigeration is proposed.
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The storage subsystem is comprised of a liquid tank and an adsorption-based vapor accumulator.
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High energy storage densities can be achieved for the system when operating with ammonia.
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Solar residential cooling is evaluated as a potential application.
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Feasibility of the proposed strategy requires development of better adsorption pairs.

Abstract

A cycle-integrated energy storage strategy for vapor-compression refrigeration is proposed wherein thermo-mechanical energy is stored as compressed liquid. A compressed-liquid tank is integrated into the liquid line of the system by means of an adsorption-based vapor accumulator in the vapor line. Energy is retrieved through expansion of the compressed liquid, which allows for a tunable evaporator temperature. A thermodynamic model is developed to assess the system performance, with storage incorporated, for solar residential cooling in two locations with contrasting ambient temperature profiles. Ammonia, R134a, and propane, all paired with activated carbon as adsorbent, are evaluated. A high cold thermal energy storage density is achieved when operated with ammonia. However, the accumulator suppresses the coefficient of performance of the system because work is required to extract refrigerant from the adsorbent. Practical feasibility of the proposed storage strategy calls for the development of nontoxic refrigerant–adsorbent pairs with more favorable adsorption behavior.

Introduction

Global concerns about the environmental impact and finite availability of conventional energy sources have motivated efforts to develop technologies that harness clean and renewable energy sources. However, renewable sources are often challenged by their inherently intermittent nature. Energy from renewable sources is not always available in a useful form when demanded, and energy storage strategies are necessary to align supply with demand. In this context, solar cooling technologies appear promising because of the direct relationship between cooling load and solar radiation intensity (Kim and Infante-Ferreira, 2008). Solar radiation intensity strongly correlates with the ambient temperature, and hence, cooling load is considerably higher during insolation hours and generally reaches a maximum value shortly after solar noon. This partial alignment of solar radiation intensity and cooling load thus reduces the required energy storage capacity.

Cold thermal energy storage (CTES), or the process of storing cooling capacity (ASHRAE, 2007), is relevant in a variety of refrigeration applications including solar cooling. Cold thermal energy storage serves to decouple cooling from power consumption. It can be used to shave and/or shift electricity peak demand in conditioned spaces, such as commercial buildings and residences (Chen et al, 2009, Reddy et al, 1991, Saito, 2002). Alternatively, CTES can supply cooling capacity when the energy source is unavailable, as may be the case for refrigeration systems powered by variable renewable energy sources or in the transportation of temperature-sensitive items.

Established CTES technologies include storage systems using chilled water, ice, and other phase change materials (PCM). Water-based storage technologies are mature and commercially available in view of the advantageous thermal properties, chemical stability, wide availability, and low cost of water (Oró et al, 2012, Rismanchi et al, 2012). Sensible cold energy storage in water demands few modifications to conventional refrigeration systems and has a lower initial cost; however, large system volumes are required due to the low energy storage density (Rismanchi et al., 2012). To bring about stratification inside the chilled water storage tank, charging temperatures should exceed the water-density maximum of 4 °C (ASHRAE, 2007, Saito, 2002), restricting the temperature range and reducing the storage density. Ice storage systems, which have much higher storage densities, require charging temperatures below the freezing point of water, between −12 °C and −3 °C (Rismanchi et al, 2012, Wang, Kusumoto, 2001). This temperature range is significantly colder than the typical evaporator temperature in air-conditioning systems (Oró et al, 2012, Saito, 2002), and has an adverse effect on thermal performance. Moreover, ice storage systems have other technological challenges, such as the need for methods to control ice nucleation, processes that prevent adhesion of the ice to the cooling surface, approaches for maintaining the fluidity of ice-water mixtures, and methods to effectively melt the ice, among others (Saito, 2002). Other PCMs, such as eutectic salt solutions and organic compounds, can offer a range of different charging temperatures, but have other limitations. In general, eutectic salt solutions have good thermal properties and low cost, but are chemically unstable and corrosive (Oró et al., 2012). Organic PCMs are chemically stable, but are more expensive and have less favorable thermophysical properties (such as low thermal conductivity, low latent heat of fusion, and large change in density between solid and liquid phases) (Oró et al., 2012).

In air conditioning systems, CTES technologies are beneficial due to the inherently variable nature of the cooling load, which is dominated by daily and seasonal variations in environmental conditions and by user habits. In many areas in the United States, the maximum electrical peak demand occurs during the summer time due to air-conditioning demand. This is especially so in regions where winter demand is met in part by the use of gas or oil for space heating (Reddy et al., 1991). In the absence of energy storage, electricity must always be produced on demand, and electrical power plants need to be oversized accordingly, leading to inefficient operation of expensive facilities (Chen et al., 2009).

The residential sector, which has a 24% share of the final energy consumption worldwide (Ürge-Vorsatz et al., 2015), is a viable niche for solar cooling with energy storage. Residences are usually spread out over large areas and have considerable roof area, traits that are compatible with distributed solar energy collection. Furthermore, energy consumption for worldwide residential heating and cooling is projected to increase by around 80% between 2010 and 2050 due to an increase in the number of households, and rising income levels leading to increased ownership of cooling equipment (Isaac, van Vuuren, 2009, Ürge-Vorsatz et al, 2015). Due to its high initial cost, the use of solar cooling is currently rare (Kim and Infante-Ferreira, 2008), and is typically restricted to commercial buildings, where total cooling demand is large and net savings offer a favorable economic return.

Although solar availability partially overlaps with air conditioning cooling load, the two are not perfectly coincident, and a compatible energy storage strategy is required to fully meet cooling demand with a solar collector of reasonable area. Solar cooling technologies include solar electric, solar thermo-mechanical, and solar thermal. In solar-electric cooling, such as electrically driven vapor compression and thermoelectric refrigeration (Kim, Infante-Ferreira, 2008, Sarbu, Sebarchievici, 2013), photovoltaic panels are used for electricity generation with batteries incorporated for energy storage (Sarbu and Sebarchievici, 2013). The cost competitiveness of this system is favored as prices of photovoltaic modules continually decrease (Bazilian et al, 2013, Lang et al, 2013); additional benefits are realized for systems with commercial vapor-compression refrigeration units, because of the standard prices and the high coefficient of performance (COP) that facilitates the use of smaller collection areas (Otanicar et al., 2012). However, electrical energy storage in batteries is still expensive compared with thermal strategies for energy storage. Solar thermal refrigeration technologies predominantly include closed and open sorption systems, but thermo-mechanical refrigeration systems that use a steam ejector have also been considered (Kim, Infante-Ferreira, 2008, Sarbu, Sebarchievici, 2013). Although this group of technologies is compatible with commercial solar collectors used for solar heating, commercial thermal refrigeration units are scarce, and those that exist are expensive and exhibit low COP (Otanicar et al., 2012). However, thermal refrigeration systems can incorporate cold thermal energy storage strategies that are less expensive compared to electrical energy storage in batteries; alternatively, cycle-integrated storage strategies can be proposed. For example, for open sorption refrigeration with desiccants, cooling capacity may be stored through storage of hygroscopic solutions with low water content (Sarbu and Sebarchievici, 2013).

In the present study, a new strategy for cycle-integrated energy storage in vapor compression systems is proposed. The basic principle of this technology is to store compressed-liquid refrigerant, which can be expanded when cooling is required. Cooling is achieved through change of phase of the expanded liquid in a conventional evaporator. The proposed strategy addresses key limitations in the available energy storage technologies for solar electric residential refrigeration through vapor compression by providing high energy storage density, tunable temperature range of energy recovery, and a potentially lower-cost solution. The performance and size of a solar electric refrigeration system with the proposed energy storage strategy are investigated for an average American house in the summer at two locations with contrasting ambient daily temperature profiles. The assessment is performed using a thermodynamic model and considering different refrigerant–adsorbent pairs.

Section snippets

System description

The cycle-integrated energy storage concept for vapor compression refrigeration uses excess available electricity, generated during low cooling load periods, to compress additional refrigerant vapor, which is condensed and stored at a constant pressure so that it can be expanded and evaporated at a later time when cooling is required in the absence of adequate electricity generation. An adsorption process allows densification and storage of the resulting discharged vapor. Fig. 1 presents a

Thermodynamic analysis

The behavior of the system is analyzed with a thermodynamic model that considers ideal quasi-equilibrium processes. In the model, it is assumed that the vapor refrigerant is saturated at the evaporator outlet, and that the liquid refrigerant is saturated at the condenser outlet. Also, the liquid storage tank and the vapor adsorption accumulator are assumed to be fixed at the ambient temperature. The heat exchangers (evaporator and condenser), compressors, and expansion valves are modeled as

Cold thermal energy storage density

The CTES density is estimated for the proposed strategy with each different refrigerant considered using Equation (12), as a function of evaporator temperature, as shown in Fig. 3. Storage density increases almost linearly with evaporator temperature: Evaporator pressure increases with temperature, enlarging the operating range of the storage subsystem, and thus boosting the refrigerant uptake in the adsorption bed. This trend is opposite to the behavior in chilled water storage, where energy

Conclusion

A cycle-integrated CTES strategy for vapor-compression refrigeration systems is proposed. The underlying principle is to store compressed liquid refrigerant so that it can be expanded when cooling capacity is required. An adsorption-based vapor accumulator is necessary to store excess expanded vapor refrigerant when the CTES subsystem is discharged. The performance and storage capacity of the refrigeration system with compressed-liquid energy storage strongly depends on the properties of the

Acknowledgments

The first author acknowledges financial support from the Colombia-Purdue Institute (CPI) and the Colombian Department for Science, Technology and Innovation (Colciencias).

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Highlights

Abstract

Introduction

Section snippets

System description

Thermodynamic analysis

Cold thermal energy storage density

Conclusion

Acknowledgments

Renew. Energy

Int. J. Refrigeration

Prog. Nat. Sci

Appl. Therm. Eng

Sep. Purif. Technol

Energy Policy

Int. J. Refrigeration

Int. J. Refrigeration

Appl. Energy

Sol. Energy

Energy

Appl. Therm. Eng

Renew. Sustain. Energy Rev