Elsevier

Solar Energy

Volume 86, Issue 11, November 2012, Pages 3417-3429
Solar Energy

Design and optimization of an air heating solar collector with integrated phase change material energy storage for use in humidification–dehumidification desalination

https://doi.org/10.1016/j.solener.2012.07.017Get rights and content

Abstract

Compared to solar water heaters, high-temperature solar air heaters have received relatively little investigation and have resulted in few commercial products. However, in the context of a humidification–dehumidification (HD) desalination cycle, air heating offers significant performance gains for the cycle. Heating at a constant temperature and constant heat output is also important for HD cycle performance. The use of built in phase change material (PCM) storage is found to produce consistent air outlet temperatures throughout the day or night. In this study, the PCM has been implemented directly below the absorber plate. Using a two dimensional transient finite element model, it is found that a PCM layer of 8 cm below the absorber plate is sufficient to produce a consistent output temperature close to the PCM melting temperature with a time-averaged collector thermal efficiency of 35%. An experimental energy storage collector with an 8 cm thick PCM layer was built and tested in a variety of weather and operating conditions. Experimental results show strong agreement with model in all cases.

Highlights

► A novel air-heating solar collector with embedded phase-change material is described. ► The design is aimed at solar thermal desalination systems requiring a stable top temperature. ► The performance characteristics of the collector are modeled and numerically simulated. ► A physical prototype of the collector was constructed and tested. ► The experimental results are in good agreement with the numerical model.

Introduction

Solar water heaters have been thoroughly investigated and developed commercially (Eggers-Lura, 1978, Garg, 1985, Kalogirou, 2004). While there are commercial air heaters (Solar Rating and Certification Corporation, 2008) their use is primarily for low-temperature (40 °C) space heating applications and do not use any form of energy storage. In the context of humidification–dehumidification (HD) desalination, heating the air high temperature (50–80 °C), as opposed to the water stream leads to significant performance gains (Narayan et al., 2010b). Heating at a constant temperature also provides more stability to the HD system and increases performance over time (Narayan et al., 2010a).

Narayan et al. (2010b) laid out a comparison of current solar air heating technologies. Summers et al., 2010, Summers et al., 2011 investigated the performance of air heaters at steady state and showed improved performance over current designs; however, this analysis did not take into account the transient nature of solar irradiation throughout the day from sunrise to sunset. Rapidly dropping temperatures at the end of the day can severely reduce performance of a HD desalination system, ultimately reducing the amount of water obtained.

Energy storage is an important aspect of collecting intermittent energy like that from the sun, especially for applications like HD desalination in which a stable warm temperature is for optimum performance. The goal of a properly optimized storage system is to deliver heat at an approximately constant temperature throughout a 24 h day. This avoids startup effects in the morning as well as energy loss when the cycle top temperature drops at night.

Section snippets

Comparison of performance

Summers et al. (2011) compared the performance of solar air heaters using the collector efficiency. This was an instantaneous power efficiency that was applicable to a collector with no heat capacity effects running in steady state with constant solar irradiation. Since this study concerns time varying radiation, which is zero at night, a time averaged value of efficiency will be used as defined by:η=∫t1t2m˙cp[Tout(t)-Tin]dt∫t1t2IApdt

This definition of performance is the time average of the

Built-in energy storage

In this design, as shown in Fig. 1, a phase change material (PCM) is placed below the absorber plate in direct contact with the absorber. This allows heat to be transferred directly from the absorber plate to the storage medium and then directly from the storage medium to the air when the sun is not shining. This eliminates the need for a separate apparatus and control systems for external storage as well as the associated heat losses. Additionally the low heat capacity working fluid is not

Mathematical model

A simple lumped parameter model was used by Summers et al. (2011) to describe the temperatures and heat fluxes in a solar collector operating at constant irradiation with no heat capacity and transient effects. Due to the inclusion of transient effects in this study, there are spatial and temporal temperature variations in the absorber plate, and, as a result, an irregularly shaped PCM melt front that varies along the length of the collector, as well as in the depth of the PCM. Therefore, more

Simulation results

A collector with the cross section in Fig. 1 was sized to produce air at 55–60 °C from inlet air at 30 °C. The collector length required to produce this temperature and store enough energy to produce heated air at night was obtained with a simple energy balance on the collector; equating the total solar energy absorbed in a day to the total energy used to heat the air for 24 h at a given flow rate. From this balance a collector length of 10 m was obtained for the simulation.

Experimental validation

The numerical model was validated using a scaled-down experimental version of the solar collector tested in Dhahran, Saudi Arabia. Due to the nature of the collector’s design and size and weight constraints for the testing site, the length of the experimental collector was reduced to 1 m from 10 m with the cross section remaining the same. Since this collector was reduced in length, and a minimum flow rate is required to achieve turbulent flow, the collector will not collect enough energy to heat

Conclusions

An air heating solar collector with integrated phase-change material storage has been designed and optimized. The computational results have been validated by experiment. A novel configuration of phase change material has been presented.

Built-in latent heat energy storage has shown great promise to maintain a consistent temperature output throughout the entire day and night. A finite element model is used to assess the performance of a built-in storage system with paraffin wax and an embedded

Acknowledgements

The authors thank Alexander Guerra for his help with the FEM software, P. Gandhidasan for helpful discussions of the solar design, and also M.K. Adham for his assistance in performing the experiments. The authors would also like to thank the King Fahd University of Petroleum and Minerals for funding the research reported in this paper through the Center for Clean Water and Clean Energy at MIT and KFUPM.

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