A review of chemical heat pumps, thermodynamic cycles and thermal energy storage technologies for low grade heat utilisation
Highlights
► The review of various thermal technologies for the utilisation of under exploited low grade heat. ► The analyses of the absorption and adsorption heat pumps possibly with performance enhancement additives. ► The analyses of thermal energy storage technologies (latent and sensible) for heat storage. ► The analyses of low temperature thermodynamic cycles to maximise power production.
Introduction
Since the onset of the ‘Great Recession’ [1] which began in December 2007, a decrease in ecologically friendly investments is generally envisaged, particularly after the withdrawal of Canada from the Kyoto treaty. However, a recent survey by Ernst & Young has reported that many firms, such as BHP and Rio Tinto, have actually increased their ecologically friendly investments [2] as the falling price of renewable energy and the rising price of crude oil have made energy efficiency and clean energy more attractive. This finding is in agreement with another survey of over 500 large companies conducted by Carbon Disclosure Project [3], which claimed that 59% of emissions-reducing investments made so far have shown a three-year payback period on average. These reports indicate that investments to improve energy efficiency can result in significant monetary savings.
A major cause of energy inefficiency is the generation of waste heat and the lack of waste heat utilisation, particularly low grade heat. The temperature range for low grade heat sources is typically between ambient temperature and 523 K [4], [5], and such low grade heat is especially abundant in industry as by-products. The market potential for surplus/waste heat from industrial processes in the UK is between 10 TWh and 40 TWh [6], [7], [8]. This represents a significant potential resource which has remained under-exploited to date, mainly because of the cost of obtaining useful exergy and energy out of low grade heat. Law et al. [9] claimed that the complete recovery of waste heat would be higher than the output of many of the renewable energy sources currently used in the UK, and that a large amount of low grade heat is available in the process industries as water from cooling towers with temperature between 308 K and 328 K, or as flue gas or vapours from stacks with a larger temperature range, between 303 K and 523 K. Opportunities to deploy low grade heat (up to 523 K) in industry are plenty as indicated in Fig. 1 which presents the range of heat supply required by industrial processes in producing different products within the EU-27. Although low grade heat generally exists in the form of waste heat from the process industries, other examples of low grade heat including renewable energy resources, e.g. solar, are available within the temperature range. It is worth mentioning that low grade heat technologies can be practically applied to other sectors such as automobile and HVAC (heating, ventilation and cooling) systems. Thus, the employment of low grade heat presented beyond the scope of the process industries should not be overlooked.
The scope of low grade heat utilisation is very broad (the scope of heat utilisation that encompasses a wider range of temperatures as shown in Fig. 1 is even broader). It covers heat exchangers, heat pipes, heat pumps, energy storage, heat recovery, process intensification and optimisation, etc. The authors are currently working on a technology roadmap on thermal energy management with an intention to cover the aforementioned technologies. The roadmap will be in much greater detail, presenting a more holistic view of the current state and the future direction of thermal related technologies across all temperature ranges. This paper will however focus on three distinct areas, i.e. thermal energy storage, chemical heat pumps (thermo-chemical energy conversion) and thermodynamic cycle (thermo-electrical energy conversion) in order to summarise and capture the spread of the challenge that lies ahead in low grade heat (<523 K) thermal energy management.
Section snippets
Chemical heat pump
A chemical heat pump in principle consists of a condenser, an evaporator and one reactor (with a generator) or two reactors (or adsorber/absorber), and is used to upgrade and store thermal energy, particularly low grade heat, via the reversible reaction between chemical substances without chemical consumption or production. Wongsuwan et al. [11] and Fadhel et al. [12] state that a chemical heat pump can either involve gas–liquid absorption process [13], [14] or solid–gas adsorption process.
Conclusions
Selected low grade heat technologies were reviewed in this paper, including: (i) chemical heat pumps, (ii) thermodynamic cycles, and (iii) thermal energy storage and corresponding media to balance demand on electricity, cooling and heating cycles. To summarise, the performance improvement of chemical heat pumps can be achieved by various approaches applicable to different cycles, i.e. (gas–solid) adsorption cycles and (liquid–gas) absorption cycles. A number of promising techniques and
Acknowledgements
This work is carried out in line with the research projects funded by the Research Council UK (RCUK) Energy Programme entitled PRO-TEM Network: Process Industry Thermal Energy Management Network (Project No. EP/G059284/1) and Thermal Management of Industrial Processes (Project No. EP/G056706/1). The authors would like to thank the RCUK for providing the funding to sustain the network which has significantly helped to promote the interest in sustainable solutions in order to create a low carbon
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