A review of available technologies for seasonal thermal energy storage
Introduction
Rapid worldwide population growth has put a heavy burden on conventional energy resources, such as fuel, coal and oil, which are estimated to run out in several decades. These conventional resources are also blamed for CO2 and other harmful gas emissions that lead to climatic change problems, for example, global warming and the deterioration of the ozone layer. All of these severe consequences cause people to begin to reconsider what acceptable and sustainable development patterns are.
In recent years, considerable progress in renewable energy development has made new energy resources quite competitive with conventional energy in terms of both efficiency and reliability. Solar energy, as a pollution-free, inexhaustible and affordable energy resource, has received extensive study and numerous applications throughout the world. However, one of the longstanding barriers to solar energy technology lies in the noticeable misalignment between energy supply and consumption. Therefore, the energy storage concept is proposed as an essential way to address the mismatch.
The idea of thermal energy storage (TES) was first mentioned and investigated to address the energy shortage crisis in the 1970s. By means of energy storage, intermittent solar energy is able to not only meet the demands of space heating and domestic water supply but also to offer a high grade heat source all year round regardless of timing or seasonal constraints.
Energy storage can be classified into short-term storage and long-term storage according to different storage durations. Using excess heat collected in the summer to compensate for the heat supply insufficiency during the wintertime is the concept of seasonal thermal energy storage (STES), also called long-term heat storage.
Fisch et al. (1998) summarised thirteen existing and fourteen planned large-scale solar heating systems in Europe with different storage applications of short-term (diurnal) and long-term (seasonal) storage and compared the cost-benefit-ratios of them. The results showed that the pattern of seasonal storage could satisfy 50–70% of the annual heat demand, whereas the diurnal pattern could only meet 10–20%. The investigation indicated that seasonal storage was more capable of conserving energy and reducing fossil fuel consumption for the environment’s sake.
Although seasonal storage has greater potential in practical applications, it is more technologically challenging than short-term storage. It requires large storage volumes and has greater risks of heat losses, and the material chosen for implementation must be economical, reliable and ecological.
There are three different mechanisms for energy storage: sensible heat storage, latent heat storage and chemical reaction/thermo-chemical heat storage. Over recent decades, related studies addressing diverse applications and storage mechanisms have been carried out throughout the world (Dincer and Rosen, 2002). The concept of seasonal energy storage is not only realised in district heating (Schmidt et al., 2003) but also in greenhouses for space heating (Alkilani et al., 2011, Sethi and Sharma, 2008) because heating for the plants during winter nights consumes a large portion of heat input in agricultural greenhouses Bricault, 1982, Kavin and Kurtan, 1987, Santamouris et al., 1994. Many international collaborative efforts have been performed on this interesting topic, and some of them have yielded remarkable achievements. For instance, IEA-SHC Task 32 (The Solar Heating and Cooling Programme of the International Energy Agency) and Solarthermie 2000 are two such programs that focus on the topic of advanced storage concepts. In the framework of Task 32, chemical and sorption, phase change materials and water tank storage technologies were studied and discussed. The cooperative research efforts of IEA–ECES (International Energy Agency: Energy Conservation through Energy Storage) developed the storage concept of underground thermal energy storage (UTES) and investigated phase change materials for energy storage. For UTES, previous study results have already been successfully transformed into large-scale applications through the international collaborative efforts of IEA–ECES. Solarthermie 2000 is a German program in which pilot and demonstration projects of seasonal heat storage for district heating have been realised. All the large-scale plants in Solarthermie 2000 adopted the concept of sensible heat storage.
Sensible heat storage comprises water tank storage (Novo et al., 2010) and UTES (Bakema et al., 1995, Givoni, 1977, Reuss et al., 1997). The major methods employed for UTES include aquifer storage and underground soil storage, which will be elaborated in detail in Section 2. Latent heat storage stores heat nearly isothermally in phase change materials and can provide higher energy density than the sensible storage. Chemical heat storage is a newly studied technology that permits more compact storage through greater energy storage densities without heat losses, and it is mainly classified as sorption and chemical reaction storage.
Distinct from previous reviews on a particular technology or material comparison (Alkilani et al., 2011, Farid et al., 2004, Hasnain, 1998, Jegadheeswaran et al., 2010, Kenisarin and Mahkamov, 2007, Khalifa and Abbas, 2009, Novo et al., 2010, N’Tsoukpoe et al., 2009, Pinel et al., 2011, Shukla et al., 2009, Zhou et al., 2012), this article focuses on thermal energy storage on a seasonal scale and covers all three seasonal storage technologies. The aim of this paper is to provide a state-of-the-art review based on the theoretical and experimental research. The latest studies and projects are presented and analysed.
Section snippets
Sensible heat storage
The sensible heat storage method converts collected solar energy into sensible heat in selected materials and retrieves it when heat is required. The stored heat amount is determined by the specific heat of the material and its temperature increase. Sensible heat storage is considered to be a simple, low-cost and relatively mature technology for seasonal energy storage compared to the other alternatives. Due to its feature of inexpensive and reliable, it has been implemented in a significant
Materials research on PCMs
Latent heat storage (LTS) can offer higher energy densities than sensible storage and is considered to be an efficient energy-storing option. Phase change materials (PCMs) are used in this storage type, and a classification of the materials is listed in Fig. 11.
PCMs undergo phase changing processes by absorbing and releasing heat in the form of latent heat of fusion without the temperature changing in each period. The phase changing temperatures of PCMs differ across a wide range, making them
Chemical storage
Chemical storage has distinctive advantages of high energy storage and low heat losses over other storage technologies and is regarded as the most promising alternative. The storage volume for 34 m3 of water equivalent (70 °C temperature increase) is only 1 m3 by means of chemical storage (Hadorn, 2008), and Fig. 15 illustrates a comparison of the energy densities among high energy storage methods. Another attractive feature of chemical storage lies in its capability to conserve energy at ambient
Comparison of available options and prospects
From the storage mechanism aspect, sensible and latent heat storage are direct ways to store heat, whereas chemical storage is indirect by taking advantage of the endothermic and exothermic reaction processes between a pair of substances.
Table 10 provides a summary of the three storage concepts, including a comparison of each one’s advantages and disadvantages. Based on the knowledge of the reviewed studies and relevant project performance, the author also suggests work to be concentrated on in
Conclusions
The concept of seasonal/long-term heat storage presents great opportunities for making the utmost use of solar energy. Stored “excess” heat can compensate for the heat shortage when necessary. Seasonal storage offers the possibility that solar energy can cover all the heating loads without an extra heating system. As a result, there will be less and less dependency on fossil fuel resources, and the world’s carbon footprint will surely be reduced in the near future.
Three different technologies
Acknowledgements
This work is supported by the Key project of the Natural Science Foundation of China for international academic exchanges under the contract No. 51020105010 and the Shanghai Commission of Science and Technology under the contract No. 10dz1203402. The support from The Ministry of education innovation team (IRT 1159) is also appreciated.
References (116)
- et al.
A review of materials, heat transfer and phase change problem formulation for latent heat thermal energy storage systems (LHTESS)
Renew. Sustain. Energy Rev.
(2010) - et al.
Review of solar air collectors with thermal storage units
Renew. Sustain. Energy Rev.
(2011) - et al.
Heat recovery from a thermal energy storage based on the Ca(OH)2/CaO cycle
Appl. Therm. Eng.
(2003) - et al.
Modeling of thermochemical energy storage by salt hydrates
Int. J. Heat Mass Transf.
(2010) - et al.
German central solar heating plants with seasonal heat storage
Sol. Energy
(2010) - et al.
Performance analysis of a latent heat storage system with phase change material for new designed solar collectors in greenhouse heating
Sol. Energy
(2009) Energetic performance analysis of a ground-source heat pump system with latent heat storage for a greenhouse heating
Energy Convers. Manage.
(2011)- et al.
Thermal performance of a greenhouse with a phase change material north wall
Energy Build.
(2011) - et al.
Recent experience with large solar thermal systems in The Netherlands
Sol. Energy
(2001) - et al.
Materials used as PCM in thermal energy storage in buildings: a review
Renew. Sustain. Energy Rev.
(2011)