At present, the consumption of energy for heating purpose is approximately 45% to 47% in the UK, however over 80% of it is produced by burning fuel fossil (i.e. gas, solid fuel, oil). The heat use can be divided by three sectors, of which the domestic sector account for 57% of the heat use, industry and service sector are responsible for 24% and 15% respectively. The gas fuel is at the dominant position, which provides 80% of the total fuel for the domestic consumption of heat. These energy using situations indicate a big challenge to achieve the UK government target that 80% reduction of greenhouse gas (CO₂) emission by the end of 2050. (Eames et al., 2014)

The graph below indicates the dynamic change of the gas demand and electrical supply among a week. There are two peck demand periods (5-8am and 4-7pm) every day.

the gas demand is around four times higher than the electrical supply. Because the gas supply system is more flexible to provide heat to satisfy the demand by changing pressure to store extra energy in network, however the electrical grid has no inherent means of storing electricity within the delivery infrastructure itself (the electrical cable). (Wilson, Taylor and Rowley, 2018)

Figure 1 The gas demand and electical supply dynamic change curve

To reduce the greenhouse gas emissions and achieve the low carbon heating, it is necessary to reduce large amount of using of the gas, solid fuel and oil for heating purpose, and utilizing the renewable energy resource instead by using electrical heat pump. However, the maximum electrical grid load is quite limit for heat generation. The novel forms of thermal energy storage systems can play an important role to solve this challenge-help balance differences between heat generation and demand required. The thermal energy storages allow shifting the heat generation by electrical heat pump 2-3 hours earlier (the last time of charging depends on the thermal demand and the charging rate) than peak demand time, and store it, then release thermal energy at the peak demand time (Wilson, Taylor and Rowley, 2018). in addition, when the renewable power is sufficient, the storage can be charged by the electric heat pump, so that the renewable power generation and the low-carbon heat pump can be utilized on a large scale.

The figure below shows the seasonal variation in heating demand and electrical supply, the peak heating load occurs in February, which is much higher than the average heat load.

(Wilson, Taylor and Rowley, 2018)

Figure 2 The annual demend and supply curve

There are three types of thermal energy storage approaches- sensible heat storage, latent heat storage and thermochemical heat storage. The sensible heat storage is mature form of the thermal storage and currently have been commonly installed in different sector, but the volume required is significantly large to integrate into dwelling, the latter two can provide enough energy by given volume, but there are still under developing, further exploration is needed especially the thermochemical heat storage (Eames et al., 2014).

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My core research topic is： what the sizes of the selected storage systems need to be to prevent the operation of heat pump at time of peak demand, and which of them are most suitable? My literature review below force on the research about water storage, which is the most commonly used among sensible heat storage and different types of the PCMs.

2 Literature review

2.1 Advantages of using the thermal energy storage systems

better economics: reducing capital and operational costs
better efficiency: achieving a more efficient use of energy
less pollution of the environment and less CO2 emissions
better system performance and reliability.

2.2 Thermal Energy storage approaches

Figure 3 the thermal energy storage approaches

Table 1 Typical parameters of TES systems

2.2.1 Sensible Heat Thermal Energy Storage

Sensible heat storage devices store thermal energy by heating or cooling the temperature of the storage material through heat transfer. The media of the storage can be liquid like water, oil; and solid such as brick, concrete; and gas, like air etc. Generally, the available space for storage and the heat capacity should be considered for determine the material. The total heat stored equation:

$Q = ρV C_{P} (T_{2} - T_{1})$

where Q is the amount of heat stored in the material (J), m= V is the mas of storage material (kg), cp is the specific heat of the storage material (J/kg·K), and DT=T₂-T₁is the temperature change (K) (Cabeza, 2015).

The equation of quantity of charged / discharged to a sensible heat storage system (considering the case of spherical store):

$\frac{\frac{4}{3} π r^{3} ρ C_{p} (T_{2} - T_{1})}{4 π r^{2} U (T_{2} - T_{1})} = \frac{rρ C_{P}}{3 U}$

Where U is the heat loss coefficient.

This equation explains the reason for larger stores can achieve low value of percentage heat loss and can effectively store heat over long period of time (Eames et al., 2014).

Among all potential media for sensible heat storage, water is used most commonly, Due to its high heat capacity (4.2 kJ/kg · K), low cost and abundance, it is often used in storage devices over the temperature range of 20-70 ℃. Also, as a liquid storage medium with high convective heat transfer, water allows the storage device to have higher heat injection and extraction rates compared to other solid heat storage media (Avghad, Keche and Kousal, 2016). However, sensible heat storages are not useful for long-term or automobile applications because:

Low energy storage density (~100 kJ/kg),
Heavy insulation required to minimize heat loss
Require large space.

2.2.2 Latent Heat Thermal Energy Storage

Latent heat storage relies on the phase change enthalpy of material to store heat within a narrow temperature range. The most common change of phase applied is the change from solid to liquid is the most common change of phase applied but using the change of phase from liquid to gas can also be feasible. The choice of material used within a PCM heat store is dependent on the required temperature for the TES. The energy stored in phase change material equation:

$Q = m C_{p 1} (T_{pc} - T_{1}) + m h_{pc} + m C_{p 2} (T_{2} - T_{pc})$

Where Q is the energy stored above the datum temperature T₁, m is the mass of phase change material, C_p1 the specific heat capacity of the phase change material when solid, C_p2 the specific heat capacity of the phase change material when liquid, T₁ the datum temperature, T₂the final temperature, T_pc is the phase change temperature, h_pc is the enthalpy of phase change/heat of fusion (Eames et al., 2014).

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The range of potential PCMs that can be used is very wide and PCM applications range from freezing to high temperature storage. PCMs can be divided into three categories based on their malting point – Low temperature, Medium temperature and high temperature, but their widely classified based on their physical transformation for heat absorbing and desorbing capabilities (Sarbu and Sebarchievici, 2018).

Figure 4 Classification of phase-change materials

Figure 5 Categories based on their malting point

2.2.2.1 Organic PCMs

Organic compounds characterized by having carbon atoms in their structure, which have a wide range of melting points, higher safety, high latent heat and negligible super-cooling, but are costly and have low energy density, suffer from flammability and extremely low thermal conductivity (from 0.1 to 0.7 W/m K), Therefore, it is necessary to have mechanisms to strength their heat transfer in order to achieve reasonable rates of heat output (Department for Business, Energy & Industrial Strategy, 2016).

Paraffin waxes is a type of Organic PCM which can release a large amount of latent heat during the crystallization of the (CH3)– chain. And as its chain length increasing, the melting point and latent heat of fusion will increase. Because of cost consideration, however, only technical grade paraffin’s may be used as PCMs in latent heat storage systems. Paraffin is safe, reliable, predictable, comparatively less expensive, non-corrosive, and available in a large temperature range (5–80 ◦C) (Sarbu and Sebarchievici, 2018).

2.2.2.2 Inorganic PCMs

Inorganic compounds are mainly salt hydrates or molten salts with the advantages of lower costs, high latent heat, high specific density and a melting point range from about 5-120℃. The disadvantages are that for long term stability these must be sealed to prevent water loss, chemical decomposition and corrosion of casing materials (Department for Business, Energy & Industrial Strategy, 2016).

2.2.2.3 Current and future technological potential and development

Currently, PCMs is only in early commercialization phase with only few products in market. From a technical point of view, it is prospective to develop of PCM based heat stores. At the present, the PCMs are likely to be used only when space is limited for using sensible heat storages. Research and development are focused on integrating PCM storages into heating systems and buildings internal temperature control (Department for Business, Energy & Industrial Strategy, 2016).

The constraints to the wider development and install of PCM storages are relatively high costs and applications is suitable for where a wider temperature range is required. This is a key factor because the benefits of PCM can be achieved when the temperature range of the desired application is close to the temperature of the material change phase. However, a wider temperature range of operation is allowing sensible heat storage approaches provide a stronger financial proposition. This also indicates why PCM is generally not considered for using in district heating (Department for Business, Energy & Industrial Strategy, 2016).

Figure 6 A comparison of the stored energy with temperature for a PCM and water

It is likely the cost of PCMs will decrease in the future, even in next 5 year. The follow reasons will drive the cost down.

 Further materials research driving down costs

 Increased experience and improved manufacturing techniques driven by production volume driving down costs

 Improved system design and component integration (e.g. heat exchangers)

There are many areas of technology development and exploration of applications for PCM. (Department for Business, Energy & Industrial Strategy, 2016).

In the future, the PCMs can be developed in many technology aspects and application of it can be further explored. For example:

Integration of PCM into small and larger hot water tanks for improved performance.
Integration into building materials – such as integration within walls (plasterboard) and building materials(concrete) improving the thermal mass of buildings and flattening heat consumption through this. The Knauf’s ‘comforboard’ (using two plasterboards filled with PCM) experiment indicates this new application can reduce the consumption of energy of heating systems in building, and also saving the cost. (Department for Business, Energy & Industrial Strategy, 2016).

2.2.3 Thermochemical Heat Storages

Thermochemical energy storages use reversible chemical reaction to store and release energy. The heat is stored during of dissociation reaction and released during exothermic process of the reversible chemical reaction. Thermochemical energy storage has not been commercialized yet, but its advantages have attracted great attention to develop it (Avghad, Keche and Kousal, 2016).

Figure 7Schematic Diagram illustrating the Thermochemical Heat Storage Process

No or low heat losses.

High energy storage density (~2MJ/kg)
Long-term storage period.
Long distance transport possibility.
The required space is small.

2.2.4 Comparison of TES types

Figure 8Storage capacities of PCM and TCM compare to water

Figure 9 Comparison of different storage technologies for solar space heating and hot water production application

2.3 Thermal energy storage modelling method

case study: Potential for Thermal Energy Storage in UK Housing Stock

A paper from the UK energy research center presents a case study, analyzing of the thermal loads for a domestic family dwelling in Derby and assuming it met the Building code for the 1980s, 1990s and 2010s. In order to obey with each group of building regulations, the thermal energy storage capacity required to shift the peak demand of the day and night winter space heating for 3 hours was analyzed, and the storage size was calculated based on the hot water storage and the phase change material storage. The result of this case study shows that to satisfied 3 hours demand, the size of the Phase change material storage required is much smaller than the water storage, the PCM storage is likely to become a feasible technology in next few years.

This case study is a good example of modeling the short-term thermal energy storage systems, only the daily consumption of energy is considered. It is one-dimensional steady- state analysis, the temperature difference between internal and externa of the building is determined, therefore the design heat loss rate here is constant. And the required thermal demand is determined by using the maximum daily electrical energy consumption appears during the period of January-February. In order to make a dynamic model of thermal energy storage, more variable design parameters need to be considered for more accurate result. For example, the heat loss of the storage will be changed by the dynamic temperature difference. Meanwhile, the charging and discharging rates need to be think about, which affect the lasting of charge time. The detail of setting up dynamic model of thermal storage systems need further study.

3 Conclusion

The water storage currently is one of the most common and mature form of the sensible storages because of its low cost, abundance and relatively high heat capacity. However, its huge volume requirement constraint it to be integrated in building.
The latent heat storage in PCMs have much higher storage density than the water tank storage, which is likely to become a feasible technology in next few years.
Comparing with sensible heat storage, although PCMs are able to store much more heat within a small required size, there are also some barriers for widely apply the PCMs due to relatively high costs, extremely low conductivity. The further research on it is necessary. The cost issue needs to be considered for assessing feasibility of the storages

My current reading is almost about fundamental knowledge of different kinds of storage, their advantages and disadvantages, and the simple equation of total heat stored. However, for modelling more parameters (i.e. Charging and discharging rate) need to think about and do more research to get relative data. And more case study about modelling I need to read for getting inspirations of model setting up. For getting size of water tank storages, the temperature effects may need to consider, it will influent density of water and heat capacity and other parameters (research the more complex equation). How to assess the feasibility of the storage systems need to do further research. If time available, the research for dynamic demand model of building can think about. So that the storage size for different sector can be work out.

4 References

Eames, P., Loveday, D., Haines, V. and Romanos, P. (2014). The Future Role of Thermal Energy Storage in the UK Energy System: An assessment of the Technical Feasibility and Factors Influencing Adoption. [online] London: UKERC, pp.1-49. Available at: http://www.ukerc.ac.uk [Accessed 13 Dec. 2018].
Wilson, G., Taylor, R. and Rowley, P. (2018). Challenges for the decarbonisation of heat: local gas demand vs electricity supply Winter 2017/2018. [online] London: UKERC, pp.1-4. Available at: http://www.ukerc.ac.uk [Accessed 13 Dec. 2018].
Cabeza, Luisa F.. (2015). Advances in Thermal Energy Storage Systems – Methods and Applications. (pp. 1-45). Elsevier. Retrieved from
https://app.knovel.com/hotlink/toc/id:kpATESSMA1/advances-in-thermal-energy/advances-in-thermal-energy
Avghad, S., Keche, A. and Kousal, A. (2016). Thermal Energy Storage: A Review. IOSR, [online] 13(3 Ver. II (May- Jun. 2016), pp.72-77. Available at: http://www.iosrjournals.org [Accessed 14 Dec. 2018].
Department for Business, Energy & Industrial Strategy (2016). Evidence Gathering: Thermal Energy Storage (TES) Technologies. London, pp.6-86.
Sarbu, I. and Sebarchievici, C. (2018). A Comprehensive Review of Thermal Energy Storage. [online] Available at: https://www.researchgate.net/publication/322757998_A_Comprehensive_Review_of_Thermal_Energy_Storage [Accessed 14 Dec. 2018].