Fossil fuel plays a major role in the society, and they are commonly used for the heat and power production for the commercial and residential applications. According to UKERC Research Report, heat accounts for around 47% of the total energy consumption in the UK and 33% of carbon emissions. Approximately, 80% of this heat account for space and water heating. At the same time, a significant amount of heat is being used by the industry sector 24%, for industrial processes. The corporation of thermal energy storage system is very beneficial for CO2 emission reduction and energy saving. And, it is the key technology to balancing the energy system stable with the increasing capacity from wind.
This research report uses the concept of Wind-Powered Thermal Energy System (WTES) recently proposed by (2015, Okazaki et al) with Latent-Thermal Energy Storage (TES) design to produce stable heat for industrial application and also, looked at the economics of h-WTES (Direct heat production) and electrical-WTES (Indirect heat Production using electrical heater) with thermal energy storage for heat generation and compared the results with the traditional gas boiler.
This paper demonstrates the potential and probable costs of different WTES with thermal energy storage and a design of tube-in-tank phase change thermal storage system by using a previously developed an effectiveness-NTU method potentially applicable for the industrial sector.
From the results, it was identified that it is more economically feasible to generate heat from h-WTES (Direct heat production) with thermal storage than e-WTES (indirect heat production). 56.2 £/MWh-t and 63.36 £/MWh-t, the levelized cost of heat for h-WTES and e-WTES respectively. Also, the result suggested that the renewable technologies with TES present the most expensive LCOE for heat production compared to the industrial gas boiler which has the LCOE of 15-20 £/MWh.
The result from the second part of the research shows that the technique effectiveness-NTU can accurately predict the average effectiveness of the system (performance) with high heat transfer rate for discharging and charging processes (115kW). Therefore, the design can effectively be integrated into WTES to provide a significant amount of heat demand for industrial applications. It was also, found that latent-TES can hold up to 6.5 times more heat than sensible-TES.
Table of Contents
Abstract
1.0 Introduction
1.1 Research overview
1.2 Research Aims and Objectives
1.3 Project Description
1.4 Review of the literature
2.0 Levelised Cost of Energy
2.1 Methodology
2.2 Results and Discussion
2.21 Estimated energy cost
3.0 An effectiveness-NTU technique
3.1 Description of the Latent-Thermal Energy Storage system
4.2 Mathematical Model
4.3 Results and Discussion
5.0 Conclusion
6.0 References
Appendices
List of Figures
Figure 1: Heat Use by Sector, 2015, UK. (Eames et al., 2014)
Figure 2: Energy consumption by End Use, 2015. (Eames et al., 2014)
Figure 3: tank storage system. (Herrmann, Kelly and Price, 2004)
Figure 4:The basic configuration of Wind Powered thermal Energy System (WTES) with PCM-Thermal storage for industrial processes
Figure 5: Heat wind turbine with thermal energy storage
Figure 6: Electrical Wind Turbine with thermal energy storage to meet heat demand
Figure 7:This shows the LCOE for h-wind turbine with storage for 2020 and 2025
Figure 8:This shows the LCOE for e-wind turbine with storage for 2020 and 2025
Figure 9:Schematic of one tube tank
Figure 10:a Simple schematic unit of PCM-TES system Right) Single heat transfer tube and left) unit cross-section and all the dimensions are present in cm. (Colella, Sciacovelli and Verda, 2012)
Figure 11:a) simplified model of the storage unit. b) thermal circuit
Figure 12:Graph shows the average effectiveness vs mass flowrate of two different HTF water and mineral-oil for Latent/PCM-Thermal Energy Storage Device. Obtained using the ɛ-NTU technique as shown above
Figure 13:Graph shows the average outlet temperature 0C vs mass flowrate of two different HTF water and mineral-oil for Latent/PCM-Thermal Energy Storage Device. Obtained using the ɛ-NTU technique as shown above
Figure 14: Shows the Stable output of the thermal energy (PCM)
Figure 15:Comparison between the PCM storage vs the Sensible Storage tank
List of Tables
Table 1: Phase transformation material comparison
Table 2: Capital costs and Operating & Variable costs from literature. (NICOLÁS NITTO, 2017)
Table 3:This shows the net present value calculated using the above equation for e-wind turbine and h-wind turbine with thermal energy storage
Table 4:shows the LCOE costs for h-wind turbine with thermal energy storage
Table 5:shows the LCOE costs for e-wind turbine with thermal energy storage
Table 6:PCM properties
Table 7:HTF properties (Therminol.com, 2017)
Table 8:Results obtained from the effectiveness-NTU technique
Table 9:further Results obtained from the effectiveness-NTU technique
Table 10: shows the temperature range of industrial sector
Table 11:Sensible heat storage specifications
Abstract
With the increasing world population growth, energy supply can face many challenges in future regarding a sustainable dispatch and in mitigating climate change.
Fossil fuel plays a major role in the society, and they are commonly used for the heat and power production for the commercial and residential applications. According to UKERC Research Report, heat accounts for around 47% of the total energy consumption in the UK and 33% of carbon emissions. Approximately, 80% of this heat account for space and water heating. At the same time, a significant amount of heat is being used by the industry sector 24%, for industrial processes. The corporation of thermal energy storage system is very beneficial for CO2 emission reduction and energy saving. And, it is the key technology to balancing the energy system stable with the increasing capacity from wind.
This research report uses the concept of Wind-Powered Thermal Energy System (WTES) recently proposed by (2015, Okazaki et al) with Latent-Thermal Energy Storage (TES) design to produce stable heat for industrial application and also, looked at the economics of h-WTES (Direct heat production) and electrical-WTES (Indirect heat Production using electrical heater) with thermal energy storage for heat generation and compared the results with the traditional gas boiler.
This paper demonstrates the potential and probable costs of different WTES with thermal energy storage and a design of tube-in-tank phase change thermal storage system by using a previously developed an effectiveness-NTU method potentially applicable for the industrial sector.
From the results, it was identified that it is more economically feasible to generate heat from h-WTES (Direct heat production) with thermal storage than e-WTES (indirect heat production). 56.2 £/MWh-t and 63.36 £/MWh-t, the levelized cost of heat for h-WTES and e-WTES respectively. Also, the result suggested that the renewable technologies with TES present the most expensive LCOE for heat production compared to the industrial gas boiler which has the LCOE of 15-20 £/MWh.
The result from the second part of the research shows that the technique effectiveness-NTU can accurately predict the average effectiveness of the system (performance) with high heat transfer rate for discharging and charging processes (115kW). Therefore, the design can effectively be integrated into WTES to provide a significant amount of heat demand for industrial applications. It was also, found that latent-TES can hold up to 6.5 times more heat than sensible-TES. Finally, it was identified that by having a 2MW-WTES with latent-TES for the food industry, can reduce 625 tons of CO2 emission per year from burning fuel such as natural gas.
1.0 Introduction
1.1 Research overview
Fossil fuel plays an important role in the society, and they are commonly used for the heat and power production for the commercial and residential applications (Li and Zheng, 2016). According to UKERC Research Report, heat accounts for around 47% of the total energy consumption in the UK and 33% of carbon emissions. Approximately, 80% of this heat account for space and water heating. At the same time, a large amount of heat is being used by the industry sector 24%, for industrial processes (Eames et al., 2014). To achieve the UK target of 80% reduction in carbon emission by 2050, a significant drive towards decarbonise heat production is required. The Scottish Government have introduced a ‘Heat Hierarchy’ to meet the emission target which includes: reducing heat demand, supplying heat efficiently at least costs to consumers and using low carbon/renewable sources (Radcliffe, 2015).
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In recent years, there has been a significant drive towards renewable technologies such as solar, wind power, biomass, hydro and also nuclear energy. These can lower the carbon emission and allow UK to achieve their European target of 20% energy from the renewable by 2020 (EU, 2008). However, most of the renewable energy sources are intermittent and can’t be used as the baseload energy source because of this, thermal energy storage (TES) has become a crucial technology for energy crisis and, playing an important role for renewable energy applications as it makes the operation more efficient (Tian and Zhao, 2011). In brief, TES system stocks the thermal energy by cooling or heating a storage substance for later usage.
Integration of TES with renewable sources can balance the difference between the global energy demand and supply in various applications. This has attracted the industry and scientific community, and it is seen as one of the innovative solutions for energy management. In general, TES is the warehouse between production and consumption of energy and this allows. (Radcliffe and Li, 2015)
Energy which is generated by renewables at times when demand is low, to be provided as heat or electricity at peak times.
Thermal Energy Storage (TES) Technologies
TES system stocks the thermal energy by cooling or heating a storage substance so that the energy can be used later for power generation or heating and cooling applications. TES can be classified as sensible heat storage, latent heat storage and thermochemical storage. (IRENA, 2013)
Sensible Heat Storage System
In this system, the energy is stored/removed by lowering/raising the storage medium temperature in a solid or liquid with no phase change or chemical reaction taking place. The quantity of energy stored in the medium depends upon the amount of the storage substance (m), specific heat capacity of the material (Cp) and the temperature difference (dT) (Liu et al., 2016). A substance with high C p can store a large amount of heat for a given temperature change. Storage materials include water, granite, concrete, oil, molten salt, rock, etc. (Eames et al., 2014). The main advantages of this system are its simplicity and low cost (cost for sensible heat storage ranges between 0.1-10 €/kWh). Disadvantages are low energy density (3-5 times less than PCM and require large volumes) and heat losses.
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Latent Heat Storage Systems
Latent heat storage systems, store and release thermal energy through the state change process of the material which is called phase change materials (PCM). PCMs have an advantage over the sensible heat storage materials as they have high energy storage density at a constant temperature (Liu and Rao, 2017). Due to the phase change, the PCM is typically different from HTF, where PCMs are encapsulated in containers with HTF flowing over them or by using a heat exchanger, inserted into a store full of PCM materials (Eames et al., 2014). The advantages of PCM are larger energy densities, higher efficiencies and constant discharging temperature. The disadvantages of PCM are low thermal conductivity and requires a large heat exchange area. Other drawbacks for PCMs are expensive and corrosive (Sharma et al., 2009). These can be designed for long term energy storage, applicable for industrial application. Storage cost range between 10-50 €/kWh.
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Thermochemical storage
A full review/summary of thermal energy storage with the various application is present in the appendix.
1.2 Research Aims and Objectives
The purpose of this research project is to use the idea of the Wind Powered thermal energy system (WTES) proposed by (Matsuo and Okazaki, 2017) for generating electricity, to directly use the thermal energy (generated) to supply heat demand in the industrial sectors such as food and paper industries which require heating of a fluid stream (hot water, hot air streams) and heating of some reservoir. Currently, heating systems in the industries are based on heating of a fluid/air streams from a boiler, which uses a significant amount of fossil fuels like coal, oil and gas. According to (Eames et al., 2014), UK is considered to be the windiest country in the Europe and could power itself several times over using wind and a 2MWth of wind power energy system is considered to be enough to supply the average heat demand of an industrial plant (Gov.uk, 2017). The limiting factor for the wind energy is the non-continuous supply of energy. Therefore, accurate design and sizing of the thermal storage system are required, taking into consideration the particular demand profile.
Overall, there are various studies which have investigated the integration of Thermal Energy Storage (TES) with technologies such as CSP, CHP, wind power, etc., for the generation of electricity. However, there has been an insufficient investigation on TES integration with renewable technologies to provide decarbonised heat for industries. Taking the idea from (Matsuo and Okazaki, 2017) and (Okazaki, Shirai and Nakamura, 2015) this research will investigate the integration of PCM-Thermal Energy Storage (high energy storage density at a constant temperature) into Wind-Powered Thermal Energy System (WTES) for industrial application to provide decarbonised heat. Which has not been discussed by the author yet. This research consists of two different parts:
1. The potential and probable costs of different Wind thermal energy system with thermal energy storage and comparing it with the traditional heating source to provide useful heat.
2. The designing of the latent-TES using the effectiveness-NTU modelling suitable for industrial application. And, comparing the results with the sensible -TES.
1.3 Project Description
Overview of WTES with Thermal storage
A WTES with thermal storage system can convert wind power efficiently into thermal energy which could be stored for low cost and stable heat generation. Integration of WTES with thermal storage into industrial application consists of low-cost heat generator employing inductive heating technology (electromagnetic induction), a PCM thermal storage system for stabilisation and a boiler for further heating. According to (Matsuo and Okazaki, 2017), this is an economical (the cost is 10% lower) solution than the combination of thermal plant and wind power.
The novel configuration of the process is shown in the figure below Fig 4. The heat generator employing inductive heating technology converts the rotating energy to the thermal energy directly at the top of the tower without the use of fossil fuel. The heat energy produced by WTES is indirectly stored in the PCM storage system with sCO2 (20MPa) as an HTF, because sCO2 is chemically stable, low-cost, reliable and can work under high temperature making it a desirable candidate. Further, the stored heat is transferred in the Cold-Mineral Oil (As it provides reliable heat transfer and has the performance benefits of low fouling, high thermal stability, practically non-toxic and environmentally friendly) coming from the industrial process as an HTF which is used for drying, washing, sterilising, bleaching, etc. The fossil fuel boiler is used when the plants heating demand is not meet. The minimum inlet temperature of 1400 C was considered for mineral-oil, and the design inlet temperature into the industrial process was defined as 1750 C, which is the average temperature of the food and paper industry.
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1.4 Review of the literature
Overall, there are various studies which have investigated the integration of Thermal Energy Storage (TES) with technologies such as CSP, CHP, wind power, etc., for the generation of electricity (IEA-ETSAP and IRENA, 2015) (Liu et al., 2016) (Li and Zheng, 2016). However, there are not many studies on TES integration with Wind-Powered Thermal Energy system (WTES) which is recently proposed by Okazaki et al. (directly converts wind energy to heat), and their economic and technical feasibility is also unknown compared with other conventional wind systems.
Direct conversion of the wind to heat using a Joule Machine was developed by (Chakirov and Vagapov, 2011) which converts the wind energy directly into cost-effective thermal energy using heat generator driven by a wind turbine for domestic users. Further, (Meibom et al., 2007) this study analysed the economic value of using heat pumps and electrical heat boiler integrated into wind power for heat production. Moreover, (Hedegaard and Münster, 2013) showed the significance of integrating individual heat pump and hot water tank as storage system into wind power. There are also few patents presents on the concept of WTES. (Lee; Jean L, 2012), patented a novel wind turbine WTES concept to produce electricity from heat. Recently, (Matsuo and Okazaki, 2017) (Okazaki, Shirai and Nakamura, 2015) proposed a WTES system with sensible heat storage (not in detail) which directly converts wind energy into heat for electricity generation.
Additionally, (Colella, Sciacovelli and Verda, 2012) (Liu et al., 2015) (Belusko et al., 2016) these studies show the investigation/design of tube-in-tank latent thermal storage for CSP, solar tower plants, CHP and district heating applications by the use of effectiveness-NTU numerical modelling technique.
All of these studies have shown that the concept of WTES with latent thermal energy storage to produce heat for industrial applications has not been studied yet and also the economics of WTES and e-WTES (Wind Turbine with electrical Heater) with thermal energy storage for heat generation is unknown.
2.0 Levelised Cost of Energy
Wind thermal energy system (WTES) with thermal energy storage for heat generation has not yet been studied in depth, and their economic and technical feasibility is unknown compared with other conventional wind systems. Since there is very little research on economic benefits, this part of the thesis will include the potential and probable costs of different Wind thermal energy system with thermal energy storage and comparing it with the traditional heating source to provide useful heat at 1750 C.
The heat can be produced in two different ways indirectly or directly. The indirect generation typically requires an electrical generator to convert rotating energy into electrical power and utilises this to generate heat Fig 6. On the other hand, direct generation produces thermal energy without any intermediate like electric generators Fig 5. The full configuration of both systems is shown below in the diagram.
Direct Heat Production (WTES)
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Indirect Heat Production
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2.1 Methodology
This section describes the economical parameter which has been used to fulfil the economic assessment of two different WTES concepts. The methodology which has been used to assess these technologies are the Levelized Cost of Energy (LCOE). This method allows different renewable energy technologies to be compared and it is widely used for policy and modelling development.
Levelized Cost of Energy
The LCOE method which has been used in this research follows the procedure described by (gov.uk, 2016). LCOE for a certain technology is the ratio of the total costs of that particular technology which includes the operating and capital costs, to the total amount of energy which is expected to be produced over the lifetime of the plant. Expressed in net present value term and converted into an equivalent unit of cost of generation in £/MWh (gov.uk, 2016). The main benefit of this method is its transparency and simplicity.
This approach does not consider wider costs such as network investment, the full cost of system balancing. And also, does not recognise the revenues from the sale of energy or other sources. The formula and the assumptions which are used for calculating the levelized cost of energy is described below.
The capital costs include: Pre-development costs, Construction costs. The operating expenses include: Fixed opex, Variable opex, insurance and connection costs. The other data required: the capacity of the plant (5MW) and a life time of the plant (20 years). The data is given below taken from (IEA-IRENA, 2015), which also shows that h-WTES can save 12% on construction and 50% on fixed opex.
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2.2 Results and Discussion
This current section shows the results obtain from the economic assessment of the wind heat & electrical turbine with thermal energy storage. It is composed of two parts: firstly, the LCOE results from 2017 data and second part shows the LCOE for 2020 and 2025.
General Results - LCOE Estimation
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Table 3: This shows the net present value calculated using the above equation for e-wind turbine and h-wind turbine with thermal energy storage.
LCOE results for 2020 and 2025
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Table 4: shows the LCOE costs for h-wind turbine with thermal energy storage e-wind turbine
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Table 5: shows the LCOE costs for e-wind turbine with thermal energy storage
LCOE for h-Wind Turbine with Thermal Energy Storage
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Figure 7: This illustrates the LCOE for h-wind turbine with storage for 2020 and 2025.
LCOE for e-Wind Turbine with thermal energy storage
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Figure 8: This illustrates the LCOE for e-wind turbine with storage for 2020 and 2025.
2.21 Estimated energy cost
The estimated energy cost for the electrical-wind turbine (e-WTES) with storage & electrical heater and heat-wind turbine (h-WTES) with storage is shown in Fig 7 and 8. The result indicates that the LCOEHEAT of direct heat production h-WTES is 12% lower than indirect heat production e-WTES, with exact same thermal energy cost. The reason for this is that e-WTES employ electrical driven heater (100% efficiency) to convert the electricity into heat which increases the cost of e-WTES and further, according to (IEA-ETSAP and IRENA, 2015) h-WTES can save around 12% compared to a traditional wind turbine on construction which decreases the cost of h-WTES. Therefore, it can be concluded that it is more economically feasible to generate heat from h-WTES with thermal storage for industrial application than e-WTES with thermal storage.
According to (Okazaki, Shirai and Nakamura, 2015), the cost of thermal energy storage is 60-70% cheaper compared to other storage application such as (Backup thermal and Battery) and is favourable for WTES since wind repetition time is longer than 24hr and requires larger storage capacity.
Additionally, table 3 shows the levelized cost of heat range for industrial boiler, which is commonly used to produce heat in the industries, the LCOEHEAT ranging between 15-20 £/MWh (ec.europa.eu, 2016). As expected and according to the literature reviewed, the renewable technologies with thermal storage present the most expensive LCOE for heat production purposes compare to the industrial boiler. Further, the graph shows there is a decrease in the thermal energy cost, 10% reduction every 5years, which is also forecast by (SunShot target 2016), this could be due to further developments in thermal energy storage. Also, the graph shows that there is an overall decrease in the LCOE of h-WTES 12% and e-WTES 8% by 2025. Moreover, it can be assumed that the technologies such WTES can meet the LCOE of the traditional gas boiler in future, mainly due to the current energy crisis, CO2 tax, Carbon capture and reduction in fossil fuel.
3.0 An effectiveness-NTU technique
An effectiveness-NTU technique for characterising PCM thermal storage system for WTES application This part of the research includes the design of thermal storage system with the phase change material using mathematical formulation. A simplified method which was proposed by (Tay, Belusko and Bruno, 2012) is being used to characterise the phase change thermal storage system based on 1D phase change. An analytical method developed using the effectiveness-number of transfer units (ɛNTU) technique for a tank filled with PCM, with HTF flowing through tubes inside the shell as shown in Fig 9. (Tay, Belusko and Bruno, 2012)
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Figure 9: Schematic of one tube tank
Previous research by (Chan and Tan, 2006) (Tan, 2008) have empirically demonstrated that effectiveness-NTU can be used for the PCM thermal storage system to characterise the heat transfer and the result showed that the heat exchange effectiveness clearly correlates with the number of transfer units (NTUs). The CFD model of the PCM within a tube in tank developed by (Tay et al., 2014) also validate the result of effectiveness-NTU. From the experimental result of (Tay, Belusko and Bruno, 2012), it was found that the tube-in-tank with sufficient heat transfer area has the highest energy storage density with capacity factor above 90%, identified by the average heat exchange effectiveness which defines the performance of the storage system.
3.1 Description of the Latent-Thermal Energy Storage system
The system consists of a metal shell and a 15 x 15 matrix of copper pipes (see figure below Fig 10). The heat transfer fluid HTF in this work is mineral oil as it provides reliable heat transfer and the properties are presented in table 7. The HTF flows through the tubes exchanging heat with the PCM filled inside the shell (Fig 11). The dimensions of HTF tube is (1mm thick and 12.5mm i.d.), and 4.0m long. NaNO3- NaOH has been used as a PCM (Pereira da Cunha and Eames, 2016), its properties are presented in table 6.
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Figure 10: a Simple schematic unit of PCM-TES system Right) Single heat transfer tube and left) unit cross-section and all the dimensions are present in cm. (Colella, Sciacovelli and Verda, 2012)
The Phase Change Material PCM
The selected PCM was identified from the Phase Change Material Review (Pereira da Cunha and Eames, 2016) and the properties of the material are presented in Table 6. The selected PCM was based on being representative of typical eutectic mixture and salt hydrates, with a melting point suitable for Food and Paper industrial sector. According to the review, salt hydrates are promising for applications below 1000 C and eutectic mixtures from 100 to 3000 C. Appropriate selection of the phase change material is beyond the scope of this research.
Organic compounds were not selected as a PCM material as they have very low thermal conductivity (range from 0.1 to 0.7 W/mK), they usually require a mechanism to enhance their heat transfer rate.
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Table 6: PCM properties
Heat Transfer Fluid Properties
Mineral-oil has been chosen as a Heat transfer fluid as it provides reliable heat transfer and has the performance benefits of low fouling, high thermal stability, practically non-toxic and environmentally friendly.
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Table 7: HTF properties (Therminol.com, 2017)
4.2 Mathematical Model
The formulation of the heat flow is a one dimensional between the PCM and the HTF at the phase change profile. It is based on the internal temperature measurement of PCM by (Tay, Belusko and Bruno, 2012) where it was found that the phase change was uniform throughout the PCM tank. The NTU is calculated from the thermal resistance to heat flow within the tube wall, HTF and the part of PCM which undergoes a phase change.
Assumptions
-The phase change occurs in a cylindrical pattern and one-dimensional Fig 11.
-Constant inlet temperature and velocity of the HTF and the outer wall of the thermal storage system remaining adiabatic.
-System initial temperature is uniform. Natural convection has been ignored.
-The thermos physical properties of the PCM, the tube wall and the HTF are constant. Single length tube arrangement. Bend impact is ignored.
-Sround - represents the conduction between two concentric cylinders so in this case the heat flow from the tube to the phase change front Eq 5.
-Rmax - The phase change front varies with time, start at tube wall and increase to max radius Rmax.
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Figure 11: a) simplified model of the storage unit. b) thermal circuit
The NTU can be presented by the following equation at any point in time.
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To formulate the storage effectiveness, the thermal resistance needs to be determined which is a function of the overall heat transfer coefficient between the PCM and HTF, and the heat transfer area. The overall thermal resistance RT is given by the equation below (2) and (3), where RPCM is the resistance in the PCM, RWALL resistance in the tube wall and RFLUID resistance of the heat transfer fluid defined by forced internal convection. (Tay, Belusko and Bruno, 2012)
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The shape factor varies with time; this can be calculated based on phase change fraction ߜ for PCM, which is the amount of material that has yet to change phase. For tube surrounded by PCM (cylindrical volume), the phase change fraction ߜ is define by the following equation (6). Where, Ao outer tube area, AMAX maximum area after phase change and Ar is the changing area of PCM. (Tay, Belusko and Bruno, 2012)
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The overall thermal resistance RT is defined by the following equation. Substituting Equation (7) into (5).
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Therefore, the effectiveness can be defined by the following equation (9) in terms of RT, at certain phase change fraction and mass flow rate.
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Set of equations below are used for the calculation of hf. Prandtl number, Pr μ f C p
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Nusselt’s number,
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Where: Ac is the cross-section area of the inner tube, di inner diameter, m is the HTF mass flow rate,
Cp is the HTF specific heat capacity and μ
f
is the dynamic viscosity.
The heat exchange effectiveness changes with the phase change process, therefore it is important to determine the average effectiveness to obtain the performance of the system. The average effectiveness is defined as follows.
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4.3 Results and Discussion
The ɛ-NTU technique has been used for characterising a latent/PCM-TES system integrated into WTES for industrial application such as Food and Paper sector, which requires heating of a fluid stream (hot water, hot air streams) and heating of some reservoir for various applications. MATLAB was used to perform all the calculation, and the results are presented in the graphs below.
The maximum effectiveness of the system typically occurs when the HTF outlet temperature equal to the PCM temperature as this is illustrated in Fig 12 and Fig 13 where at mass flow rate of 0.001kg/s, the effectiveness of the system is around 95% with the outlet temperature of HTF is 2450 C which equals to the PCM temperature. The effectiveness is a very useful design parameter, for both discharging and charging processes. In a thermal storage system with PCM, the exit temperature of HTF during the discharging process should meet the system specifications, (In this case, the average temperature of the food and the paper industrial sector should be around 1700 C (IEA-ETSAP and IRENA, 2015)). Therefore, the minimum required effectiveness after which the thermal storage system is ineffective is a performance parameter of the system design (Iqbal, 2014) (Kreith and Bohn, 2001). However, for both discharging and charging processes different parameters can be changed such as mass flow rate, mass flux and length of the tube to maximise the effectiveness, minimise the exergy losses and maximise the useful energy that is stored. The Fig 12, clearly illustrates that the average effectiveness of the system is the function of mass flow rate of the HTF. As the flow rate of the HTF decreases the average effectiveness of the thermal storage system increases so does the average outlet temperature of the HTF (Fig 13). Therefore, the above parameter has been optimised to satisfy the minimum outlet temperature and the average effectiveness of the industrial processes (effectiveness = 31% and outlet HTF temperature = 1750 C) (Belusko, Halawa and Bruno, 2012)
The analysis of the average effectiveness of thermal energy storage only considers the latent energy stored in the PCM and ignores the sensible energy stored, the reason for ignoring the sensible energy is because most of the energy is stored in the latent phase for the PCM storage system and the maximum exergy efficiency typically occurs when the melting point of the phase change material is the average of heat sink and heat source temperature. Therefore, the system is only applicable to the low-temperature difference.
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Figure 12: Graph shows the average effectiveness vs. mass flowrate of two different HTF Air and mineral-oil for Latent/PCM-Thermal Energy Storage Device. Obtained using the ɛ -NTU technique as shown above.
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Figure 13: Graph shows the average outlet temperature0 C vs mass flowrate of two different HTF air and mineral-oil for Latent/PCM-Thermal Energy Storage Device. Obtained using the ɛ -NTU technique as shown above.
Figure 12, shows the calculated average effectiveness for the melting process using Eutectic Compound NaNO3-NaOH as a phase change material with the melting point of 2500 C, using two different heat transfer fluid, Air (blue line) and Mineral Oil (orange line), overall showing good agreement with an error of around 14%. A sudden change in Blue curve occurs at ṁ = 0.006kg/s. This is because of the change of condition of Air from the laminar regime to turbulent regime. At the point ṁ = 0.006kg/s, the Reynolds number reached > 2000 values of turbulent flow, making the Nusselt to increase and causing the overall thermal resistance to decrease. Therefore, water is not suitable to be used as an HTF for this process.
The result from the ɛ-NTU technique also shows good agreement with the experimental result, conducted by (Lik ab., 2017) for Solar Tower Power Plants using the same PCM and HTF. The result has an average absolute error of around 10%; this is mainly due to the natural convection which occurs during the melting process of the experiment. The report by (Bédécarrats et al., 1996) also shows that the effectiveness-NTU technique does not take into account the natural convection which depends on the system temperature difference but, the effectiveness-NTU method is intended to be temperature independent therefore no natural convection. (The graph of experimental result and modelling result present in appendix 1).
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Table 8: Results obtained from the effectiveness-NTU technique
By analysing the existing literature and the results of the model, it can be concluded that the technique ɛ-NTU proposed by (Tay, Belusko and Bruno, 2012), can approximately (since it only consider the average temperature/effectiveness of the system) predict the average effectiveness of the system (which describes the performance of the device) with a high heat transfer rate for discharging and charging processes for PCM-Storage system (115kW) Table 8. Therefore, this design can effectively be integrated into WTES to provide a large amount of heat demand for industrial applications. Further, the table 4, below shows some of the important parameter obtained from the ɛ-NTU mathematical model which includes the energy capacity (total amount of energy that can be stored), Maximum charge/discharge power, energy storage duration and round-to-round efficiency.
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Table 9: further Results obtained from the effectiveness-NTU technique
Wind Renewable system such as Wind thermal energy system (WTES) are intermittent and cannot be used as the baseload energy source as there inevitably exists a time discrepancy between demand and generation. Therefore, thermal energy storage (TES) has become a crucial technology for energy crisis and, plays an important role for renewable applications as it makes the operation more efficient. The above result shows that PCM-Thermal Storage system integrated with WTES can store large amount of surplus latent energy at times when the industrial heat demand is low and can provide stable heat at peak times for longer period (9.4 hours) with discharge power of 115kW and average outlet temperature of 1750 C, which is enough for industrial sectors such as Food, Beverages and Paper Table 10. This means that by having TES integrated into the system can provide a significant amount of energy for a longer period as illustrated in Fig 14.
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Table 10: shows the temperature range of industrial sector
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Figure 14: Shows the Stable output of the thermal energy (PCM)
Latent/PCM - Thermal energy storage system has the total storage energy capacity of 1.41GWh per year with the average effectiveness of 31% to achieve the specified temperature. 2MW-WTES with PCM-TES has a system capacity of 440kW (considering the UKs wind capacity factor 0.22 (Gov.uk, 2017)) with the storage discharge rate of 115kW. According to (Law, Harvey and Reay, 2017) research report, food industries in the UK consumes around 28.767GWh of energy per day, which is around 2 - 2.8GWh/yr for a single plant in the UK. Therefore, a PCM-Storage system with WTES has enough energy capacity (440kW) to supply heat demand for the food plant for application such as drying, washing, pasteurising, boiling, bleaching, sterilising and heat treatment (IEA-ETSAP and IRENA, 2015).
Sustainability is increasing concern in the food industry, and the various report shows that the food industry is lagging in the environmental performance compared to other sectors. This is mainly because the food/drink industry plays a significant role in environmental impact as they currently account for 7 million tonnes of carbon emission each year (Gov.uk, 2017). Interestingly, by having a 2MW WTES with a PCM-storage system for the food industry, can reduce 625 tons of CO2 emission per year from burning fuel such as natural gas (the value of carbon saved is calculated using the current DECC guidelines). It should be noted that by having this system in the different sector can help the UK to meet their emission target. (Gov.UK, 2017)
Comparison of PCM storage vs. Sensible storage tanks
Sensible heat storage specifications
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Table 11: Sensible heat storage specifications
As expected, the calculated result shows that the sensible heat storage system requires larger storage unit (5 x bigger) than PCM - Storage Unit to store the same amount of energy (1.085MWh). The reason being, the PCM has very high energy storage densities at a constant temperature (Liu et al., 2016) and are an ideal candidate for heat storage. An experiment conducted by (McKeever, 2015) shows that the PCM-TES can hold 6.5 times more heat than the SH-TES which allowed the CHP to run longer at a peak period and also eliminated the use of backup gas boilers.
Result
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Figure 15: Comparison between the PCM storage vs. the Sensible Storage tank
More thermal properties of the commonly used liquid material for sensible thermal storage are present in the appendix.
5.0 Conclusion
Fossil fuel plays an important role in the society, and they are commonly used for the heat and power production for the commercial and residential applications. According to UKERC Research Report, heat accounts for around 47% of the total energy consumption in the UK and 33% of carbon emissions.
Approximately, 80% of this heat account for space and water heating. At the same time, a large amount of heat is being used by the industry sector 24%, for industrial processes. The corporation of thermal energy storage system is very beneficial for CO2 emission reduction and energy saving. And, it is the key technology to balancing the energy system stable with the increasing capacity from wind.
Limited research on the concept of Wind-Powered Thermal Energy System (WTES) recently proposed by (2015, Okazaki et al.) with Latent-Thermal Energy Storage to produce heat for industrial application and also, the economics of WTES and electrical-WTES with thermal energy storage for heat generation is unknown.
A study of the potential and probable costs of different WTES with thermal energy storage and the designing of the latent-TES using effectiveness-NTU modelling suitable for industrial sector was conducted.
It was identified that it is more economically feasible to generate heat from h-WTES (Direct heat production) with thermal storage than e-WTES (indirect heat production). The reason for this was that e-WTES employed electrical driven heater to convert the electricity into heat which increased the LCOEthermal of e-WTES to 63.36 £/MWh-t and further, h-WTES saved around 30% on construction which decreased the LCOE of h-WTES to 56.2 £/MWh-t. Additionally, the LCOEthermal was compared with the traditional industrial gas boiler (LCOEthermal = 15-20 £/MWh), the results suggested that the renewable technologies with TES present the most expensive LCOE for heat production compare to the industrial gas boiler. However, these technologies can meet the LCOE of the traditional gas boiler in future, mainly due to the current energy crisis, CO2 tax, Carbon capture and reduction in fossil fuel.
For the second part of the research latent-TES (present higher energy storage density) was characterised using the effectiveness-NTU technique based on 1D phase change. It was investigated that the technique effectiveness-NTU can accurately predict the average effectiveness of the system (performance) with high heat transfer rate for discharging and charging processes (115kW). Therefore, the design can effectively be integrated into WTES to provide a significant amount of heat demand for industrial plants such as food and paper. It was also, found that latent-TES can hold 6.5 times more heat than sensible-TES. Finally, it was identified that by having a 2MW WTES with latent-TES for the food industry, can reduce 625 tons of CO2 emission per year from burning fuel such as natural gas. It should be noted that by having this system in different sector can help UK to meet their emission target.
Further research is needed to examine the impact of combining WTES with other thermal plants such as geothermal, CSP, biomass with different thermal energy storage material or technologies (as it can be shared) to generate heat for industrial applications.
6.0 References
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Appendices
Appendix 1: Experimental Result of NTU from literature.
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Appendix 1: The result from the ɛ-NTU technique also shows good agreement with the experimental result, conducted by (Lik ab., 2017) for Solar Tower Power Plants using the same PCM and HTF. The result has an average absolute error of around 10%.
Appendix 2: Thermal Properties of Sensible Thermal Storage materials
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Appendix 3: summary of TES with various application
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This section shows the integration of thermal energy storage into different applications.
- Citar trabajo
- Sharyar Ahmed (Autor), 2017, Integration of Thermal Energy Storage into Energy Network, Múnich, GRIN Verlag, https://www.grin.com/document/384572
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