Economic Analysis of Mobile Thermal Energy Storages as Complement to District Heating

Environmental and Climate Technologies 2023, vol. 27, no. 1, pp. 516–531 https://doi.org/10.2478/rtuect-2023-0038 https://content.sciendo.com 516 ©2023 Author(s). This is an open access article licensed under the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0). Economic Analysis of Mobile Thermal Energy Storages as Complement to District Heating Alois RESCH1*, Harald DEHNER2 1,2University of Applied Sciences Upper Austria, School of Engineering, Stelzhamerstrasse 23, A-4600 Wels, Austria Received 29.03.2023; accepted 30.06.2023 Abstract – Urban areas are increasingly supplied by district heating networks (DHN) because this technology is reliable, provides easy handling for the customer and contributes to the required reduction of greenhouse gas emissions if it is operated from renewable sources. Waste heat from the industrial sector can serve as such, however, industrial plants are often not in the meaningful range of DHN, as they are mostly located in the periphery. For this reason, the application of mobile thermal energy storages (M-TES) is investigated by the present research work. M-TES systems are technically capable of exchanging heat between a DHN and heat sources or heat sinks, as previous studies have shown, but economic viability could not be reached with former energy prices. However, geopolitical incidents of 2022 resulted in massive fluctuations on the energy markets and unpredicted price increases. Therefore, this paper provides an updated analysis of M-TES, considering the premises of 2022. An economic model according to VDI2067 was developed for calculating the costs of transported heat for different storage technologies and materials. Moreover, transportation by a Diesel driven truck was compared to an electric driven one. The updated analysis yielded economic feasibility for specific M-TES configurations, achieving minimum heat costs of € 89.5 per MWh. This is equivalent to a reduction of 40.3 % related to the prices of conventional district heating in Austria by end of 2022. Keywords – Economic evaluation; heat transfer network; industrial excess heat; waste heat recovery. 1. INTRODUCTION District heating networks (DHN) are an important backbone of today´s heat supply with high potential to contribute to a reduction of greenhouse gas emissions if their heat sources are increasingly transformed into renewable ones [1]–[4]. However, an economically viable application of such networks requires adequate occupancy rates and power density along the lines, because the infrastructure causes significant effort in terms of investment, operation, and maintenance [5]–[7]. This is one of the reasons, why DHN are mostly implemented in urban regions. On the other hand, industrial plants are not always located within the meaningful range of DHN, although they require huge amounts of heat as well as release significant amounts of waste heat, depending on the specific industrial sector [8], [9]. The utilisation of mobile thermal energy storages (M-TES) can be a possibility to close the gap of energy transfer between a DHN and remotely located industry or even directly between two or more industrial plants. The supply of residential heat demand can also be a potential * Corresponding author. E-mail address: [email protected] Environmental and Climate Technologies ____________________________________________________________________________ 2023 / 27 517 scenario for the application of M-TES. The intention of this approach is to transfer heat by charging a mobile heat storage at the producer and transfer it to the consumer by means of common transport and available infrastructure, e.g. by a truck on the road, as illustrated in Fig. 1. Fig. 1. Schematic illustration of the M-TES concept. In this way, the M-TES concept could serve as a complementary heat supply technology for regions without DHN or it could even be a competitor to DHN for the case that it might be economically advantageous. M-TES was already investigated in the past with different approaches and various material configurations, also by experimental implementation, and mostly with a focus on the economic viability of this concept [10]–[12]. An M-TES system working with the phase change material (PCM) Erythritol for providing heat energy for detached houses was investigated by Li et al. [13]. One conclusion of their study was that the storage material costs had the highest influence on the total costs of delivered heat, which resulted in $ 30 to $ 60 per MWh in the year 2012. Deckert et al. [14] considered another type of PCM for their research on M-TES, which was Sodium Acetate Trihydrate (SAT). Compared to Erythritol, SAT offers a melting point of 59 °C, which meets the requirements for supplying residential heat demand. With a distance of 6 km between a biogas plant as heat source and a small heat network with residential heat sinks, the heat generation costs were calculated to € 50 per MWh, which was a clear benefit compared to the average price of district heat of € 74 per MWh in 2014 in Germany. Although, the profitability of M-TES depends on the transported storage capacity, the number of transportation cycles and the availability of low-cost or even costless waste heat. A comparison of potential storage materials and different types of transportation containers was provided by Guo et al. [15], combined with an economic evaluation of different M-TES configurations. This overview shows the wide temperature range of 50 °C to 350 °C that can be addressed by M-TES with available storage materials. Furthermore, the bandwidth of resulting heat costs was calculated by € 20 to € 80 per MWh in 2018, under the premise that the required waste heat is available for free. Guo et al. also provided two potential improvement aspects for M-TES. Firstly, not only waste heat should be considered for being transported by mobile solutions, but also any other renewable heat, like solar thermal generated heat, biomass heat, or geothermal heat. Secondly, zero-emission vehicles should be applied in the future for transporting emission-free heat. By contrast to this technical research, Yang et al. [16] focussed on the economic performance of M-TES Environmental and Climate Technologies ____________________________________________________________________________ 2023 / 27 518 approaches and investigated their dynamic supply chain by numerical modelling. In 2022, Fritz et al. [17] presented an economic evaluation of innovative forms of heat transportation, e.g. excess heat distribution through sewer networks or ammonia-water absorption cycle technology. Both grid-bound and grid-free approaches of heat supply were considered and compared to conventional DHN. The calculated levelised costs of transported heat ranged from € 4 to € 260 per MWh, depending on the heat demand and the transportation distance. DHN appears to be the most economical solution for long distances and high amount of heat, whereas M-TES using PCM is favourable for distances below 7 km and transported heat amounts of lower than 700 MWh/a. Guo et al. [18] provided an insight into policies and regulations with regard to M-TES in China. PCM were seen as the most promising storage materials, especially Erythritol and SAT, transported by a truck on the road. As Erythritol has a melting point of 118 °C, the charged M-TES is transported with at least this temperature and is therefore subject to the legal regulations of dangerous goods in China. This increases the effort for such M-TES configurations, compared to the utilisation of SAT as storage material with a melting point of 59 °C. Remarkably, M-TES is already supported by national funding schemes of China. By contrast to PCM, Fujii et al. [19] worked on the investigation of thermochemical storage materials (TCM) for M-TES, which was Zeolite in this case. A new design of this storage system was provided, containing an amount of 4 t of Zeolite, and assuming a transportation distance of 3 km. The conducted life cycle assessment confirmed that the considered M-TES configuration can reduce greenhouse gas emissions, based on the given assumptions. Practical experience with M-TES could be obtained by the development and operation of a demonstration plant in Germany, as reported by Krönauer et al. [20] and Hauer et al. [21]. In this case, also Zeolite was used as storage material for transporting heat from a waste incineration plant over a distance of 7 km to an industrial consumer for supporting a drying process. The amount of transported heat was 1092 MWh/a, resulting in total energy costs of € 73 per MWh in 2014. At this time, the M-TES concept competed with low costs of conventional energy of € 36 per MWh, and therefore, the demonstration plant was economically not feasible. As energy markets had to face unknown fluctuations in 2022, the research work presented by this paper had the aim to analyse the M-TES concept for the current situation. Therefore, a comprehensive economic evaluation was performed, based on VDI2067 [22], for calculating the costs of transported heat (COTH) for M-TES. This investigation was done for the three main types of heat storing mechanisms, namely sensible, latent and thermochemical storages. In each category, several material types were considered to meet possible requirements of the specific application, e.g. in terms of temperature demand. This updated analysis of M-TES yielded positive results for thermochemical and latent storage materials, as the COTH are significantly lower than the heat costs of DHN in Austria in 2022. 2. METHODS AND METHODOLOGY Compared to other studies, Krönauer et al. [20] presented a realised M-TES system based on Zeolite and monitored its operation over one year. Furthermore, the calculation of the costs of heat is documented in a detailed way, providing the possibility to reproduce the economic evaluation. Due to these reasons, the system of Krönauer et al. was chosen as a basis for the present analysis of the M-TES concept for the energy prices in 2022 and for various technical configurations. Environmental and Climate Technologies ____________________________________________________________________________ 2023 / 27 519 2.1. Overview of methodological approach The detailed methodological process of the performed investigation is depicted in Fig. 2. The economic data of Krönauer et al. was used to develop a replication of the heat cost calculation for 2014 according to the annuity method described by VDI2067 [22]. Some details of the economic calculation parameters, e.g. personnel costs of the truck driver, had to be adapted within a loop, until the results of the developed calculation model were equal to the reference results. For transferring the heat costs of 2014 into the year 2022, the customer price indices (CPI) for Austria from 2014 to 2022 [23] were applied to all cost categories affected by inflation. From this point, the developed and updated cost calculation model was used to analyse different configurations of the M-TES concept. On the one hand, thermochemical, latent and sensible storing of heat was considered, and on the other hand, various kinds of storage material were analysed for each storage technology. Furthermore, a comparison between electric and Diesel driven trucks was implemented in the model. Fig. 2. Visualisation of the applied methodological process. 2.2. Calculation of heat costs The annuity method described by VDI2067 [22] is the basis for the presented calculation of heat costs caused by M-TES systems, combined with slight adaptations to consider the specific premises of this application. The total costs for implementing and operating an M-TES system can be split up into three different categories: − Capital-related costs Cc are investment amounts for the construction of M-TES facilities. − Demand-related costs Cd are caused by energy demand for operation. − Operation-related costs Co cover financial efforts for maintenance and inspection, as well as transportation costs caused by personnel and vehicle. Environmental and Climate Technologies ____________________________________________________________________________ 2023 / 27 520 These cost categories are transformed into annuities by considering an observation period T and a calculative interest rate q. Using these two parameters, the annuity factor a can be calculated as follows in Eq. (1). 1 1T q a q− − =− (1) The annuity Ac of the capital-related costs Cc is Ac = Cc·a (2) For the case of ongoing costs, which are Cd and Co, the price-dynamic cash value factor b has to be introduced. This factor considers discounting of future costs to the initial point in time and takes into account possible increase of energy or personnel costs r in the following years, as displayed in Eq. (3). 1 T r q b qr − =−      (3 ) Therefore, the annuity Ad for the demand-related costs Cd results in Ad=Cd·a·bd, (4) and the annuity Ao for operation-related costs Co is calculated by Ao=Co·a·bo, (5) where two different kinds of cash value factor are considered for demand-related and operation-related annuity, which are bd and bo, respectively. The total annuity Atot adds up the three partial annuities: Atot=Ac+Ad+Ao (6) Dividing Atot by the amount of transported heat Qth between the heat source and the heat sink over a period of one year results in the requested costs of transported heat COTH: tot th A COTH Q = (7) In order to be comparable to other forms of supplying heat, the results of COTH are given in €/MWh. 2.3. Boundary conditions 2.3.1. Definitions for the M-TES operation The following boundary conditions are derived from the reference system of Krönauer et al. and are also valid for all investigated M-TES configurations: − Transported amount of heat: 1092 MWh/a; − Transportation distance: 7 km; − Transportation on the road by a semitrailer truck with 40 t gross weight, using two trailers in cyclic operation mode; Environmental and Climate Technologies ____________________________________________________________________________ 2023 / 27 521 − Costs for waste heat: € 5.0 per MWh. Further applied boundary conditions specific for each case are given in the section “Results” within the corresponding sub-sections. 2.3.2. Customer price index (CPI) The transformation of the calculated heat costs from 2014 into the year 2022 was done by applying the CPI2010 for Austria [23]. Table 1 in the Appendix lists the CPI2010 for the relevant years from 2014 until 2022 and provides the annual change compared to the previous year. The total change between 2014 and 2022 results in a relative price increase of 21.78 %, which was included in the developed cost model by increasing all relevant cost categories with this factor. In this way, the results of Krönauer et al. could be replicated and updated for 2022. 2.3.3. Technical properties and price of storage materials The technical properties and prices of the investigated TCM, PCM and sensible storage materials were taken from [15], [20], [24]–[28] and are documented in Tables 2 to 4 in the Appendix. As some material costs were given for previous years, they were updated for the year 2022 by applying the CPI2010, accordingly. 2.3.4. Economic parameters for COTH calculation Similarly to the reference case of Krönauer et al., a value of 5 % was defined for the calculative rate of interest q, and the observation period T was chosen with 15 years, valid for all performed calculations. The detailed assumptions of investment costs are given in Table 5 in the Appendix, required for the calculation of the capital-related costs. Table 6 in the Appendix provides the summary of all demand-related premises and costs, as well as the applied annual increase of waste heat costs. Finally, the operation-related premises and costs for the calculation of COTH are condensed by Table 7 in the Appendix. 2.3.5. Boundary conditions for the comparison of Diesel driven and electric driven trucks The conducted comparison between Diesel driven and electric driven trucks bases on the calculation of the truck operation costs, given in €/km, which are part of the operation-related costs Co for calculating the total COTH. The annuity method described by VDI2067 [22] was used to derive the truck operation costs, based on data of [29], [30]. General boundary conditions were chosen with a truck power of 400 kW, a kilometrage of 100000 km/a, a calculative rate of interest of 5 % and a truck life span of 8 years. The specific capital-related and operation-related premises and costs of both types of drive are given in detail in Tables 8 and 9 in the Appendix. 3. RESULTS This section provides the resulting COTH for different configurations of M-TES systems. The first three sub-sections are dedicated to the specific storing technologies TCM, PCM and sensible storage, each of them considering different kinds of storage materials. The calculated operation-related costs Co are split up into two segments, which are Co,m+i for maintenance and inspection, and Co,p+t for personnel and transportation, because Co,p+t is varying significantly between the different M-TES configurations. Therefore, its impact becomes more obvious when displayed in a separate cost category. Subsequently, the COTH Environmental and Climate Technologies ____________________________________________________________________________ 2023 / 27 522 calculation results are summarised and combined with a benchmarking value for conventional district heating networks. Furthermore, an economic comparison between Diesel driven and electric driven trucks yields the impact of fluctuating energy prices on the specific transportation costs. 3.1. Costs of transported heat with TCM Investigating the M-TES concept with TCM storing technology was done on the one hand for the storage material Zeolite 13X, as it was also used for the demonstrated reference system of 2014 [20]. On the other hand, the COTH were also calculated for M-TES using Zeolite 4A, as it is cheaper than Zeolite 13X [24], [25] (see Table 2 in the Appendix). 3.1.1. Replication of the reference system using Zeolite 13X from 2014 and updated calculation for 2022 The verification of the developed cost model for M-TES systems was done by replicating the reference case documented by [20], [21]. The calculated COTH resulted in € 73 per MWh, what exactly corresponds to the original investigation of 2014. The segmentation into the single cost categories showed a slight deviation from the results of [20], [21], as the capitalrelated costs caused 62 % of COTH and the costs for personnel and transport were calculated by 23 % of COTH. Compared to the replicated results as shown in Fig. 3 (left), this deviation of 1 percentage point each appears to be acceptable. Fig. 3. COTH of M-TES using Zeolite 13X and segmentation into cost categories in 2014 (left) and in 2022 (right). Fig. 3 (right) provides the calculation results of COTH for the same M-TES configuration and the same boundary conditions as in the reference system but updated for the year 2022 by including the influence of inflation. In this case, the transported heat had a price of € 89.5 per MWh, while the distribution into cost categories remained unchanged. 3.1.2. M-TES using Zeolite 4A The use of Zeolite 4A as M-TES material instead of Zeolite 13X reduces the investment costs for the storage material by around 15 %, however, also the energy density decreases from 286 kWh/t for Zeolite 13X to 132 kWh/t for Zeolite 4A [24], [25]. Therefore, this MTES configuration requires 591 cycles per year for transporting the defined energy amount of 1092 MWh, resulting in COTH of € 114.3 per MWh. The financial effort for personnel and transport causes 41 % of the total costs, as illustrated in Fig. 4. Specific boundary conditions: Storing cycles: 273 per year 14 t of storage material per trailer Temperature of heat source: 130 °C Temperature of heat sink: 75 °C Environmental and Climate Technologies ____________________________________________________________________________ 2023 / 27 523 Fig. 4. COTH of M-TES using Zeolite 4A and segmentation into cost categories. 3.2. Costs of transported heat with PCM Besides the TCM storing technology, also PCM can be used for realising the M-TES concept. In this study, the materials Erythritol and Sodium Acetate Trihydrate were considered, with melting points of 118 °C and 59 °C, respectively. Compared to TCM, the charging and discharging of PCM storages is done with fluid-based facilities, which were assumed to have reduced investment costs, related to the air-driven charging and discharging stations. Furthermore, heat losses of 5 % by forced convection on the surface of the storage tanks are considered during transportation. 3.2.1. Erythritol as latent storage material With a melting point of 118 °C, Erythritol could be used for the same application as described in the reference case with TCM, where the heat source supplies 130 °C and the heat sink requires 75 °C. Additionally to the latent heat capacity, also the sensible heat capacity of the phase changing material is covered by the developed model, in the liquid phase between 118 °C and 130 °C and in the solid phase between 75 °C and 118 °C. The investment costs of the charging and discharging stations are considered with a reduction of 15 % compared to TCM. Fig. 5 illustrates the results of the economic calculation with Erythritol, with respect to the given specific boundary conditions. Although 19.5 tons of storage material can be transported per trailer, 492 storing cycles are required per year in order to deliver the defined 1092 MWh. This results in COTH of € 105 per MWh. Fig. 5. COTH with Erythritol as storage material for M-TES. Specific boundary conditions: Storing cycles: 591 per year 14 t of storage material per trailer Temperature of heat source: 130 °C Temperature of heat sink: 75 °C Specific boundary conditions: Storing cycles: 492 per year 19.5 t of storage material per trailer Temperature of heat source: 130 °C Temperature of heat sink: 75 °C Heat losses during transport: 5 % Environmental and Climate Technologies ____________________________________________________________________________ 2023 / 27 524 3.2.2. Sodium Acetate Trihydrate (SAT) as latent storage material By contrast to Erythritol, SAT has a melting point of 59 °C, and therefore, it would be suitable for M-TES to supply low-temperature heat sinks, e.g. residential or industrial in-floor heating systems with a flow temperature of 35 °C. The volumetric energy density of SAT is lower than the one of Erythritol, resulting in a higher number of storing cycles per year for transporting the same amount of energy. However, SAT is significantly cheaper, which reduces the COTH to € 94.9 per MWh, as displayed in Fig. 6. As the temperatures of heat source and heat sink are below 100 °C, the charging and discharging stations can be driven with water, compared to the M-TES system using Erythritol, and therefore, their investment costs are considered with a reduction of 30 % compared to TCM. Fig. 6. COTH with Sodium Acetate Trihydrate as storage material for M-TES. 3.3. Costs of transported heat with sensible storage This sub-section summarises the investigation of the M-TES concept with the sensible storing materials water, thermal oil and gravel. 3.3.1. Water as sensible storage material As water is the most common storage material so far, it can also be considered for mobile heat storages. For the sake of reduced effort, only unpressurised water is investigated for utilisation in M-TES systems, which limits the maximum temperature to 95 °C. Analysing the same low-temperature application for residential or industrial in-floor heating as mentioned above using SAT with a heat source temperature of 80 °C and a heat sink temperature of 35 °C, the COTH for M-TES with water results to € 152.7 per MWh. As depicted by the pie chart in Fig. 7, the costs for personnel and transport account for two thirds of the total costs, because the transportation of 1092 MWh per year requires 1284 cycles in this case. Specific boundary conditions: Storing cycles: 555 per year 22 t of storage material per trailer Temperature of heat source: 80 °C Temperature of heat sink: 35 °C Heat losses during transport: 5 % Environmental and Climate Technologies ____________________________________________________________________________ 2023 / 27 531 TABLE 7. OPERATION-RELATED PREMISES AND COSTS FOR THE CALCULATION OF COTH Description Value Personnel costs for truck driver € 48.7 per h Annual increase of personnel costs 2 %/a Transportation distance (one-way) 7 km Average speed of truck 35 km/h Time duration for trailer handling 1 h/cycle Operation costs for Diesel driven truck (including fuel costs) € 0.97 per km Maintenance costs related to total investment costs 0.8 %/a TABLE 8. CAPITAL-RELATED AND OPERATION-RELATED PREMISES AND COSTS OF DIESEL DRIVEN TRUCKS [29], [30]. Description Value Invest for chassis of tractor unit € 58 000.0 Invest for engine and gearbox € 71.0 per kW Invest for Diesel tank € 2.0 per lt. Size of Diesel tank 1000 lt. Invest for after-treatment of exhaust gases € 19.8 per kW Diesel consumption 32.2 lt. per 100 km Maintenance costs € 0.07 per km Insurance costs € 4500.0 per year CO2 toll € 0.187 per km Motor vehicle tax € 929.0 per year TABLE 9. CAPITAL-RELATED AND OPERATION-RELATED PREMISES AND COSTS OF ELECTRIC DRIVEN TRUCKS [29], [30]. Description Value Invest for chassis of tractor unit € 58000.0 Invest for motor € 19.8 per kW Invest for power electronics € 29.0 per kW Invest for battery € 261.0 per kWh Battery capacity 500 kWh Electricity consumption 1.56 kWh per 100 km Maintenance costs € 0.0595 per km Insurance costs € 4500.0 per year CO2 toll € 0.187 per km Motor vehicle tax € 373.0 per year