Combined heat and power plant integrated with mobilized thermal energy storage (M-TES) system

Abstract

Energy consumption for space and tap water heating in residential and service sectors accounts for one third of the total energy utilization in Sweden. District heating (DH) is used to supply heat to areas with high energy demand. However, there are still detached houses and sparse areas that are not connected to a DH network. In such areas, electrical heating or oil/pellet boilers are used to meet the heat demand. Extending the existing DH network to those spare areas is not economically feasible because of the small heat demand and the large investment required for the expansion. The mobilized thermal energy storage (M-TES) system is an alternative source of heat for detached buildings or sparse areas using industrial heat. In this paper, the integration of a combined heat and power (CHP) plant and an M-TES system is analyzed. Furthermore, the impacts of four options of the integrated system are discussed, including the power and heat output in the CHP plant. The performance of the M-TES system is likewise discussed.

Keywords

Mobilized thermal energy system / district heating / thermal energy storage / combined heat and power / detached houses

Introduction

In Sweden, 35% of the final energy use is consumed as heat and electricity in residences [1]. Among these, detached houses share 42% of the 89 TWh in 2005 for space heating and hot water usage [2-8]. In the downtown or in neighborhoods within the cities, biomass-based district heating (DH) systems from combined heat and power (CHP) plants that reduce CO₂ emissions and primary energy use are responsible for heat supply and electricity production. However, it is a tremendous challenge for the CHP plant to distribute heat to sparse areas with quite low heat densities due to two main obstacles: the high capital investment required for extending the DH network, and the low benefits obtained from the small heat demand of such end users [9, 10]. In some sparse areas, the existing local heat supply system supplies hot water mostly through wood/oil-based boilers in the small local network. However, these boilers contribute to local air pollution and have quite low efficiencies. Therefore, the CHP plant management must look for other ways to supply heat to large detached buildings or sparse areas with no connection to its DH network.

For this purpose, the mobilized thermal energy storage (M-TES) system has been proposed to supply heat to large detached buildings or to cover parts of the heat demand in sparse areas<FootNote>

ECES 18 Annex. Kick-off Meeting in Bad Tölz. Germany, November 14^th-15^th, 2005

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Transheat. http://www.transheat.de; Access on Dec. 12^th, 2009

</FootNote> [11,12]. The M-TES system includes the TES container, heat exchangers in a charging or discharging station, and circulated pumps. TES materials are packed in a specific container carried by a lorry (in some cases, heat can be delivered through a train or vessel). The high heat density is important in the M-TES system; in addition, TES technology, especially latent thermal energy storage (LTES) or chemical thermal energy storage (CTES), can provide this capacity that has been studied for the last three decades [13-16]. Related TES knowledge and experiences have also been reported in previous works [17-19].

In this paper, a case study of M- TES system is presented, where heat is charged in the CHP plant and then delivered by a lorry to a distant location. However, using steam to charge the TES container will affect the electricity output of the plant. Hence, in this paper, the analysis of the CHP plant combined with the M-TES system is made to address various needs to supply heat. The case study reveals the impacts of the integrated system with different options and various heat supplies.

Methodology

To investigate the integrated M-TES system in an existing CHP plant in Sweden, computational simulations were conducted to analyze different options for charging the M-TES system in the plant. First, information on the operating process were collected from the existing CHP plant in Eskilstuna, including data on operating conditions, heat and power productions and heat supply load, to simulate and validate the modeling. Second, computational models of the CHP plant were developed using steady-state Aspen Plus software and validated with the data collected from the plant. Finally, the performance of the integrated CHP plant was evaluated with the different options based on power and heat output.

Description of the existing CHP plant and end users

The CHP plant studied in this work is located in a city in the middle of Sweden, and has a population of 90000. The plant is owned by the municipal energy company called Eskilstuna Energi and Miljö AB (EEM). Its annual fuel consumption is over 700000 m³ of biofuel, consisting mostly of wood chips or waste. A storage tank with 1200 MWh heat capacity supplies hot water with temperatures of over 90°C to supply heat during the peak period [20]. In some sparse areas and detached buildings throughout the area, heat is still totally or partly supplied by either conventional or low-efficient heat supply systems, such as oil boilers, electric heaters, and small wood-fired boilers. The owner of EEM prefers to directly use the heat of the power plant to cover the heat demand of this group of end users. Thus, an M-TES system has been introduced for this purpose.

Description of the integrated CHP plant and M-TES system

The existing CHP plant has been integrated with the M-TES system, using the plant heat to cover the heat demand of specific end users. The TES container of the M-TES system is heated in the CHP plant and delivered to supply heat to end users. The option of integrating the CHP plant with the M-TES system is an important one. In the steam cycle of the CHP plant, live steam drives turbines to produce electricity, and various kinds of steam are extracted from different stages. The extracted steam can then be used to heat the TES container. Meanwhile, the hot water in the accumulator of the CHP plant is also used as a heat source. Thus, four options (I to IV) of the different heat sources are proposed using hot water or extracted steam to heat the TES container. This is presented in Table 1.

The scheme of the integrated CHP plant and M-TES system is also illustrated in Fig. 1. It shows two sites (Site 1 and Site 2) that have been chosen to extract live steam. In Option II and Option IV, steam was extracted through a valve 1 (V1). In Option II, the TES container was heated by Exchanger 1. In Option IV, part of the hot water from the accumulator controlled by valve 3 (V3) was first heated by the extracted steam through Exchanger 1, and then the TES container was heated by Exchanger 2. In Option III, the steam was extracted through valve 2 (V2) to heat the TES container.

Simulation and validation of the existing CHP plant

Data on the practical operating conditions collected from the Eskilstuna CHP plant were used to model and validate the CHP plant model. Steam (540°C and 140 bars) was produced by bubbling a fluidized bed boiler with a flow rate of 28.04 kg/s in the plant. The essential simulated data were compared with the designed data (Table 2), which mainly focused on steam extraction sites to investigate the impacts on the CHP plant by extracting steam to charge the TES container. The simulation results on the operation parameters of this study are close to those of the designed case from the CHP plant, indicating that the model can be used to further simulate the integrated CHP plant with the different options.

Results and discussion

Analysis of the integrated CHP plant and M-TES system

Option I is much different from the other three, because it uses hot water with a temperature of 90°C from an accumulator as a heat source to heat the TES container directly. The hot water in the accumulator is a potential abundant heat source and is low-cost as well. In addition, without any steam extraction, it has minimum impact on the internal processes of the CHP plant. In Option II, the steam extracted from HPST was used to heat the TES container with a temperature of 175°C. Through Exchanger 1, the steam became hot water with a temperature of 157°C and returned to Condenser 1. In this option, the latent heat and part of sensible heat of the steam served as the major heat source to heat the TES container. In Option III, the extracted steam from LPST heated the TES container with a temperature of 124°C, which became the hot water of 124°C flowing back to Condenser 1. In this option, only the latent heat of the steam was used in heating the TES container. In Option IV, the water was first heated from 73°C to 140°C using extracted steam with a temperature of 175°C from HPST. This was used to heat the TES container. The water temperature dropped to 58°C, and then returned to the Accumulator. According to the above analysis of the simulation results, the four options heated the TES container in different temperatures, resulting in different TES container designs and different practical applications.

Table 3 shows the characteristics of the M-TES system based on the four options. According to the above simulation results, the TES container can be heated with the different temperatures following the order Option II>Option IV>Option III>Option I. From the viewpoint of heat source temperatures, Options II and IV are better due to their higher temperatures. Such high temperatures can increase the heat exchange rate during the heat storage and release processes of the M-TES system. The different options also affected the M-TES system in terms of choosing the TES materials, which were selected according to different melting temperatures based on the different heat source temperatures. The performance and cost of the M-TES system also varied due to the different thermal properties and cost of the TES materials. Meanwhile, more TES materials are available for the higher charging temperatures. Consequently, the different options decide the M-TES system charged by the different temperature heats to meet the demands of the different types of end users. In Options I and III, the M-TES system covered the daily heat demands of detached buildings, such as office buildings, schools, etc. In a small sparse area, the heat demands were covered by a combination of the M-TES system and other boiler heating systems, such as pellet boilers. The water returned from the end users could be preheated by the M-TES system and then continually heated to the required temperatures in the boilers. From this viewpoint, the M-TES system with a higher charging temperature can be seen as a suitable method to supply heat to the sparse areas in Options II and IV.

Impacts on the existing CHP plant

The integrated CHP plant was simulated to obtain the operating conditions for each case and to evaluate the performance of the system. A design case was set for the same amount of heat extracted to heat the M-TES system, after which comparisons were made among different options. Three important parameters were discussed, including power output, heat output of the DH and flow rate of extracted steams, to evaluate the impacts of the different options on the system. Among the four options, only Option I had no steam extraction from the steam cycle. Subsequent comparisons were then made among the other three Options (II to IV) based on the same amount of heat (3.66×10⁴ (MW·h)/a) extracted for charging the TES container. The simulation results are shown in Table 4.

Table 4 demonstrates that the three options decreased the power output by 4.6%, 3.0% and 3.8%, respectively, compared with the base power output of the plant. Based on the comparisons, the flow rate of the extracted steam was similar in Options II and III, because the latent heat of saturated steam at 124°C in Option III was equivalent to the released heat of live steam cooled from 175 to 157°C. However, steam extracting positions influence exergy losses during the heat exchange process. The higher amount of live steam extracted at higher temperatures resulted in greater exergy losses. Therefore, the power output in Option II became less than that in Option III. In Option IV, steam was extracted from the same position as Option II. However, Option IV used steam to heat water to 140°C, after which the heated water was then used to heat the TES container. Compared with Option II, hot water from the accumulator decreased the steam consumption in Option IV. Thus, the extracted flow rate accounted for 9.8% in Option IV compared with 11.7% in Option II, resulting in a higher power output of 25433 kW in Option IV.

Sensitivity study

Figures 2 (a) to (d) depict the impacts of the three options (Options II to IV) on the CHP plant with different amounts of heat supply to the M-TES system, including power output, flow rate of steam extraction, heat supply and income of power, heat supply in the DH, and heat supply of the M-TES. Figures 2 (a) to (c) indicate that the power output and heat output in the DH network decreased and the flow rate of steam extraction increased with an increase in charging heat needed from the CHP plant. In comparing these three options of steam extraction, the power followed the order Option III>Option IV>Option II, while heat supply from the DH network followed the order Option IV>Option II>Option III. In addition, income served as the important indicator for the power plant. The impacts on the incomes of the CHP plant were thus analyzed as well; these incomes consisted of incomes of electricity, heat supply in the DH, and heat supply of the M-TES. Assuming that the price of electricity and the heat are 540 and 520 SEK/(MW·h), respectively, and then Option IV obtained the largest incomes among the three options (Fig. 2(d)). Therefore, from an economic perspective, Option IV can be considered the best for the integrated system.

Conclusions

In this paper, four different options for heating the TES container of the M-TES system from the existing CHP plant have been proposed. The performance of the integrated CHP plant and the impacts of the different options have been analyzed, and comparisons were made between these and the stand-alone plants based on the results of the simulation conducted using Aspen Plus. TES containers were charged with different temperature heats in four options, which affected the performance and cost of the M-TES system due to the different TES material selections. Based on the temperatures in the four options, the M-TES system is deemed a suitable method to supply heat to sparse areas in Options II and IV, and to detached buildings in Options I and III. Only Option I had no impact on the existing CHP plant because it only used hot water from the accumulator without any steam extraction. Compared with the other three options of steam extraction, the electricity production and heat supply in the DH network decreased and the flow rate of extracted steam increased with an increase in heat supply from the CHP plant. The power output followed the order Option III>Option IV>Option II, while the heat supply from the DH network followed the order Option IV>Option II>Option III. From the viewpoint of income from electricity, heat supply to the DH and heat supply to the M-TES system, Option IV can be considered the best option for the integrated system.

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