Numerical simulation of underground seasonal cold energy storage for a 10 MW solar thermal power plant in north-western China using TRNSYS

Abstract

This paper aims to explore an efficient, cost-effective, and water-saving seasonal cold energy storage technique based on borehole heat exchangers to cool the condenser water in a 10 MW solar thermal power plant. The proposed seasonal cooling mechanism is designed for the areas under typical weather conditions to utilize the low ambient temperature during the winter season and to store cold energy. The main objective of this paper is to utilize the storage unit in the peak summer months to cool the condenser water and to replace the dry cooling system. Using the simulation platform transient system simulation program (TRNSYS), the borehole thermal energy storage (BTES) system model has been developed and the dynamic capacity of the system in the charging and discharging mode of cold energy for one-year operation is studied. The typical meteorological year (TMY) data of Dunhuang, Gansu province, in north-western China, is utilized to determine the lowest ambient temperature and operation time of the system to store cold energy. The proposed seasonal cooling system is capable of enhancing the efficiency of a solar thermal power plant up to 1.54% and 2.74% in comparison with the water-cooled condenser system and air-cooled condenser system respectively. The techno-economic assessment of the proposed technique also supports its integration with the condenser unit in the solar thermal power plant. This technique has also a great potential to save the water in desert areas.

Graphical abstract

Keywords

seasonal cold energy storage / borehole heat exchangers / typical meteorological data / TRNSYS / condenser cooling / techno-economic assessment

1 Introduction

Today the traditional energy resources are being replaced by the renewable energy systems in order to overcome the challenges related to global warming and energy shortage. Renewable energy technologies have attracted much attention from the researchers and the business community because of their useful characteristics. However, these technologies have also certain drawbacks such as instabi-lity and discontinuity because of different weather conditions at day and night [1]. Thermal energy storage (TES) is particularly important these days in achieving the continuous operation of renewable energy such as solar energy [2,3]. TES systems contain temporary storage of heat or cold energy for later use in order to fulfill heating and cooling requirements of domestic and industrial buildings [4]. There are several TES systems that are available these days including aquifer TES, borehole TES, and pit TES. The choice of required TES systems relies on local geological and hydrogeological conditions [5].

Concentrating solar thermal power plants (CSPs) are mostly used these days to generate electricity because they can easily be coupled with TES systems [6]. Most of the solar thermal power plants in the USA, Africa, China, and Middle East countries are located in the desert areas which are rich in solar potential [7]. Many parts of the solar thermal power plants need proper cooling arrangements during the working hours and it is evident that if the temperature of condenser cooling water can be decreased with sustainable energy source, not only can the efficiency of power plant be increased but also the electricity be saved that is used in the form of forced cooling of condenser. Usually, power plants have cooling tower mechanism to provide cooling water for condenser. However, some of the water is evaporated through this process and these systems also consume water from nearby river or lake. This problem is disastrous in desert areas where there exists a water shortage. Therefore, it urges to find a unique method to provide low-temperature water to the condenser by saving both electricity and water.

The previous research literature [8] reveals that there exists the potential in desert areas under typical environment conditions to store cold energy in the winter, but to date, it has not been verified either experiments or simulations that include a storage system to fulfill the cooling requirements for a solar thermal power plant located in a desert area. Mostly, a condenser is cooled by using high power electrical fans at solar thermal power plant [9]. This dry cooling of condensers at power plants not only affects the thermal efficiency of the power plant but also increases the unit price of electricity, especially in the peak summer season when high power is required. The main objective of this paper is to study the feasibility of borehole seasonal TES system for the 10 MW solar thermal power plant located in typical west China under typical weather conditions.

The novelty of the proposed seasonal cooling technique is threefold: Until now there have been no studies that illustrate the use of seasonal storage of cold energy at the solar thermal power plant. Besides, the design, modeling, and simulation of a BTES system to meet the cooling requirements of a condenser in the summer season by replacing the forced cooling mechanism of the condenser has not been studied yet. Moerover, the proposed seasonal cooling technique demonstrates the cost-effective cold energy storage concept for solar thermal power plants that will increase the efficiency of the solar thermal power plant in the summer season. Furthermore, the technique proposed in this paper is capable of saving a sufficient amount of water to overcome the water shortage issues in desert areas.

2 Literature review

TES consists of different technologies. There are three methods for TES with operating temperature ranging from –40°C to more than 400°C, which are categorized as sensible heat, latent heat linked with phase change materials (PCMs), and thermo-chemical heat storage (TCS) related to chemical reactions [10]. Sensible heat is a suitable and easy technique to store thermal energy and can be achieved by introducing a temperature gradient to the storage medium in order to store and release heat energy. Most of the sensible heat storage systems use water as a heat carrier fluid. Seasonal storage of sensible heat in both liquid and solid media using borehole heat exchangers is also used for typically large-scale applications [11]. PCMs are used to charge and discharge a large amount of thermal energy at a constant temperature which makes the latent TES more prominent and viable storage mechanism. Thermochemical energy storage systems utilize the heat energy stored to stimulate the reversible endothermic process [12]. These days, sensible heat storage systems are commercially available while TCS and PCMs heat storage systems are mostly under development. Moreover, PCM storage and TCS storage systems are more complex and expensive than sensible heat storage systems [13]. Borehole thermal energy storage (BTES) is one of the most common methods used for seasonal TES currently employed around the world [14]. Most of the BTES systems are designed based on numerical simulation results. Lundh and Dalenback [15] worked on the Annenberg project (Sweden) and proposed the final design of the project containing 100 boreholes that were inserted in series and parallel connections. In Canada for the Drake Landing Solar Community project, a BTES field was designed using 144 boreholes with each borehole 37 m deep [16]. Many BTES systems focused on the heat storage in the summer season and to provide it to the buildings in the winter season. However, very few projects are designed in the world to investigate the cold energy storage potential [17]. Eicker and Vorschulze proposed two cases to study the geo cooling mechanism in German buildings [18]. They analyzed the seasonal performance factor and the heat transfer rate of the system during the operation time. According to Lund et al. [19], geo cooling were introduced to domestic users, but it attracted little interest. Recent literature reveals the fact that geo cooling has very few practical applications and ground-source heat pumps (GSHPs) are mostly used instead, to extract underground thermal energy available and to use it not only for small homes but also on a large-scale to satisfy district cooling and heating demands [20]. Pahud et al. [21] analyzed the geo cooling potential using concrete slabs for an office building in Switzerland. They inserted plastic pipes inside concrete slabs and water was flowed from ground heat exchangers to supply cooling.

According to the Renewable Energy Policy Network for the 21st Century (REN21) global status report 2018, China was ranked at the top position in total renewable power capacity, solar water heating collector capacity and geothermal heat capacity [22]. China’s rapid economic growth is facing challenges related to climate change, pollution, and energy shortage [23]. The current statistical energy data reveal that the building sector utilizes 27.8% of the total energy consumption in China, which has little difference from industrial consumption [24]. China has planned to manage more than 35% of the national energy utilization till 2020, where heating and cooling systems account for 65% of the total energy used in residential and office buildings [25]. China has also planned to increase non-fossil fuel energy generation, with the target to achieve 15% of total energy consumption from renewable and sustainable energy resources by 2020 [26]. Many Chinese researchers and intellectuals have been working to investigate the thermal performance of ground heat exchangers in recent years. Li et al. studied the thermal performance of U shaped vertical ground-coupled heat exchangers by performing experiments and analyzing simulation models [27].

However, the aforementioned studies indicate that the concept of seasonal BTES is mostly utilized to manage the heating and cooling requirements of the offices and domestic buildings. No analysis has been conducted on the use of BTES system integrated with the solar thermal power plants that are located in the desert areas under typical climate conditions. In some desert areas at day time in the summer, ambient temperature is very high but at night it decreases to a very low value and the winter season is extremely cold. Therefore, two types of seasonal energy storage systems can be applied in order to enhance the efficiency of the solar thermal power plant by replacing the forced cooling of the condenser with the proposed seasonal cooling method. One type of storage system will work at evening while the other will be the seasonal energy storage system. This paper aims to model and analyze the performance of the seasonal thermal energy storage system in warm summer and cold winter in Dunhuang, Gansu province, in north-western China. The proposed seasonal cooling technique will not only increase the efficiency of the solar thermal power plant but also presents a cost-effective method to store cold energy in the winter season and to use it in the peak summer season.

3 System description and case study

The BTES system is designed in order to store the cold energy from October to March according to the active weather conditions of Dunhuang, China. The heat transfer process starts from heat transfer fluid to the borehole wall after completing the certain number of circulations, which includes both the convection and the conduction process. The cold energy is then conducted to the storage material that is usually sand which starts interacting with other surrounding boreholes. As the energy storage system is exposed to the surrounding atmosphere, certain energy losses are observed. The thermal interaction between the borehole wall and heat carrier fluid is characterized by the thermal resistance of a borehole, which includes the heat transfer carried out by the convection between the heat transfer fluid and the walls of borehole channel. To achieve a maximum heat transfer, these boreholes play an important role and act as a medium between underground soil and the heat carrier fluid. Therefore, in this way, a large amount of cold energy can be efficiently stored in the winter season that can be used in the summer season. The whole BTES system with all its accessories is shown in Fig. 1 where the exhaust from the steam turbine enters into the water-cooled condenser that is connected with the cooling tower and the seasonal underground storage system. The surface condenser is used to condense the exhaust steam from the steam turbine and to convert this steam into water which can be used as boiler feed water to achieve the maximum power plant efficiency. The underground BTES system, as shown in Fig. 1, is charged with cold energy during the winter season and condenser cooling water is allowed to circulate through the storage system in the peak summer months in order to achieve a required cooling temperature in the condenser.

The entire storage system can operate in three modes as schematically illustrated in Fig. 2. In mode 1, at night in the summer season when the ambient temperature decreases to a certain value, the storage system starts its operation to store the cold energy using the cooling tower. During this operation, valves A,C, and D are closed while valves B and E are opened to ensure the heat transfer process. In mode 2, the storage is completely charged with the cold energy during the entire winter season and power plant condenser is directly cooled by using the cooling tower with wet-recirculating or closed-loop mechanism. In the winter season, the ambient temperature is very low. Therefore, it is quite feasible to use the cooling tower mechanism to decrease the condenser water temperature. During the cold energy storage operation, valves B, D, and E are closed while valves A and C are opened to perform the direct cooling of the condenser. In mode 3, during the summer season, the high-temperature water in the condenser is allowed to pass through the underground storage. As a result, heat transfer takes place between the storage walls and the heat transfer fluid. This low-temperature fluid will be transferred back to the condenser to condense the steam. During this operation, valves A, B, and E are closed while valves C and D are opened to perform the cooling operation using a storage unit. As the ambient temperature at day time remains high during entire summer season, the storage system will work to cool the power plant condenser.

The underground cold energy storage system is designed to fulfill the cooling requirements of the 10 MW solar thermal power plant located in Dunhuang, Gansu province near the Gobi Desert in north-western China. It can be seen from Fig. 3 that the underground BTES proposed is integrated with the condenser of the solar thermal power plant. The solar tower thermal system consists of the heliostat field, the receiver, the storage tanks, the water pump, the transportation pipeline, and other components as depicted in Fig. 3.

To determine the required design parameters to build a BTES storage unit, the solar thermal power plant is simulated for one year and the required heating load is calculated. Transient system simulation program (TRNSYS) has been used to establish the model of the solar tower thermal system. After establishing a model in TRNSYS, the local weather data are imported to simulate one year’s operation of the solar tower steam generation system under the designed working conditions. Figure 4 demonstrates the complete simulation model of the solar thermal power plant generated in TRNSYS.

Table 1 describes the design parameters for the BTES system. At first, the total heating load is calculated to be 3200 GJ for the peak summer months from June to August, and then the total storage volume is estimated to be 467027 m³. The number of boreholes is calculated using Eq. (11). The borehole depth has been chosen to be 50 m and spacing between the two boreholes is 4 m. The 354 boreholes are estimated to store the cold energy during the peak winter months from October to March. The BTES parameters are selected after the rigorous study of the already existing BTES systems in the world. The parameters have been chosen after examining the hydrogeological inspection data of the location and considering the guidelines for vertical borehole field design to optimize underground energy storage [28]. The simulation procedure is followed according to design parameters and the storage unit is simulated at 30 m, 50 m, 70 m, and 90 m respectively. The highest ground temperature variations are observed at a depth of 50 m in the borehole and the lowest at 90 m. It is found that a depth of 50 m is an optimum value for borehole depth for the proposed research work. The drilling cost is also proportional to the borehole depth. The spacing between boreholes is also investigated and a spacing of 2.5 m to 5 m is recommended to increase the heat transfer between the vertical boreholes. After considering various factors influencing borehole storage design and cost, an array comprising 354 boreholes to 50 m depth and spaced at 4 m are chosen in order to optimize the efficiency of the borehole heat storage system and to increase the heat transfer rate between the boreholes and the surrounding soil. The proposed system is simulated using different flow rates in order to determine the suitable flow velocity inside buried pipes. It is observed that as the flow rate inside the buried pipes increases from 0.1 to 0.5 m/s the input heat, the heat stored, and the heat dissipation increases first and then tends to stabilize. The increase in flow rate also leads to an increase in soil temperature. Besides, the heat storage rate initially decreases and then stabilizes but the overall change in heat storage rate is very small. Hence, the simulation results show that increasing the flow velocity of the buried pipe can effectively increase the heat storage into the soil. In actual practice, increasing the flow rate will reduce the design parameters of the buried pipe, and increase the pump consumption. Therefore, by considering the economic parameters and conditions required for the storage unit, a flow rate of 0.5 m/s and design parameters of the pipe buried, such as the inside and outside diameter of 0.026 m and 0.032 m, are selected accordingly. To reduce the heat losses and to ensures the maximum heat injection and extraction rates, certain simulations are performed and the soil thermal conductivity is considered 2 W/mK with a 4 m borehole spacing. Figure 5 shows that the borehole spacing is the function of the soil thermal conductivity.

Figure 6 demonstrates the direct normal irradiance (DNI) data of a typical year in Dunhuang, which indicates that it has enough direct beam irradiance throughout the year that can be used for solar thermal power plant. In this paper, a DNI value of 2499 kJ/(h·m²) (equal to 694 W/m²) is considered for heliostat field design point. The maximum and minimum annual average temperature profiles for Dunhuang are given in Fig. 7. The weather conditions are very severe in the summer season and extremely cold in the winter season due to the precipitation caused by evaporation. It can be seen from Fig. 7 that the maximum temperature in the summer season is 35°C in July and the minimum temperature is –21°C in January that is a peak winter month. The annual average temperature is 9.3°C.

From Fig. 8 it can be observed that the ambient temperature is very low from October to March in the winter season, and that the ambient temperature decreases to –21°C in January, showing a huge potential to save cold energy using the underground thermal energy storage technique, based on vertical ground heat exchangers.

4 System model and operation strategy

4.1 Model description

The vertical ground heat exchangers are placed at a constant distance into the storage unit. The inlet temperature of the heat transfer fluid is the function of time and written as T_fin(t). The temperature variation of the heat carrier fluid during the circulation inside boreholes is calculated by using a heat balance equation. When there exists a certain temperature difference between the heat carrier fluid and the surrounding atmosphere, heat transfer will take place. As a result of heat transfer, the heat carrier fluid will gain or lose energy depending upon surrounding weather conditions, and these temperature variations will be monitored by the sensors along the flow path inside the storage volume [29]. TRNSYS uses a DST model to simulate an underground soil thermal storage system. The DST model is mainly composed of a steady flow model, a global heat conduction model, and a regional heat conduction model [30]. The calculation of the global thermal model takes into account the effects of the other two models. It calculates the thermal conductivity of the entire soil. The numerical calculation formula for the global temperature T_g can be calculated using

(1)C{\displaystyle \frac{\partial T_{\mathrm{g}}}{\partial t}}=\nabla ·\left(\lambda \nabla T_{\mathrm{g}}\right)+q_{\mathrm{sf}}+q_{\mathrm{l}},

where C is volumetric heat capacity, T_g is global heat conduction temperature, q_l is regional heat conduction heat flow, q_sf is steady-state heat flow, and λ is thermal conductivity.

Regional heat conduction mainly considers the effect of heat transfer of the pipe buried on the surrounding soil. The pipe buried is regarded as a straight line, and the regional heat conduction temperature T_l can be obtained by the difference of the preceding term according to Eq. (1).

(2)C{\displaystyle \frac{\partial T_{\mathrm{l}}}{\partial r}}=\lambda \left({\displaystyle \frac{{\partial }^{\mathrm{2}}{T}_{\mathrm{l}}}{\partial r}}+\frac{\mathrm{1}}{r}{\displaystyle \frac{{\partial }^{\mathrm{2}}{T}_{\mathrm{l}}}{\partial r}}\right)-q_{\mathrm{l}},

where T_l is regional heat conduction temperature, and r is the radial distance of buried pipes.

The steady flow model calculates the heat source q_sf and the steady-state heat flow temperature T_sf. The heat source q_sf can be calculated by

(3)q_{\mathrm{sf}}={\displaystyle \frac{C_{\mathrm{f}}{Q}_{\mathrm{f}}}{V_k}}\left(\mathrm{1}-{\beta }_{\mathrm{sf}}^k\right)\left(T_{\mathrm{g}}^k-T_{\mathrm{g},i,j}^k\right),

(4)T_{\mathrm{sf}}=(T_{\mathrm{g}}^k-T_{\mathrm{g},i,j}^k)·{\displaystyle \frac{r_{\mathrm{1}}{}^{\mathrm{2}}}{\mathrm{2}{l}^{\mathrm{2}}}}·\left(h{\left({\displaystyle \frac{r}{r_{\mathrm{1}}}}\right)}^{\mathrm{2}}/\mathrm{2}-\ln \left({\displaystyle \frac{r}{r_{\mathrm{1}}}}\right)-{\displaystyle \frac{\mathrm{3}}{\mathrm{4}}}\right),

where q_sf is heat source, C_f is the specific heat capacity of the fluid, Q_f is fluid flow rate, V_k is the volume of grid k, {\beta }_{\mathrm{sf}}^k is correction factor, T_{\mathrm{g}}^k is the average temperature in grid k, T_{\mathrm{g},i,j}^k is the temperature of the grid (i, j), l is the characteristic length of heat transfer, and r₁ is the outer diameter of buried pipe.

The temperature of the heat storage unit can be obtained by combining the above three points, and the value is the sum of the steady flow temperature, the global temperature, and the regional temperature that is

(5)T=T_{\mathrm{g},i,j}^k+T_{\mathrm{l},j}^k+T_{\mathrm{sf},j}^k.

The inlet and outlet fluid temperatures of borehole heat exchangers are combined by

(6)T_{\mathrm{fin}}\left(t\right)=T_{\mathrm{fout}}\left(t\right)+\mathrm{\Delta }T\left(t\right),

where T_fin is inlet fluid temperature, T_fout is outlet fluid temperature, and ΔT is temperature difference.

(7)\mathrm{\Delta }T\left(t\right)={\displaystyle \frac{Q}{{\rho }_{\mathrm{f}}{C}_{\mathrm{f}}{V}_{\mathrm{f}}}},

where V_f is the volumetric flow rate of fluid, Q is the flow rate through the pipe, and {\rho }_{\mathrm{f}}is the density of the fluid.

The duct storage model (DST) deals with the number of boreholes that are connected in series per parallel loop. In parallel loops, the number of boreholes remains the same connected in series. The BTES volume is associated with V_BTES and has a cylindrical shape. In the BTES system, Q_charge is the total amount of cold energy accumulated in the storage volume in which vertical ground heat exchangers are inserted. The storage medium (soil) in the system directly contacts with the heat source. The total amount of Q_charge can be estimated using Eq. (8). Q_discharged is related to the total amount of heat extracted from the BTES system, which consists of not only the heat supplied to the distribution system but also the total heat losses to the surrounding environment. The total amount of Q_discharged is calculated by observing the temperature change of the storage system as expressed in Eq. (9). The major quantity of cold energy in circulating fluid (Q_in) is accumulated in the soil and the minor amount of energy is also transferred to surrounding as heat loss. Therefore, a heat balance analysis is necessary, which is performed in Eq. (10).

(8)Q_{\mathrm{charge}}={\displaystyle {\int }_V\rho c_{\mathrm{ps}}\mathrm{d}{V}_{\mathrm{s}}\mathrm{\Delta }T},

(9)Q_{\mathrm{discharge}}={\int }_{c_{\mathrm{o}}}^{c_{\mathrm{e}}}{\rho }_{\mathrm{s}}\mathrm{d}{V}_{\mathrm{s}}{c}_{\mathrm{ps}}\mathrm{\Delta }T,

(10)Q_{\mathrm{in}}=Q_{\mathrm{stored}}+Q_{\mathrm{loss}},

where V is storage volume, c_o shows the start of cooling, and c_e shows the end of cooling.

The spacing between the boreholes is determined by analyzing the heat capacity and thermal conductivity of the soil in the specific area for underground storage. The total storage volume can be estimated if the spacing, the number, and the length of boreholes are already determined. The total volume for the underground storage system can be calculated by using

(11)V_{\mathrm{BTES}}=\mathrm{\pi }·\mathrm{0.525}·B^{\mathrm{2}}·H·N_{\mathrm{b}},

where B is borehole spacing, N_b is the number of boreholes required, and H is the length of boreholes.

TRNSYS is used to simulate the storage system. Apart from the DST model described above, standard components presented in TRNSYS (pump, weather file, differential controller, etc.) are also used in the simulation program. The velocity of the fluid in the pipes can be calculated by using

(12)v={\displaystyle \frac{Q}{A}}.

The area of the pipe can be calculated as

(13)A={\displaystyle \frac{\mathrm{\pi }{D}^{\mathrm{2}}}{\mathrm{4}}},

where D is the diameter of pipe.

So the total flow rate in ‘N’ number of vertical ground heat exchangers will be

(14)Q=N·v·A,

(15)Q=N·v·{\displaystyle \frac{\mathrm{\pi }{D}^{\mathrm{2}}}{\mathrm{4}}}.

To analyze the performance of the cold energy storage system, it is necessary to calculate the coefficient of performance (COP) of the system, which is the ratio of the total amount of cooling that must be provided to perform the required work, and can be calculated by using

(16)\mathrm{COP}={\displaystyle \frac{Q_{\mathrm{c}}}{W}}.

The value of COP is calculated by using Eq. (16) and the pump power is calculated by using

(17)P_{\mathrm{h}}\left(\mathrm{k}\mathrm{W}\right)={\displaystyle \frac{q\rho gh}{\mathrm{3.6}\times {\mathrm{10}}^{\mathrm{6}}}},

where q is flow capacity, g determines gravitational acceleration, and h is head loss.

The flow capacity is calculated by using Eq. (14). The head losses can be expressed as

(18)h_{\mathrm{loss}}={\displaystyle \sum h_{\mathrm{major}\mathrm{-}\mathrm{loss}}+{\displaystyle \sum h_{\mathrm{minor}\mathrm{-}\mathrm{loss}}}}.

So the total head loss in the duct system is evaluated using the mathematical relationships

(19)h_{\mathrm{loss}\mathrm{-}\mathrm{single}}=\kappa \left({\displaystyle \frac{l}{d_{\mathrm{h}}}}\right)\left({\displaystyle \frac{v^{\mathrm{2}}}{\mathrm{2}g}}\right)+{\displaystyle \sum \xi {\displaystyle \frac{v^{\mathrm{2}}}{\mathrm{2}g}}},

where \kappa is friction coefficient and x is minor loss coefficient.

4.2 Operation strategy

The pump will start working as the ambient temperature decreases to certain values. The storage will be charged with cold energy from October to March. Therefore, based on the typical meteorological year data obtained from TRNSYS type (TMY2), some reference inlet fluid temperatures are selected. Table 2 explains the reference fluid temperature for the BTES system. The minimum fluid temperature is –9°C for January and the ambient temperature is selected as –13°C. These reference fluid temperatures are selected according to the number of hours in which the ambient temperature is very low. It provides an opportunity to store more cold energy into the storage and to reduce the pump work. After January, the ambient temperature starts rising but the inlet fluid temperature is still very low in order to store maximum cold energy into the storage system.

It is important to calculate the number of hours for each winter month with a low ambient temperature. By using typical metrological year data type TMY109 in TRNSYS, the number of hours is calculated for each month at a low ambient temperature as shown in Fig. 9.

From Fig. 9, it is clearly observed that pump will operate for the hours in which ambient temperature will be less than –1°C during the winter season and it will transport the heat transfer fluid into the underground storage. The maximum number of hours with the low temperature is calculated in January. After examining the temperature profile of each month, the system is designed for underground thermal energy storage.

4.3 Simulation tool

TRNSYS is a useful and famous system simulation program to simulate transient thermal systems. Type 557a, consisting of either a U-tube ground heat exchanger or a concentric tube ground heat exchanger, is selected in TRNSYS 16 to simulate the underground storage. A heat carrier fluid is circulated through the ground heat exchanger, which either rejects the heat to or absorbs the heat from the ground depending on the temperatures of the heat carrier fluid and the ground. The weather data for Dunhuang is analyzed by type TMY109 (Typical metrological year data) in TRNSYS. This component serves the main purpose of reading weather data at regular time intervals from a data file, converting it to a desired system of units and processing the solar radiation data to obtain tilted surface radiation and angle of incidence for an arbitrary number of surfaces. In this mode, Type 109 reads a weather data file in the standard TMY2 format. The TMY2 format is used by the National Solar Radiation Data Base (USA) but TMY2 files can be generated from many programs, such as Meteonorm.

The overall system is simulated from October to March in a charging mode and from June to August in a discharging mode. Type 114 in TRNSYS is selected for single stage single suction standard centrifugal pump. It models a single (constant) speed pump that is able to maintain a constant fluid outlet mass flow rate. The pictorial view of the TRNSYS model is plotted in Fig. 10.

5 Results and discussion

In this section, the total cold energy stored into the system during the entire winter season is analyzed and total thermal energy losses caused by different reasons are discussed. The average storage temperature is examined and its behavior in the charging and discharging mode is investigated. In the end, the fluid temperature profile, the COP of the storage system, and the techno-economic assessment analysis are elaborated.

5.1 Total cold energy stored in the system

A TRNSYS model is simulated according to the ambient weather conditions at Dunhuang in the winter season and BTES design parameters as mentioned in Table 1. Simulation results consist of the total amount of cold energy that is stored in the soil during the winter season. The total heat losses are also calculated for each month. These results are analyzed in origin Pro software. The total internal energy of the storage unit is the sum of the energy stored in the soil during each working hour of the system from October to March. The total amount of energy that is stored in each month is analyzed in Fig. 11. The entire BTES system is linked to its size. The system performance and behavior is examined by changing the design parameters in the complete cycle of operation.

In Fig. 11, the vertical bars show the total energy losses in each hour of operation when the system is working to store cold energy. It can be observed from Fig. 11 that the internal energy of the storage system is increasing as the outside air temperature at Dunhuang decreases. Hence, the design proposed shows the most promising results for cold energy storage. This internal energy value can be increased by increasing the depth of underground pipes but it will affect the drilling cost of the vertical ground heat exchanger. Moreover, the heat capacity of storage volume also affects the optimum storage design. A storage system with increasing heat capacity is capable of storing energy more densely and as a result, the total storage volume can also be reduced. The benefit of the smaller storage volume is to have less annual heat losses. It can be observed that the maximum input energy to the storage system can be provided in the peak winter season when the outside air temperature is very low because the pump will work to circulate the low-temperature fluid into the storage pipes to enhance the heat transfer between the fluid and the soil. Moreover, the thermal conductivity of the soil plays an important role in this case as it controls the internal heat flows of the storage system. It is easy to inject and extract energy from the underground storage system with the high thermal conductivity of the storage material. In this way, the optimum spacing between boreholes can also be increased, which means, few boreholes are required with a high thermal conductivity to achieve the required heat transfer rate. The storage system with a high value of thermal conductivity can sufficiently reduce the drilling cost. In the design proposed, the soil thermal conductivity has the optimum value of 2 W/mK which ensures the maximum heat injection and extraction rates with minimum heat losses.

It is noticed that the large value of the top soil layer thermal conductivity can cause an increment in total heat losses from the storage unit. The ideal design of underground thermal energy storage should be capable of reducing the heat losses from the top area of the storage system. So in this paper, the top soil layer thermal conductivity is selected as 1.2 W/mK. The heat losses are analyzed in Fig. 11 and it can be examined that constant borehole spacing with a topsoil layer thermal conductivity of 1.2 W/mK can reduce the heat losses, which will ultimately improve system performance. The total heat loss can be further reduced by increasing the top soil layer depth but this will affect the drilling cost and other parameters.

5.2 Average storage temperature

When the cold energy is charged into the storage volume in the winter season, the temperature of soil starts decreasing gradually due to the interaction of the borehole heat exchanger with heat carrier fluid circulating inside boreholes. It can be observed from Fig. 12 that the average storage temperature decreases. After six months of charging, the average storage temperature decreases significantly.

Figure 12 shows that the initial storage temperature is selected as 9°C in the analysis as it is also an average air temperature of Dunhuang. In the winter season, when ambient temperature decreases, the temperature of a storage unit also decreases accordingly. In March, the average storage temperature is analyzed at 2.5°C. This reveals that the storage system is working efficiently with the given parameters including borehole spacing, depth, and inlet fluid temperature. In this way, a certain amount of cold energy can be stored which can be utilized in summer months to fulfill cooling requirements of a solar thermal power plant and its accessories. In April and May, the storage unit cannot be operated to store cold energy due to the constant increase in the ambient temperature. It is observed that the maximum temperature is respectively 20°C and 24°C in April and May, which is lower than the maximum temperature from June to August. Therefore, in April and May, storage is not operated and condenser water is cooled by using the cooling tower with wet-recirculating or closed-loop mechanism as the ambient temperature is not so high in these two months and capable of exchanging the heat with warm water coming from the condenser exit valve with the minimum evaporation losses. In the summer season when the hot water of 40°C circulates through the BTES system, the storage temperature starts increasing. Figure 13 suggests that from June to August in the peak summer season, the storage temperature gradually increases because the cold energy is discharged from the storage unit. From October to March, the storage is working in the charging mode, while the system operates in discharging mode from June to August.

It can be seen from Fig. 13 that the storage temperature rises to 4°C in May because the ambient temperature starts rising from April and as a result, some heat is lost from the storage unit. The optimum design of underground thermal energy storage should be capable of reducing the heat losses from the top area of the storage system in these two months. Although the storage unit is covered with the insulation layer, there always occur some heat losses and the storage temperature rises from 2°C to 4°C in May. When the underground storage starts its operation from June and the hot fluid circulates through the borehole tubes, the temperature between the tubes rises slowly, and the soil between the tubes is also heated at the same temperature. Initially, due to the larger temperature difference between the storage unit and the inlet fluid from the condenser, the storage temperature rises up to 7°C in June and as a result, more heat is discharged in this month. However, increasing the insulation thickness and insulation hight fraction can further reduce the heat losses in April and May and as a result, the storage temperature will remain low till the start time of the discharging period. Further, during the initial years of operation, the BTES system is charged up with cold energy according to the operating temperatures. This process often involves recharging more cold energy to the ground than extracted. During these years the temperature field around the BTES changes; the ground surrounding the BTES gets colder and the temperature difference between BTES and surrounding ground decreases. At some stage, typically after 3–5 years, some kind of quasi-steady-state [31] will be achieved where the annual heat loss to the storage surface and surrounding ground will stabilize. The discharged energy is calculated using TRNSYS. It can be noticed from Fig. 14 that the maximum heat is discharged from the storage in June due to the high-temperature difference between the water and the storage unit. The negative values in Fig. 14 shows that the storage is charged with the cold energy in the winter season (October to March) as the air temperature decreases in this season and positive values show that the storage is discharged as the hot water from the condenser is circulated through the underground pipes in the peak summer season (June to August). The total cold energy stored in the charging mode is 5946.45 GJ, and the cold energy extracted in the discharge process is 3195.96 GJ. Approximately, 53.7% of the cold energy is used in the one-year operation. The percentage of charging and discharging of the storage unit varies according to the weather conditions such as the solar radiations and the ambient temperature.

The heat losses are calculated in the discharge period and Fig. 15 shows that the maximum heat loss occurs in June due to the highest ambient temperature conditions and the larger temperature difference between the storage and the surrounding atmosphere.

In the charging process in the winter season, the inlet fluid temperature is very low but it rises at the outlet of U-tube pipes after completing the cooling cycle. The temperature profile of the injected fluid at the inlet and outlet of the storage system is shown in Fig. 16.

5.3 Fluid temperature

In Fig. 16, the blue line shows the inlet temperature profile for circulating fluid in U-tube underground pipes and the red line shows the outlet temperature profile of the fluid for each month. The BTES system proposed operates in each winter month according to the given conditions in Table 2 and Fig. 9. So the inlet fluid temperature has a constant value and it will be circulated through the borehole pipes in the low ambient temperature hours for each winter month, which can be noticed during the storage working hours, as shown in Fig. 16.

It can be observed that the outlet fluid temperature increases after passing through borehole heat exchangers and also increases in the hours when the storage is not in operation. As the ambient temperature is not uniform in each hour and sometimes there are abrupt changes in ambient temperature, there are some oscillations in the outlet fluid temperature of the down-state of the system. However, in order to achieve accuracy, such abrupt changes have been ignored in the TRNSYS simulation. There is a certain difference between the inlet and the outlet fluid temperature, and in general, the storage efficiency increases with lower charging temperature and higher discharge temperature in the case of cold energy storage. For the months with the lowest ambient temperature, such as from December to February, an antifreeze brine solution can be mixed with water so that it would not freeze in the vertical ground heat exchanger. In this paper, ethylene glycol is mixed with water, which freezes at –42°C. In this way, the maximum cold energy can be stored in the peak winter season without facing the water freezing problem in the pipes. Figure 17 shows the outlet fluid temperature profile in the summer season from June to August. The inlet fluid temperature is a constant 40°C in the discharge period and outlet fluid temperature decreases after passing through the storage unit. It can be seen from Fig. 17 that the outlet fluid temperature at the end of August is not as low as it is at the beginning of June. To maintain the condenser performance, the fixed set point is considered to control the storage leaving water temperature in the summer season. The heat rejection varies with ambient temperature. The BTES system starts working in the summer season when the inlet water temperature is greater than 25°C. Moreover, the set point for the condenser inlet temperature is 30°C and condenser water outlet temperature is 40°C. This means that the condenser water temperature will vary between 25°C to 40°C depending upon the outdoor temperature. Temperature stabilization is useful to fully utilize the storage system in order to cool the condenser water temperature in the summer season.

5.4 System coefficient of performance

After calculating the head losses and total flow capacity, the pump power is calculated using Eq. (19). The 37 kW single stage single suction vertical centrifugal pump ISL200-150-250-Z is selected for the storage unit. Rated parametric details of the pump is given in Table 3.

The COP of underground storage system for each month is calculated separately, whose results are given in Fig. 18.

From Fig. 18 it can be analyzed that the COP value is more than 10 from October to March, which means that the underground storage system is working efficiently in the entire winter season. Moreover, the storage is able to accumulate more cold energy as the inlet fluid temperature is low and the maximum heat transfer occurs. It also reduces the pump work and as a result, the COP is increased.

6 Comparison analysis

A comparison analysis based on electrical power requirements among the seasonal cooling technique is proposed and the conventional dry cooling and wet cooling techniques of condenser are discussed in this section. The comparison analysis is performed to calculate per MW electricity consumption to fulfill the condenser cooling requirement at the solar thermal power plant. The method proposed is compared with the already available data in the literature based on the per MW electricity consumption among the seasonal cooling technique proposed, the wet heat rejection system equipped with draft cooling towers, and the air-cooled condenser at 137 MW parabolic trough solar power plant located at South West desert site Barstow, California [32]. The results are tabulated in Table 4.

It can be noticed in Table 4 that the electrical power requirement for per MW cooling is the lowest in the case of using BTES seasonal cooling mechanism as compared to the water-cooled and air-cooled condenser. Therefore, it can be concluded that the seasonal cooling systemproposed is 1.13% more efficient than the water-cooled condenser and 2% more efficient than the air-cooled condenser. This means that the seasonal cooling technique can enhance the efficiency of the solar thermal power plant up to 1.54% in the comparison with the water-cooled condenser and 2.74% in comparison with the air-cooled condenser cooling. The BTES system proposed is capable of reducing the electrical power consumption for cooling purposes and reducing the per megawatt generation cost of electricity in the peak summer season. Besides, it is also capable of saving certain amount of water that is used in the wet cooling technique of the condenser. Table 5 describes the comparison based on water consumption between the water-cooled condenser [32] and the cold energy storage system proposed.

It can be observed from Table 5 that the water consumption for per megawatt cooling is 568.8 g/min in the case of conventional wet cooling technique while it is 165.68 g/min for the seasonal cooling technique proposed, which verifies the practical feasibility of the seasonal cold energy storage technique proposed in reducing the water consumption required for the cooling purpose up to 403 g/min for per MW electricity generation. It can be inferred that the seasonal cooling technique proposed is efficient, cost-effective, and water saving.

6.1

The optimum BTES system sizing is based on system performance and economic evaluation. A good measure for the comparison of overall system performance is the levelized cost of heat (LCOH) [33]. For the system studied here, the LCOH can be defined as

(20)\mathrm{L}\mathrm{C}\mathrm{O}\mathrm{H}={\displaystyle \frac{\mathrm{T}\mathrm{otal}\hspace{0.17em}\mathrm{cost}\hspace{0.17em}\mathrm{over}\hspace{0.17em}\mathrm{lifetime}\hspace{0.17em}\mathrm{of}\hspace{0.17em}\mathrm{system}}{\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}\hspace{0.17em}\mathrm{energy}\hspace{0.17em}\mathrm{d}\mathrm{e}\mathrm{l}\mathrm{i}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{e}\mathrm{d}\hspace{0.17em}\mathrm{o}\mathrm{v}\mathrm{e}\mathrm{r}\hspace{0.17em}\mathrm{l}\mathrm{i}\mathrm{f}\mathrm{e}\mathrm{t}\mathrm{i}\mathrm{m}\mathrm{e}\hspace{0.17em}\mathrm{o}\mathrm{f}\hspace{0.17em}\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{m}}},

(21)\mathrm{T}\mathrm{otal}\hspace{0.17em}\cos \mathrm{t}\hspace{0.17em}\mathrm{over}\hspace{0.17em}\mathrm{lifetime}\hspace{0.17em}\mathrm{of}\hspace{0.17em}\mathrm{system}=C_{\mathrm{c}}+{\Sigma }_{t=\mathrm{1}}^n{}^{}{\displaystyle \frac{\left(C_{\mathrm{m}t}+C_{\mathrm{o}t}\right)}{{(\mathrm{1}+r)}^{{}^t}}},

(22)\mathrm{T}\mathrm{otal}\hspace{0.17em}\mathrm{energy}\hspace{0.17em}\mathrm{delivered}\hspace{0.17em}\mathrm{over}\hspace{0.17em}\mathrm{lifetime}\hspace{0.17em}\mathrm{of}\hspace{0.17em}\mathrm{system}={\Sigma }_{t=\mathrm{1}}^n{\displaystyle \frac{\left(E_{{}_t}\right)}{{\left(\mathrm{1}+r\right)}^{{}^t}}},

where C_c is capital cost, C_mt is the maintenance cost in a year, C_ot is the operational cost in a year, E_t is the energy delivered in year t, and r is discount rate.

The estimated capital cost and the annual cost during the first year of operation and the LCOH are given in Table 6, in which borehole drilling and installation costs is estimated to be $60/m [33]. The lifetime of the system is taken as 20 years with a discount rate of 5%. An electricity rate of $11.2 GJ is utilized with an annual increase of 3%. The maintenance costs are assumed to be 1% of the capital cost per year.

It can be noticed in Table 6 that larger sized BTES systems require more capital cost but much lower operational cost. The payback time of the BTES system can be calculated by using

(23)N_{\mathrm{p}}={\displaystyle \frac{l_{\mathrm{n}}\left[C_{\mathrm{s}}\right(i_{\mathrm{f}}-d)/F{Q}_{\mathrm{L}}{C}_{\mathrm{F}})+\mathrm{1}]}{l_{\mathrm{n}}\left[\right(\mathrm{1}+i_{\mathrm{f}})/(\mathrm{1}+d\left)\right]}},

where N_p is payback period, C_s is the total cost of equipment, Q_L is the total heat load (GJ), i_f is electricity inflammation rate, C_F is the cost of input electrical energy, and d is down payment.

The payback period for the BTES system proposed is estimated to be 20–25 years.

The net present value (NPV) of the BTES system proposed is estimated by using

(24)\mathrm{NPV}(i,N)={\displaystyle \sum _{t=\mathrm{0}}^N\frac{R_t}{{(\mathrm{1}+r)}^t}},

where R_t is net cash flow, N is the total number of periods, and t is the time of cash flow.

The NPV of the proposed BTES is estimated to be $3899533, which reveals that it is profitable to install the BTES system in order to cool the condenser of the CSP power plant in the summer season. Figure 19 is the comparison of levelized cost of energy for each cooling technique at the solar thermal power plant and it can be seen that the BTES cooling system provides low-cost cooling energy in order to cool the condenser water. Therefore, the techno-economic study of the BTES system proposed indicates that it is profitable and feasible to implement this technique to save water and to enhance the efficiency of the power plant.

7 Conclusions

An underground seasonal cold energy storage system for 10 MW solar thermal power plant is designed in the desert area in north-western China under typical weather conditions. The seasonal cooling mechanism proposed is designed to enhance the efficiency of the solar thermal power plant and to reduce the condenser water temperature in the peak summer season. The whole BTES system is designed and modeled using an operational strategy to utilize the lowest ambient temperature in the winter season and to store cold energy into the storage unit. In the analysis, many designs of solar thermal power plants and climatic conditions of respective areas are considered. Essential design conditions such as borehole spacing, total storage volume, the thermal conductivity of soil, and inlet fluid temperature are analyzed. The total number and depth of borehole heat exchangers are also affected by some important parameters, such as the thermal conductivity of the storage material. The maximum coefficient of performance of the underground TES system is 35 in January when the ambient temperature is very low. The average storage temperature decreases from 9.5°C to 2.5°C as the cold fluid is transported through the pipes from October to March. The proposed BTES system is capable of storing (6000±500) GJ of cold energy in the entire winter season. Approximately, 53.7% of the cold energy is used in the one-year operation to cool the condenser water in the summer season from June to August. The comparison analysis of various cooling techniques also verifies the practical implementation of the seasonal cooling technique proposed at a large scale. The seasonal cooling system proposed is 1.13% more efficient than the water-cooled condenser and 2% more efficient than the air-cooled condenser in term of electrical power requirements for different cooling techniques. Besides, it can also reduce water consumption to a large extent. Therefore, the cold energy storage technique proposed can be applied to cool the condensers at solar thermal power plants located in typical weather condition areas and to replace the forced cooling of the condenser with the seasonal cold energy storage mechanism. Moreover, the BTES storage unit is limited to operate for three months per year in the peak summer season as it needs specific BTES design to deal with exact boundary conditions and ambient temperature requirements. The payback period for the current BTES system is larger as compared to the conventional cooling methods but it will be suitable in designing future storage system based on the vertical ground boreholes for large-scale solar thermal power plants, residential, and office buildings in order to take the maximum advantage of the geo-coolingpotential.

References

[1] Berroug F, Lakhal E K, El Omari M, Faraji M, El Qarniac H. Thermal performance of a greenhouse with a phase change material north wall. Energy and Building, 2011, 43(11): 3027–3035

[2] Najjar A, Hasan A. Modeling of greenhouse with PCM energy storage. Energy Conversion and Management, 2008, 49(11): 3338–3342

[3] Bouadila S, Kooli S, Skouri S, Lazaar M, Farhat A. Improvement of the greenhouse climate using a solar air heater with latent storage energy. Energy, 2014, 64: 663–672

[4] Sarbu I, Sebarchievici C. Solar Heating and Cooling: Fundamentals, Experiments and Applications. Oxford: Elsevier, 2016

[5] Schmidt D, Mangold D, Mülller-Steinhagen H. Central solar heating plants with seasonal storage in Germany. Solar Energy, 2004, 76(1–3): 165–174

[6] Kuravi S, Trahan J, Goswami D Y, Rahman M M,Stefanakos E K.. Thermal energy storage technologies and systems for concentrating solar power plants. Progress in Energy and Combustion Science, 2013, 39(4): 285–319

[7] Liu M, Steven Tay N H, Bell S, Belusko M, Jacob R, Will G, Saman W, Bruno F. Review on concentrating solar power plants and newdevelopments in high-temperature thermal energy storage technologies. Renewable & Sustainable Energy Reviews, 2016, 53: 1411–1432

[8] Xu X, Luo F, Wang W, Hong T, Fu X. Performance-based evaluation of courtyard design in China’s cold-winter hot-summer climate regions. Sustainability, 2018, 10(11): 3950

[9] Moore J, Grimes R, O’Donovan A, Walsh E. Design and testing of a novel air-cooled condenser for concentrated solar power plants. Energy Procedia, 2014, 49: 1439–1449

[10] de Gracia A, Cabeza L F. Phase change materials and thermal energy storage for buildings. Energy and Building, 2015, 103: 414–419

[11] Kumar A, Shukla S K. A review on thermal energy storage unit for solar thermal power plant application. Energy Procedia, 2015, 74: 462–469

[12] Sarbu I, Sebarchievici C. A comprehensive review of thermal energy storage. Sustainability, 2018, 10(2): 191

[13] International Renewable Energy Agency (IRENA). The Energy Technology Systems Analysis Programmes (ETSAP): Technology Brief E17. Paris, France, 2013

[14] Cruickshank C A, Baldwin C. Sensible thermal energy storage: diurnal and seasonal. In: Letcher T M, ed. Storing Energy, 2016: 291–311

[15] Lundh M, Dalenback J O. Swedish solar heated residential area with seasonal storage in rock: initial evaluation. Renewable Energy, 2008, 33(4): 703–711

[16] Sibbitt B, McClenahan D, Djebbar R, Thornton J, Wong B, Carriere J, Kokko J. The performance of a high solar fraction seasonal storage district heating system–five years of operation. Energy Procedia, 2012, 30: 856–865

[17] Wu W, You T, Wang B, Shi W, Li X. Evaluation of ground source absorption heat pumps combined with borehole free cooling. Energy Conversion and Management, 2014, 79: 334–343

[18] Eicker U, Vorschulze C. Potential of geothermal heat exchangers for office building climatisation. Renewable Energy, 2009, 34(4): 1126–1133

[19] Lund J, Sanner B, Rybach L, Curtis R, Hellstrom G. Geothermal (ground source) heat pumps, a world overview. Oregon: Oregon Institute of Technology, 2004, available at the website of oit

[20] Sanner B, Karytsas C, Mendrinos D, Rybach L. Current status of ground source heat pumps and underground thermal energy storage in Europe. Geothermics, 2003, 32(4–6): 579–588

[21] Pahud D, Belliardi M, Caputo P. Geocooling potential of borehole heat exchangers’ systems applied to low energy office buildings. Renewable Energy, 2012, 45: 197–204

[22] REN 21. Renewables 2018 Global Status Report (Paris: REN21 Secretariat). 2018, available at the website of ren21

[23] Li Z S, Zhang G Q, Li D M, Zhou J, Li L J, Li L X. Application and development of solar energy in the building industry and its prospects in China. Energy Policy, 2007, 35(8): 4121–4127

[24] Crompton P, Wu Y. Energy consumption in China: past trends and future directions. Energy Economics, 2005, 27(1): 195–208

[25] Yang L, Lam J C, Tsang C L. Energy performance of building envelopes in different climate zones in China. Applied Energy, 2008, 85(9): 800–817

[26] Geng Y. Improve China’s sustainability targets. Nature, 2011, 477(7363): 162

[27] Li X G, Chen Z H, Zhao J. Simulation and experiment on the thermal performance of U-vertical ground-coupled heat exchanger. Applied Thermal Engineering, 2006, 26(14–15): 1564–1571

[28] Lanini S, Delaleux F, Py X, Olivès R, Nguyen D. Improvement of borehole thermal energy storage design based on experimental and modeling results. Energy and Building, 2014, 77: 393–400

[29] Pahud D, Hellström G, Mazzarella L. DUCT GROUND HEAT STORAGE MODEL: Manual for Computer Code. Lund: University of Lund, 1989

[30] Ingersoll L R, Plass H J. Theory of the ground pipe heat source for the heat pump. Heating, Piping, and Air Conditioning, 1948, 54(7): 339–348

[31] Beier R A, Smith M D, Spitler J D. Reference data sets for vertical borehole ground heat exchanger models and thermal response test analysis. Geothermics, 2011, 40(1): 79–85

[32] Kelly B. Nexant parabolic trough solar power plant systems analysis, task 2 comparison of wet and dry Rankine cycle heat rejection. Technical Report: National Renewable Energy Laboratory NREL/SR-550–40163, 2006

[33] Semple L, Carriveau R, Ting D S K. A techno-economic analysis of seasonal thermal energy storage for green house applications. Energy and Building, 2017, 154: 175–187

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