Solar community heating and cooling system with borehole thermal energy storage – Review of systems

Renewable and Sustainable Energy Reviews 60 (2016) 1550–1561

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Solar community heating and cooling system with borehole thermal energy storage – Review of systems Farzin M. Rad n,1, Alan S. Fung 1 Department of Mechanical and Industrial Engineering, Ryerson University, 350 Victoria Street, Toronto, Ontario, Canada M5B 2K3

art ic l e i nf o

a b s t r a c t

Article history: Received 14 April 2015 Received in revised form 5 September 2015 Accepted 4 March 2016

There is a substantial need to accelerate the advancement and implementation of advanced clean energy technologies to solve challenges of the energy crisis, climate change, and sustainable processes. Solar heating and cooling technologies are feasible solutions among clean energy technologies. This paper presents a detailed literature review on studies performed around the solar district energy systems with integrated thermal storage. They are mainly either for heating or cooling. The combined district heating and cooling system with both systems integrated with borehole thermal energy storage (BTES) has not been fully explored. A low-temperature distribution fluid, suitable for use in distributed heat pumps around the community with BTES, has also not been practically installed yet. Such system, could reduce the transmission/distribution heat loss within the community, and lower the required amount of energy production and storage, compared to the other systems. This could make the entire system technoeconomically more attractive while not compromising energy efficiency of the system. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Solar community heating and cooling Seasonal thermal storage Solar community cooling Modeling and design

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Underground seasonal thermal energy storage technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Hot Water Thermal Energy Storage (HWTES) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Gravel-water thermal energy storage (GWTES) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Aquifer thermal energy storage (ATES). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Borehole thermal energy storage (BTES) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Comparison of underground seasonal thermal storage concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Seasonal thermal storage through BTES – high and low-temperature design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Field experiences with BTES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Solar communities with cooling systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Examples of installation of solar assisted cooling system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Solar assisted heating and cooling system with seasonal thermal energy storage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Conclusion – further research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Solar heat can substantially contribute to the world energy need. In 2009, International Energy Agency (IEA) reported that n

Corresponding author. E-mail address: [email protected] (F.M. Rad). 1 Tel.: þ1 416 979 500x4917.

http://dx.doi.org/10.1016/j.rser.2016.03.025 1364-0321/& 2016 Elsevier Ltd. All rights reserved.

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“global energy demand for heat represented 47% of final energy use, higher than final energy for electricity (17%) and transportation (27%) together.” IEA (2012) technology roadmap for solar heating and cooling predicts that, by 2050, more than 16% of the total final energy use for low-temperature heat ( o100 °C) and 17% of the total energy use for cooling will be from solar source [1].

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Nomenclatures C DT DTL DTV DT Dθ DθL DθV Dθ Dε Dε HL K Kn L Lf ku n qadv qet qh qli qlr qn qsi qsr

volumetric heat capacity of soil (J/m3K) thermal moisture diffusivity (kg/msK), DT ¼DTL þDTV thermal water diffusion (kg/msK) thermal vapor diffusion (kg/msK) thermal moisture diffusivity in freezing soil (kg/msK) isothermal moisture diffusivity (kg/msK), Dθ ¼DθL þDθV water diffusion caused by moisture gradient (kg/msK) vapor diffusion caused by moisture gradient (kg/msK) isothermal moisture diffusivity in freezing soil (kg/msK) ¼ LερlDθ ¼ LερlDθn specific evaporation enthalpy of water (J/kg) soil hydraulic conductivity (m/s) soil hydraulic conductivity in freezing soil (m/s) latent heat of vaporization of water (kJ/kg) latent heat of fusion of water (kJ/kg) unsaturated water conductivity (m/s) normal outward direction advection heat gain due to rainfall (W) heat flow density by evapotranspiration (W) heat flow density by sensible heat from air (W) heat supplied by long-wave radiation from atmosphere (W) heat emission and reflection of long-wave radiation (W) heat flow density from the ground (W) heat supplied by incoming short-wave radiation from sun (W) heat loss due to reflection of incoming short-wave radiation (W)

According to European Solar Thermal Technology Platform report (2007), around 9% of the total heating needs in Europe are supplied by community and district heating systems. Based on this report, in renewable heating and cooling portfolio, solar thermal (ST) has following unique specific benefits [2]: 1. ST always helps to reduce primary energy consumption; 2. ST can be used as a hybrid with almost all kinds of back-up heat sources; 3. ST is a source of renewable heating and cooling technologies and relies on infinite resource; 4. ST can reduce electricity demand, which could reduce investments and increase power generation and transmission capacities; 5. ST could be nearly everywhere with some limitations at very high latitudes; 6. ST prices are almost predictable and do not depend on future of other energy prices such as oil, gas, biomass, or electricity prices and; 7. The life-cycle environmental impact of ST systems is substantially low. For these reasons, solar thermal could be the absolute best option for satisfying the long-term heating and cooling energy supply. In Europe, in large-scale, the first solar heating plants were built 30 years ago, and the first solar cooling plants were constructed over ten years ago. From 1979 to the middle of 2011, 141 heating, and 13 cooling plants were built all of which have more

R S t T Tf Ts Tsp Z

ε Φ λ λn θ ρ υet υm υn υr

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thermal resistance (m2K/W)

ρi ∂θi ρl ∂t

time (s) temperature (°C, K) ground pipe fuel temperature (°C, K) average soil temperature at two radius distance away from the pipe (°C, K) temperature of the soil at soil-pipe boundary (°C, K) elevation (depth) (m) phase conversion factor total potential for moisture flow thermal conductivity of soil (W/mK) λ þ LερlDT (W/mK) volumetric moisture content (m3/m3) density (kg/m3) flux density of evapotranspiration (kg/m3s) snow melt water flux (kg/m3s) moisture flux from ground (kg/m3s) flux density of rainfall (kg/m3s)

Subscripts c co f i l pf pw s tc v

cooling convective frozen ice liquid partly frozen pipe wall soil contact at soil-pipe interface vapor

than 500 m2 solar collector area or greater than 350 kW nominal thermal power [3]. The development began two decades ago when large-scale systems started to be built in number of countries. The new solar heating plants have large collector areas. As a result, the markets for large-scale solar collectors have been increasing since 2007 in spite of a modest increase of such plants per annum. In community scale, when solar thermal collectors are used, for some period of times, during the season or a year, there is an inconstant availability of the solar energy which is required for heating or cooling system. There are times when solar energy is not needed, or thermal energy production is more than the demand. In order to capture the thermal energy produced during the unwanted time, thermal energy storage would be a solution as an integrated part of the whole district system. The thermal energy stored could be used during the time when the solar fraction is low, like winter time or during the nights. Thermal storage helps to utilize the maximum solar energy harvested in seasons. It also could balance the community energy demand versus thermal heat generation through solar collectors. In the Combined Heat and Power (CHP) system, thermal energy storage could decouple the thermal production from the electricity production. While the CHP system generates heat and electricity simultaneously, the unwanted or surplus heat production can be stored and saved for the time when the thermal energy is required [4]. Since 1970, the seasonal thermal storage technology, as part of a district heating system, has been under exploration and inspection. The purpose of all investigations and studies was storing heat at the

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time that is not needed and use it for the time that is required. Schmidt et al. had a detailed review of advances in seasonal thermal energy storage in Germany [5,11]. Based on their research, with reference to Dalenback and Lundin, the first solar plant was constructed in Sweden in 1978/79 [6,7]. Several countries, participated in central solar heating plants with seasonal storage working group under International Energy Agency (IEA) Task VII, since 1979. The aim of this group was to boost the progress of large-scale solar heating technologies like district heating systems. Under this working group, participant members exchanged their experiences and shared their activity results. In Germany, the first seasonal heat storage was built at the University of Stuttgart in 1985. The storage at this facility was a gravel-water heat store type [8]. Solar heat is collected by a large area of solar collectors and then transferred to the central heating plant. The excess heat from solar panels, in summer, is directed to the thermal storage. In the heating season, the stored heat will be directed to the central plant to supply to the district heating system. Following this project, “Solarthermie-2000,” a governmental program in Germany was introduced for researching in solar heating plants with large seasonal thermal energy storage since 1993. Fig. 1 shows a system schematic with direct and indirect space heating. Eight of such plants have been built in Germany for demonstration within “Solarthermie-2000” since 1996. They were all designed for 35–60% solar fractions of the total heat demand for space heating and domestic hot water of homes annually [9]. The objective of this study is to review and investigate the previous studies around the solar communities integrated with seasonal thermal storage. The main focus is on seasonal borehole thermal storage that has been used in such communities for both heating and cooling applications separately and simultaneously. The result would

be finding the gaps and opportunities for the further research on possible system configuration to enhance the performance and reduce the initial and operational costs of the system.

2. Underground seasonal thermal energy storage technologies Seasonal thermal energy storage stores heat in a sensible form. The main parameters that need to be dealt with, for finding the heat transfer and mainly losses through the storage are thermal properties of the storage medium, time of storage, storage temperature, storage geometry, and volume. In community and district solar-energy heat-storage, the storage volumes are relatively large. Therefore, ground storage due to their lower cost as well as the ability to deal with large time scale makes this technology the most promising medium [10]. Schmidt et al. and later many other researchers, presented four types of sensible seasonal energy storage that have been in operation in different plants in Germany. They are; 1) hot-water thermal energy storage (HWTES), 2) aquifer thermal energy storage (ATES), 3) gravel-water thermal energy storage (GWTES) and 4) borehole thermal energy storage (BTES) [5,11–13]. 2.1. Hot Water Thermal Energy Storage (HWTES) HWTES are built almost independently from geological conditions; the storage is usually constructed of reinforced concrete or steel. The storage media is water, which has good values for specific heat capacity and the power rate for charging and discharging. Fig. 2 shows two methods that were used in HWTES construction in Friedrichshafen and Hannover in Germany. The one in

Fig. 1. Central solar heating plant with seasonal storage [5].

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Fig. 2. Hot water heat stores in Friedrichshafen (left) and Hannover (right) [5].

Hannover was built recently with a new high-density concrete without inner steel-liner [5]. In the Friedrichshafen storage, which is older than Hannover model, there are only two locations, at top and bottom, for charging and discharging whereas in the Hannover model, another point of charging or discharging has been introduced. This point is located at one-third of the distance from the top of the storage medium height and provides an optimized flexibility for using different water temperatures at various layers of the stratified store. In Hannover, a granulated foam glass in textile bags is used for insulation. This kind of insulation material, comparing to the regular mineral wool, has an excellent drying capability plus easier and faster installation work. As high-density concrete is not able to prevent steam diffusion, a layer of the steam barrier is installed between insulation and concrete layer. 2.2. Gravel-water thermal energy storage (GWTES) Gravel-water thermal storage is a less-expensive version of tank storage, which is generally buried in the ground. These kinds of storage are mostly insulated on the side and the top. The storage media are normally a gravel and water mixture, which could also be sand or soil mixture with water [65,66]. Heat extraction or injection could be either through direct water heat exchanger or by indirect heat transfer through piping installed at different layers of the store. The pipes, in this case, are made of plastic for their longevity. The storage liner is usually made of advanced polymer material backed up with insulation. Because of the construction material used, the operating temperature is limited to less than 95 °C [14]. The specific heat of this type of store media is lower than that of water and subsequently, the store size should be constructed approximately 50% bigger to store the same amount of heat compared to the water storage tanks [5,11]. Table 1 shows the thermal and physical properties of storage media for the HWTES and GWTES, and Table 2 lists the advantages and disadvantages of the two systems [12,15]. 2.3. Aquifer thermal energy storage (ATES) Aquifers are recognized as a porous media saturated with water that the media could be sand, gravel, sandstone, igneous or metamorphic rock [16]. If the aquifers are confined, means there would be low water flow (or no flow), then they could be used as a thermal storage. For injecting heat to or extracting heat from the storage media, two or several wells are drilled into the aquifer. During charging time, water extracted from the aquifer, is heated through heat-exchanger and is sent back to the aquifer through another well that is located at some distance from the supply well.

Table 1 Thermal and physical properties of water vs. gravel-water [12].

Porosity Density [kg/m3] Specific Heat Capacity [kJ/(kgK)] Thermal Conductivity [W/(mK)]

Water (at 20 °C)

Gravel-water mixture

– 992.2 4.18 0.63

0.37–0.43 1950–2050 2.0–2.2 1.8–2.5

Table 2 Hot water thermal energy storage (HWTES) vs. gravel-water thermal energy storage (GWTES) [15]. HWTES Advantages Thermal capacity Operation characteristic Thermal stratification Maintenance and repair Disadvantages Expensive sore cover High static requirements to cover loads Cost of removing the excavated soil

GWTES

Low static requirements Simple store cover

Thermal capacity Charging system Additional buffer storage Maintenance and repair not possible Gravel cost

At the discharging time, the flow is reversed from the two mentioned wells. Fig. 3 shows a typical ATES system. In order to get the most benefit from the heat storage, all the physical and chemical parameters of the aquifer should be investigated. In the hightemperature storage understanding the microbiology, geochemistry and mineralogy of the ground will play a significant role in thermal storage design. The heat loss from the ATES could be substantial, especially for the high-temperature storage media. In order to have a minimum heat loss, the surface to volume ratio should be as low as possible. This would apply especially to storage volume with more than 100,000 m3 [5]. For example, there is an ATES in Germany, which is located in Berlin that supplies to the German parliament building. It has two separate storages: 1) cold storage in depth about 60 m and 2) a hot storage in depth below 300 m [16]. Ghaebi et al. studied the integration of various ATES with heat pump and solar collectors for heating and cooling of a community. They found that the system annual coefficient of performance (COP) will increase when the ATES is used for both heating and cooling [67].

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2.4. Borehole thermal energy storage (BTES) In borehole thermal energy storage method, the ground itself would be the storage media. This would be through a number of vertical boreholes in the ground. The storage volume is not exactly separated. Geological formation plays a significant role in defining the thermal capacity of the storage. Normally rock or watersaturated soil is the most suitable. The vertical boreholes lengths are usually in the range of 30–100 m with approximately 3–4 m separation [5]. The borehole depths in recent installations have gone up to 200 m [12]. In the borehole, the heat is exchanged through a double or single U-pipes or concentric pipes. The pipe material is typically made of synthetic material like high-density polyethylene (HDPE). Fig. 4 shows common borehole heat exchangers types and sample installation. The fluid in the pipes is mostly water in some cases; to avoid freezing, the water is mixed with ethanol or glycol. The boreholes are filled with grout that normally is of bentonite, quartz with sand or only water mixture (Northern Europe). Quartz gives the grout a higher thermal conductivity whereas the bentonite provides a sealing and plugging characteristic. The grouted boreholes heat transfer properties have been studied theoretically by Bennet et al. [17] and Hellstrom [18]. They have been tested in laboratory measurements by Paul [19] and field measurements by Austin [20]. The range of thermal

conductivities of a typical filling material is: stagnate water (0.6 W/mK), Bentonite (0.8–1.0 W/mK), thermally enhance grout with quartz (1.0–1.5 W/mK), and water saturated quartz sand (1.5– 2.0 W/mK). The storage volume of BTES comparing to the HWTES is much bigger in size. Depending on the ground formation, BTES size should be three to five time larger than the HWTES [12]. For instance for a BTES with a cylindrical earth volume of 35,000 m3 that contains 144 boreholes with the 38 m depth, the equivalent HWTES storage volume of 8700 m3 can be used. Unused or rejected heat is injected into hot fluid circulation in the boreholes and transferred to the ground for storage. BTES works in seasonal and periodic modes. When the stored heat is needed, the cold fluid is circulated to absorb the heat that is required. The best and most efficient BTES is with high thermal conductivity adjacent to the boreholes and pipes and less formation thermal conductivity away from the storage volume with no groundwater flow [21]. Lower formation thermal conductivity away from the storage volume causes less storage heat loss. Claystone or water-saturated claystone are suitable media due to the high heat capacity and at the same time preventing considerable water flow. The BTES efficiency is defined as the ratio of the annual energy injected into and extract from the ground. The efficiencies of the fully charged BTES in the existing installations are mostly in the range of 40–60%. That means the BTES energy losses are around 60% to 40% of the energy injected into the ground. One advantage of BTES from the other types of the storage system is that the size of the storage can be easily extended by drilling addition boreholes and simply connecting the pipes to the existing boreholes. 2.5. Comparison of underground seasonal thermal storage concepts

Fig. 3. Aquifer thermal energy storage (ATES) [1].

The cost advantage due to the size and scale of operation is the reason that makes most of the thermal storage project, technically and economically, viable. For example, BTES is used in heating large district system in Europe since 1990. From that time, Fisch et al. studied two large-scale solar communities in the heating

Fig. 4. Different types of borehole heat exchangers [5].

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only application [22]. The investigation was on 1) short-term thermal storage system to supply 10–20% of heating demand or 50% of domestic hot-water use annually, and 2) long-term thermal storage to provide 50–70% of heating demand annually. In their report, it was concluded that large-scale solar thermal system (seasonal), because of the economy of scale, was three times more economical comparing to the short-term (diurnal) storage, used for just a single-family house. Since 1993, “Solarthermie-2000,” a research and development program in Germany, has been focusing on the study and monitoring large-scale solar thermal plant. Based on the results of this program, Lottner et al. explored the economics of such plants [23]. The study concluded that because of the high cost of the seasonal storage, more efforts should be made to achieve viable systems in both technical and economic aspects. Schmidt et al. performed an extensive study on the same subject in Solarthermie 2000 program reports. They provided some advices about how to design an optimized system to make the system more efficient and economical [5]. Bauer et al. also explored the monitoring data in the same program [24]. He described different thermal storage types about the solar district systems and compared the particular characteristics of different storage types. Schmidt et al. presented the principal components of various methods for seasonal heat storage that is summarized in Table 3 [5]. Hesaraki et al. also presented a summary table of advantages and disadvantages of different storage methods [68]. For selecting a specific storage system, all relative conditions need to be considered, such as geological requirements, storage size, heat capacities of the storage medium, etc. The final decision should be based on an optimum economical viable case, among all possibilities.

3. Seasonal thermal storage through BTES – high and lowtemperature design In the seasonal thermal energy storage, especially in solar thermal district energy system, there is a substantially large amount of energy involved. Therefore, the ground has been found to be a favorable media for storing such a large energy amount with a relatively low cost. One of the storage types, which use the ground directly, is BTES [25]. In BTES, heat transfer process in the ground can be considered from the nearby boreholes and the surrounding ground collectively. It is also called micro-scale and the macro-scale process by Nordell et al. [25]. The heat flow in the storage is responsible for the losses from the store. The amount of the heat losses depends on the storage geometry such as its size and shape, the average store temperature, and ground properties. The ground thermal resistance around the boreholes will also depend on the borehole spacing. The spacing, usually, is considered to be uniform and

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equally distanced. In reality, irregularity in borehole spacing could happen. A study by Hellstrom shows that such irregular spacing has a small effect on the ground thermal resistance if the storage volume is kept almost the same [18]. The storage heat losses will substantially increase if ground water flow exists. This flow could be either or both regional flow and flow due to natural convection. van Meurs investigated the ground water flow effect through numerical analysis on a porous medium with uniform hydraulic property [26]. He found that if the ground water flow exceeded 0.05 m/day (18 m/year), then the heat storage volume needed a protecting hydraulic screen. The ground water flow subject to natural convection will take place in the vertical direction. The flow amount due to the buoyancy primarily depends on the temperature difference between the storage and the surrounding medium. Other parameters such as the vertical depth of the store and permeability of ground material will also affect the magnitude of the natural convection flow [27]. Lund, through numerical studies, shows that if the intrinsic permeability of the ground exceeds 10  12 m2 then, the thermal storage performance will be affected. However, the natural convection flow will be reduced if impermeable layers exist in the horizontal direction [28]. Seasonal thermal storage namely BTES are categorized by 1) lowtemperature (0–40 °C) ground storage and 2) high-temperature (40– 80 °C) ground storage. Thermal energy is extracted either indirectly by the heat pump with low-temperature storage or directly with high-temperature storage, for delivering to the consumers [29]. For stores with limited thermal conductivity, Reuss et al. claims that the heat losses from the storage are rather moderate, and the storage efficiency could reach up to 70% [29]. On the contrary, a proper heat transfer rate, per unit of the area of the heat exchanger pipes, is also needed. Consequently, a good thermal contact between the exchanger and the ground is required. In loose and unconsolidated soil, the heat capacity and thermal conductivity of the soil strongly depend on the water contents, especially when the soil temperature goes beyond 60 °C. Soil water content can be lost due to the vapor diffusion and high-temperature gradient. This will lead to dry out of the soil causing cracks along the side of the heat exchanger pipes. Subsequently, the thermal resistance of this region increases and heat transfer rate will decrease. Reuss et al. studied numerically and experimentally the heat and moisture transfer in BTES in unsaturated soil [29]. High-temperature thermal energy input, in the range of 70–90 °C, into the ground can cause a substantial hightemperature gradient and simultaneous moisture flow close to heat exchanger pipes. The total heat transfer is through conduction into the soil and partly by conduction that is created by water, air and vapor movement. The theory of thermally driven soil moisture transfer was developed by Philip and De Vries [30]. This theory was never

Table 3 Comparison of different storage concepts [5].

Storage medium Heat capacity (kW h/m3) Storage volume for (1 m3 of water equivalent) Geological requirement

HWTES

GWTES

BTES

ATES

Water 60–80 1 m3

Gravel-water 30–50 1.3–2 m3

Ground material 15–30 3–5 m3

Ground material/water 30–40 2–3 m3

 Stable ground

 Stable ground conditions Preferably no groundwater 5–15 m deep

     

 Natural aquifer layer with high hydra-

conditions Preferably no groundwater 5–15 m deep

 

 



Drillable ground Groundwater favorable High heat capacity High thermal conductivity Low hydraulic conductivity Natural ground-water flow o 1 m/s 30–100 m deep

ulic conductivity

 Confining layers on top and below  No or low natural groundwater flow  Suitable water chemistry at high temperatures

 Aquifer thickness 20–50 m

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verified for very high-temperature medium. In his methodology, the effect of the soil temperature on vapor pressure and surface tension is considered. This will be the driving force for the vapor diffusion and liquid moisture flow. A computer model was developed by Reuss et al. to model and simulate soil heat and moisture transfer in high-temperature ground heat storage [29]. The model contains two parabolic differential equations with variables of volumetric moisture content and temperature [31]. For the moisture transport, Eq. (1) takes into the account the liquid moisture and vapor movement due to the moisture and thermal gradient. The energy transport Eq. (2) is based on Fourier's Law and considers the latent heat transport by condensation and evaporation of water within the porous medium [30]. 
 ∂θ ∂k ρL L ¼ ∇½ðDTL þ DTV Þ∇T  þ ∇ DθL þ DθV ∇θL þ ρL u ∂t ∂z C


∂T ¼ ∇ λ∇T þ∇ðH L DθV ∇θL Þ ∂t

ð1Þ ð2Þ

Reuss et al. developed a numerical model for these differential equations by using finite-difference method (FDM) as a general analytical solution was not possible. In addition, the contact resistance between the pipes and the ground is also included, to consider the already mentioned drying effects [29]. The model was validated for an entire temperature range of 0–90 °C with the data from several laboratory and field experiments. A new pilot plant was designed by using this method. By changing the various parameters, an optimum system from a technical and economic point was determined. The system was a 15,000 m3 seasonal storage with 140 boreholes of 30 m length each utilizing the waste heat generated in summer by a combined heat and power generation plant with 174 kWth capacity. The thermal energy of 418 MW h was charged into, and 266 MW h was discharged from the storage annually. The economic analysis shows that prices of specific energy with this system are almost the same as those of conventional energies like liquid–gas while 266 MW h/year of useful energy are saved. The minimum and maximum mean storage temperatures were 40 °C and 72 °C, respectively. The storage efficiency was found to be 64%. Tarnawski and Leong developed a computer program to simulate the performance of an entire ground source heat pump system [32,33]. A detailed numerical solution incorporates the following: 1. Simultaneous heat and moisture transfer in ground heat storage. The equations solved by the finite-element method. 2. Steady-state heat pump unit model. 3. House heating and cooling loads. 4. Detailed climatological data. 5. A profile of the initial soil moisture content and temperature. Temperature change of the fluid circulating in the closed-loop system is calculated by the energy balance and heat transfer between the circulating fluid and surrounding soil. It is assumed that the ground heat storage around the borehole has symmetry along the horizontal axis; thus, soil temperature and moisture pattern in the ground heat storage are calculated only in the axial and radial directions that change the problem to a twodimensional problem. The developed computer program can take into account a large number of issues, which are usually ignored for a simpler analysis. Main processes that have been addressed in this program are highlighted as follows: 1. In ground heat-storage coupled heat and moisture flow. 2. During heat extraction and heat injection, soil freezing–thawing and drying-rewetting.

3. With the presence of the ground water table, different soil types and layers. 4. Ground-surface effects, i.e., radiation, convection, advection, evapotranspiration and snow cover. The following governing equation, describing simultaneous heat and moisture flow, have been used for the ground buried heat exchangers [32,33]. Unfrozen soil
∂T
∂  ∇ λ ∇T þ ∇ Dε ∇θl  c þ Lρl ðεK Þ ¼ 0 ∂t ∂z

ð3Þ


∂θ ∂ ∇ðDT ∇T Þ þ ∇ Dθ ∇θl  þ ðK Þ ¼ 0 ∂t ∂z

ð4Þ

Freezing soil  
∂T ∂  cpf Lf ¼ ∇ λpf ∇T þ ∇ Dε ∇θl þ Lρl ðεK  Þ ∂t ∂z

∂ ∂θ l þ S ¼ ∇ DT ∇T þ ∇ Dθ ∇θl þ ðK  Þ ∂z ∂t

ð5Þ ð6Þ

Frozen soil   ∂T  ¼ ∇ λf ∇T cf ∂t

ð7Þ

∂θ l ¼0 ∂t

ð8Þ

Boundary condition Ground surface: ∂T þ qn ¼ 0 ∂n

λ

K

ð9Þ

∂ϕ ∂T vn þ DT þ ¼ 0 ∂n ρl ∂n

ð10Þ

qn ¼ qsr  qsi þ qlr  qli þ qet  qh  qadv

ð11Þ

vn ¼ vet  vr vm

ð12Þ

Ground heat exchanger's soil-pipe boundary (quasi-steady): T sp ¼ T f  ðT f  T s Þ

Rco þ Rpw þ Rtc Rco þ Rpw þ Rtc þ Rs

ð13Þ

The soil moisture transport model was obtained from the Philip and De Vries [30] and Thomas [34]. Clapp and Hornberger [35] and Campbell [36] provided the field experiment test. The site ground condition and weather and climate data, such as solar radiation, ambient temperature, rainfall, wind speed, snow density, snow cover, and water vapor pressure, are used for the boundary conditions at the ground surface simulation. The program can model and simulate multiple year operations of a ground source heat pump system in the heating and cooling mode. The computer program is written in FORTRAN 77 and can be run on a broad range of computers. The computer program has been modified and imported into TRNSYS 16 [37] software as a component Type 201a in 2009 [38]. For the thermal analysis of BTES, a number of tools have been developed so far. The main purpose of these tools is to design such complex systems optimally and cost-effectively. The available tools are varying from an uncomplicated design tool to advanced simulation modeling with hourly climate data and the detailed load data. The model should consider the relatively large heat flow in the ground and the heat transfer in and adjacent to the boreholes to capture the relationship between the temperature of the heat transfer fluid and the total storage heat transfer rate with a suitable time resolution [10].

F.M. Rad, A.S. Fung / Renewable and Sustainable Energy Reviews 60 (2016) 1550–1561

Eskilson and Claesson proposed a model in finite-difference, a superposition borehole model (SBM), which was a detailed model that could accept arbitrarily placed vertical or horizontal boreholes [39,40]. This model was examined and validated in several field experiments [18,40] and was used to calculate the thermal performance of the heat pump coupled system, in software such as EED [41,42] and GLHEPRO [43]. This model calculates dimensionless thermal response functions for different borehole configurations. Another simulation model, by Hellstrom, is duct ground heat storage model (DST) [44]. It is a simulation model for multiple boreholes with uniform borehole spacing. It has been used for both detailed design and field experiments evaluation extensively. Both the SBM and DST models have been modified for use as TRNSYS components. The TRNSYS version of DST model can also investigate problems within the storage volume, such as the radial stratification of the ground temperatures or the effect of the flow conditions in the borehole pipe on the thermal performance of the system [55]. The accuracy of the DST has been verified with very detailed and timely simulations of the heat transfer between a borehole storage unit and a solar collector field [45]. 3.1. Field experiences with BTES As per Nordell et al., the first BTES experiments were built around 1976 in Sweden and France [25]. The only high-temperature largescale BTES was built in Lulea, Sweden, in 1982. Table 4 lists examples of borehole heat storage systems in Sweden. Pal et al., in 1997, investigated the world's largest BTES (1,080,000 m3) in operation, which was located at Stockton State College, Pomona, NJ, USA, with 400 boreholes at the depth of 135 m. The BTES has been used for part of the college heating and cooling demand since 1995 [46]. Table 5 shows the technical characteristics of some demonstration plants with solar thermal collectors and BTES followed by Table 4 Examples of Swedish BTES [25]. Location

Sigtuna Lulea Finspang Solna Finspang Marsta Stockholm Kristinehamn Stockholm Jarfalla Storforsen

Purpose

Build Storage volume (1000 m3)

1 Res. Unit Office Supermarket Rec. Center 750 Res. Units 40 Res. Units Winter Garden Office Office Office Hotel

1978 1983 1984 1984 1985 1985 1985 1988 1989 1990 1995

10 120 42 30 220 32 26 30 110 35 100

System temperature (°C) Warm

Cold

40 65 30 35 35 14 30 35 35 35 20

10 30 15 10 10 4 15 10 10 10  10

1557

more details for each project. They are all large-scale pilot plants located in Germany, Sweden (built in 2010) and Canada. 1. Neckarsulm BTES in Germany has 528 boreholes with depth of 30 m and maximum design storage temperature of 85 °C. It was built in 1997, and the BTES was expanded in 1998 and 2001. The monitoring data from 2003 to 2007 showed the maximum solar fraction of 44.8% had achieved in 2007. The design solar fraction was 50% at which the system did not reach yet. Bauer et al. found it was because of the 10% smaller solar collector size and higher net temperatures return from the loads. In the first five years, there was no discharge from the storage for letting the BTES heat up to a usable temperature [24]. 2. Crailsheim BTES in Germany has 80 boreholes with depth of 55 m. The system partly started the operation in 2004. The monitoring data from two-year's operation (2006 and 2007) showed the solar fraction of 20%, which was far from the designed solar fraction of 50%. The buffer storage tank for this system is 480 m3, selected because the solar collectors during the summer have a high capacity rate. Due to the large buffer tank size, the heat captured from solar collectors cannot directly charge the BTES, and therefore, it takes a longer time to charge fully the BTES [47]. 3. Attenkirchen BTES in Germany is a combined HWTES and BTES. Ninety boreholes with the depth of 30 m were installed around a central concrete tank with a volume of 500 m3. Based on the temperature level in each storage system; heat pumps either use the borehole or hot-water-tank itself as a heat source to provide heat to the living area [5]. The system, commissioned in 2002, is considered as one of the smallest such systems in Germany. Based on two-year monitoring data the solar fraction for the system was reported to be 73%. 4. The BTES in Anneberg, Sweden has been in operation since late 2002. It has 99 boreholes with the depth of 65 m. It was designed for maximum storage temperature of 45 °C in contrast to the former three cases in Germany, where all were designed for maximum storage temperature of 85 °C. The heating system was designed for low-temperature (32/27 °C) and individual electric heater back up [48]. After three to four years of operation, the solar fraction for this system was reported to be 70%. The system was designed and evaluated with simulation models of TRNSYS 16, and MINSUN [37,49,50] with ground storage module DST (Duct storage model). 1. Okotoks BTES in Alberta, Canada has 144 boreholes with the depth of 37 m. This is the first solar heat storage in Canada built in 2006. The computer simulation results for this project showed that the system would achieve the 90% solar fraction after five years of operation [51]. The maximum designed borehole temperature was 80 °C. Sibbitt et al. described that the high-temperature storage had two disadvantages, 1) during the charging time, the return fluid temperature to the solar

Table 5 Central heating plant with BTES [12]. Solar plant with BTES

Neckarsulm, Germany

Heated living area

300 Apartments, 20,000 m2 Crailsheim, Germany 260 Houses School and Gym Attenkirchen, Germany 30 Houses 6200 m2 Anneberg, Sweden 90 Houses, 9000 m2 Okotoks, Canada 52 Houses, 7000 m2

Total heat demand (GJ/Year)

Solar collector area (m2)

Storage volume (m3)

Design solar fraction (%)

Design maximum storage temperature (°C)

1663

5000

63,400

50

85

14,760

7300

37,500

50

85

1753 3888 1900

800 3000 2293

10,000 60,000 35,000

55 60 90

85 45 80

1558

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panels would be relatively high, which caused to reduce the solar panels efficiencies and 2) the storage heat loss would be relatively high, which was calculated to be almost 60% [52,53]. The system showed that the solar fraction has reached to 97% after five years of operation and measured performance [49]. To minimize the thermal storage heat losses, Chapuis and Bernier, offered an alternative design approach for the Okotoks like system, to keep the storage temperature relatively low [54]. It was based on using heat pumps to raise the temperature as per space heating demands. Based on the simulation using TRNSYS with its DST module, it was concluded that by keeping the average storage temperature slightly above the annual average ambient temperature, the return water temperature to the solar collectors would be relatively low, which led to achieving higher efficiencies from the solar collectors by correspondingly reduced collector areas. Considering heat pump electricity usage in the system, 78% solar fraction could be achieved [54]. In general, solar fraction of a system does not reflect the system effectiveness. There are other metrics, such as system efficiency and cost, including initial and operating, that needed to be taken into account for comparing different installations.

4. Solar communities with cooling systems Technically, there are many achievable processes for using solar energy in cooling. Solar radiation can be converted to electrical energy by photovoltaic to run electric chillers for cooling through vapor compression cycle. Solar energy can also be captured through thermal collectors for cooling by the heat transformation or thermoelectric process. Fig. 5 shows different technologies to convert solar energy into cooling or air-conditioning presented by Henning [56]. Processes marked in gray are technologies that are well developed and available in the market and are used for solar assisted cooling. Solar heat transformation process for cooling can be an open or closed cycle. The two main categories of the closed cycle are: 1) absorption type using liquid sorbent materials such as ammonia and lithium bromide, and 2) adsorption type with solid sorbent materials such as silica gel. The maximum possible (thermodynamic limit) thermal coefficient of performance (COP) for thermally driven technologies is defined as: COPideal ¼ TT HC  TT HM TTMC where TC is the cold source

temperature, TH is the heat source temperature, and TM is the intermediate temperature at which the heat is transferred to a heat sink. Fig. 6 shows the COPideal and the real COP of such thermally driven chillers which are available in the market presented by Henning [56] and Grossman [57]. Absorption is the dominating technology for thermally driven chillers. The operation of such systems is well-documented (e.g., ASHRAE Refrigeration, 2010 [58]). Nowadays, absorption chillers are mostly used if a less expensive heat source is available, namely, waste heat, district heat or heat from co-generation plants. Absorption chillers used for air conditioning normally use a sorption pair of water-LiBr mixture where water is the refrigerant and LiBr is the sorbent. These are known as single effect machines, in which for each unit mass of refrigerant which evaporates in the evaporator then in the generator, one unit mass of refrigerant has to be desorbed from the refrigerant–sorbent solution. Typically, this equipment operates with temperatures of 80–100 °C and can deliver a COP around 0.7. Another alternative to the single effect chiller is the machines using a double effect cycle. Two generators work in series at different temperatures, where the condenser heat of the refrigerant desorbed from the first generator is used to heat the second generator. As a result, a higher COP in the range of 1.1–1.2 can be achieved. However, working temperatures in the range of 140– 160 °C are usually needed to run these double-effect chillers. These systems are normally suitable for the large cooling capacities of 100 kW and more [56,59]. In contrast to the liquid sorbent equipment, there are also machines working with solid sorption materials. In such systems, the semi-continuous operation needs minimum two sections that contain the sorption material operating in parallel. Available systems in the market use water as refrigerant and silica gel as the sorbent. Usually, they are called adsorption chiller as per Henning; only two Japanese manufacturers make this kind of chillers [56]. This equipment, under a typical operating condition with heat source of about 80 °C, the systems can achieve a COP of about 0.6. Grossman described the open-cycle absorption and desiccant system as a suitable choice for the low temperature heat source and introduced a system that makes it possible to use the solar heat with relatively low temperature for production of chilled water in variable quantities as required by the load [57]. Solar air conditioning and cooling technology is not fully established yet. More development is expected with increased competitiveness of this technology in the future. One main condition for a successful solar assisted cooling project is a good

Fig. 5. Physical ways to convert solar radiation into cooling or air-conditioning. Dark gray boxes represent technologies used for solar assisted air conditioning [56].

F.M. Rad, A.S. Fung / Renewable and Sustainable Energy Reviews 60 (2016) 1550–1561

design with the adequate solar collector and thermal storage size. To make this kind of the systems economically viable, other systems should be integrated or supplemented; such as heating system or domestic hot water generating system. Henning reported about seventy solar assisted cooling systems in Europe, which he concluded that these systems were at the early stage of growth. No standard design procedures or common practices for design exist [56]. Delorme et al. report fifty-three commercial buildings operated by solar cooling plants in seven European countries [61]. Operation of the eleven solar assisted air conditioning plants in six countries was also investigated by IEA Task 25. Some important outcomes of these investigation results were summarized by Henning [56,59,60]. The main problems with most of the plants were in hydraulic and control design and inappropriate commissioning process. 4.1. Examples of installation of solar assisted cooling system Table 6 shows examples of realized solar cooling in Europe, presented by Delorme et al. [61]. The system components in different projects are quite different from each other, and there is no generalized established solution in design available yet. Following a four-year monitoring of operation data and optimizations in controls in the university hospital in Freiburg Germany, it was found that the chiller operated with an acceptable COP (i.e., 0.42) but the cooling tower consumes relatively high amount of electricity. Average COP decreased through the partload operation during the summer nights with low cooling demand. The annual specific collector yield was 365 kW h/m2 [62].

1559

In Canada, few solar cooling systems have been put into practice recently. In 2010, one project was commissioned in one retirement home in Woodstock Ontario [63]. The system has 162 solar collectors with a total of 3240 evacuated tubes, which could produce up to 364 kW of thermal power. Other system components were a 105 kW absorption chiller and a 13,600 L thermal storage tank. Additional heater and chiller have also been installed as the backup of the system. The system provides heating and cooling to 9900 m2 area. After two years of operation, the system generated 740 GJ heating and 420 GJ cooling. As such, this system greatly reduces the operating cost and CO2 emission. Another system was installed in 2011 at a hospital in Thornhill, Ontario. The system consists of 131 evacuated tubes solar thermal collectors and ten 10 kW small size absorption chiller plus 4364 L of thermal storage tank. The initial calculation and system modeling estimated that the system would be able to offset 36% of the cooling loads and 44% of the heating loads as well as 91% of domestic hot-water use, which resulted in solar fraction of 56%. The system operation data are under fully monitoring system and is still to be examined. A newly developed triple-state absorption chiller was also tested in a single residential home as a pilot project in Vaughan, Ontario, Canada [64]. 4.2. Solar assisted heating and cooling system with seasonal thermal energy storage Hesaraki et al. conducted a comparative review of different types of seasonal energy storage systems integrated with the heat pumps for heating and in some extent cooling applications [68].

Fig. 6. COP curves of sorption chillers by Henning [56] and Grossman [57].

Table 6 Examples of solar cooling commercial projects in Europe [61]. Country

Location

Building

Cooling capacity (kWc)

Technology

Collector type

Gross collector area (m2)

In operation since

Germany Germany Germany Greece Greece Spain Portugal

Langenau Freiburg Freiburg Oinofyta Crete Arteixo Lisbon

Offices Laboratories Offices Warehouse Hotel Offices and Store Offices

35 70 60 700 105 170 36

1997 1999 2001 1999 2000 2003 1999

Pergine Valsugana Hartberg Banyuls

Business innovation centre Research house Wine cellar

108

Evacuated tube Evacuated tube Flat plate air collector Flat plate Flat plate Flat plate Compound parabolic collector Flat plate

45 230 100 2700 448 1626 48

Italy

Absorption chiller Adsorption chiller Desiccant cooling Adsorption chiller Absorption chiller Absorption chiller Desiccant cooling and heat pump Absorption chiller

265

2004

30 52

Desiccant cooling Absorption chiller

Evacuated tube Evacuated tube

12 215

2000 1991

Austria France

1560

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The paper presented the systems with low temperatures suitable for running heat pumps to satisfy heating rather than cooling loads mostly. In their study the implications of storing excess heat generated by the heat pumps in cooling season and the storage of solar heat at the same time, have not been investigated. On the other hand, the study of such systems with distribution systems other than heat pumps, e.g., fan-coil, for both heating and cooling has not been presented yet.

5. Conclusion – further research Based on the comprehensive literature review, the conclusion can be summarized as follows:

 The majority of the existing solar communities are based on











heating only. There are numerous successful projects (e.g., Okotoks, Canada) around this topic, which will be a lead to investigate further for the modifications and enhancement of the existing system. There is no research on solar communities with combined heating and cooling with integrated BTES with distributed fan coil system. The necessity of such a system will be based on the geographic location where cooling and heating are required in the course of the year, e.g., Southern Ontario and Quebec of Canada and many regions of the USA in North America. Comparing to the existing community-scale thermal storage, borehole thermal energy storage (BTES), has the most favorable condition for long-term energy storage. This is because the large amounts of energy involvement and a relatively low cost of energy storage media. In the existing solar communities for cooling, the ground energy storage systems are designed for storing unused heat from solar collectors only. In these systems, there are no standard strategy and designs for capturing the excess heat from thermally driven cooling equipment, i.e., absorption chillers. As such a new BTES for storing the low-grade heat could be a solution. The heating distribution systems in the existing solar communities are mostly based on medium-temperature (  40 °C) fancoil distribution system. The technical and economic viability of a low-temperature distribution, suitable for distributed heat pumps has not been investigated yet. Community solar heating and cooling can produce low-carbon emission energy due to the widespread solar resources.

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