Influence of Climatic Conditions on Dynamic Performance of Solar Hybrid Heating and Cooling Systems Integrating Seasonal Borehole Thermal Energy Storages: Application to School Buildings in the Campania Region of Italy

Solar energy is abundant, free and non-polluting; therefore, it is considered one of the most competitive choices among all renewable sources able to ensure a sustainable future and address the increasingly serious impacts of climate change via either solar thermal collectors (SCs) and photovoltaic panels (PVs). Coupling battery storages (EB) with PVs could help in enhancing the self-consumption of electricity generated by PVs and, therefore, reducing the operating costs for end-users, limiting the stress on electric infrastructure during peak loads and improving power quality and reliability [1]; currently batteries represent mature energy storage devices with high energy densities and large potential applications [2]. However, solar energy is intermittent and a surplus is often available in summer months; this leads to a time discordance between solar supply and heat demand. Seasonal or long-term thermal energy storages allow for thermal energy storage over weeks and months, with it being a viable solution to overcome this temporal mismatch [3]. Numerous projects and installations have seen the light of day in Europe and North America, together with a number of scientific studies [4], highlighting promising performances of these systems. Even if four main types of seasonal storages have been presented by researchers, borehole thermal energy storage (BTES) is one of the most favorable according to Rad et al. [5]. A BTES consist of closed-loops where heat is charged or discharged by vertical or horizontal borehole heat exchangers (BHEs), which are installed into boreholes below the ground surface; after drilling, a U-pipe is inserted into the borehole and the borehole is then filled with a grouting material; the BHEs can be hydraulically connected in series or in parallel. The underground is used as storage material in a BTES, so that the performance of BTESs is strongly affected by the heat exchange capacity of the ground surrounding the BHEs, i.e. the stratigraphic succession and the hydrogeological conditions [6]. Solar energy could also be effectively used to activate adsorption chillers (ADHPs), mainly thanks to the fact that they could be driven at low supply temperatures (45÷65°C) [7, 8], avoiding the utilization of electrically-driven chillers. However, ADHPs are characterized by lower coefficients of performance (between 0.4 and 0.7) and higher unit cost for the same cooling capacity [7]; therefore, their utilization and potential benefits have to be investigated and assessed in more detail. Small ADHPs are recently being transferred into the market [7]. With reference to the Italian scenario, only five studies [9] analyzed the operation of solar heating systems integrating long-term thermal energy storages. However, they are very few and generally dated, with only two of these studies referring to the climatic conditions of southern Italy; in addition, the environmental and the economic impacts are neglected in most of these works; finally, these researches focus on districts much different from that one considered in this study. To the knowledge of the authors, there aren’t even any scientific paper focusing on performance of solar heating and cooling networks integrating both seasonal thermal energy storages and adsorption chillers (CSHCPSSs) operating in Italy [10]. Taking into account that climatic conditions could greatly affect the energy demand profiles of end-users, the design as well as the overall performance of CSHCPSSs, the literature review highlights that there is a specific need to perform additional investigations for Italian applications. In this paper, the performance of CSHCPSSs has been modelled, simulated and analyzed by means of the dynamic simulation TRNSYS [11] over a period of 5 years. The analysis has been performed by considering 5 different case studies consisting of school buildings located into 5 different cities belonging to the 5 different provinces of the Campania region (south of Italy). The climatic conditions as well as the properties of underground associated to each location have been accurately determined based on 1-year in-situ measurements as well as literature data. The performance of the CSHCPSSs have been assessed from energy, environmental and economic points of view and contrasted with the operation of typical Italian heating and cooling plants in order to estimate the potential benefits.

Figure 1 shows the scheme of the proposed Central Solar Heating and Cooling Plant with Seasonal Storage (CSHCPSS).

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Figure 1. Schematic of the proposed CSHCPSS

A water-ethylene glycol mixture (60%/40% by volume) is used as heat carrier fluid. The solar energy captured by the solar thermal collectors (SCs) installed on the roofs is firstly transferred, through the heat exchanger HE1, into the vertical cylindrical short-term thermal energy storage (STTES); solar thermal energy surplus is dissipated by blowing air across a finned coil heat dissipator (HD) when the outlet temperature of SCs is higher than 95°C and, therefore, prevent the boiling of the heat carrier fluid. During the cooling period, solar energy is moved from the STTES to the adsorption chiller (ADHP) by means of the heat exchanger HE2; this allows obtaining the desired cooling energy to be stored into the vertical cylindrical short-term cooling energy storage (STCES) and then provided to the schools for cooling purposes. During the heating period, solar energy stored into the STTES can be moved, through the HE2, into the distribution network and, then, into the fan coils installed inside the buildings to cover the heating load. If the solar energy is not immediately required for space heating/cooling purposes, it can be moved from the STTES to the BTES (with vertical single U-pipes borehole heat exchangers) during the whole year (“BTES charging mode”). Only during the heating season, thermal energy stored in the BTES field can return into the STTES (“BTES discharging mode”) to integrate the temperature level. A centralized boiler (WPB), fueled with pellets, is eventually used as back-up unit with the aim of integrating the solar contribution and maintaining the desired supply temperature. The electricity delivered by the PVs is primarily utilized to satisfy the electric demand of end-users and plant components, while the excess is transferred into the batteries (EB) only when their charging status is lower than 100%; the batteries are discharged only when their charging status is larger than 10%. In the case of the electric energy generated by the PVs is greater than the overall electric load and the batteries charging level is equal to 100%, the electric production that cannot be charged into the batteries is then sold to the central grid. The central grid, as well as the batteries, are utilized to satisfy the peaks of electric load. Table 4 details the most important characteristics of the main CSHCPSS components as a function of the location.

Table 4. Characteristics of the main components of the CSHCPSS

The total aperture area of solar thermal collectors, the volume of the STTES, the volume of the STCES as well as the volume of the BTES have been determined based on energy demands of the buildings and according to the results of a huge sensitivity analysis performed by the authors in a previous study [10]. Thermal conductivity of underground is an ‘equivalent’ value representative for the entire stratigraphic sequence; it has been obtained as the weighted average of the entire depth of the BHEs calculated by taking into account the thickness of each lithology. A specifically dedicated material [20] has been used for grouting the boreholes. The area of PVs for each building has been selected in order to cover the related maximum power demand, while the total capacity of batteries has been defined based on the daily average overall electricity consumption of schools. The sizes of both WPB and ADHPs have been set according to the thermal/cooling load profiles of each school in order to guarantee the desired thermal comfort at least 90% of the time.

Both proposed and reference heating and cooling systems have been modeled, simulated and analyzed by means of the TRaNsient SYStems (TRNSYS) software platform 17. This tool uses a component-based methodology in which (i) a system is decomposed into components’ mathematical models (named “Types”), each of which is described by a FORTRAN subroutine, (ii) the user assembles the Types by linking component inputs and outputs and by assigning component performance parameters, and (iii) the program solves the resulting non-linear algebraic and differential equations to determine system response at each time step. In this study, detailed simulation models have been applied to each component which fully account for (i) transient nature of building and occupant driven loads, (ii) part-load characteristics of components, (iii) interaction between the loads and the system output, and (iv) system energy management and control. In this paper, the Types have been selected from the TRNSYS libraries and enhanced by experimental measurements and/or manufactures performance data and/or information available in current scientific literature according to the common characteristics of the components used in practice. The duration of simulation period (5 years) has been defined in order to consider that it takes time to fully charge the seasonal storage. A simulation time step of 1 minute has been used, while January 1^st has been assumed as starting date of the simulations. The Type 56 has been used to accurately model the envelopes of buildings (windows, walls, roofs and floors) as well as the occupant driven loads. The Type 557a [21], adopted to model the BTES, is considered to be the state-of-the-art in dynamic simulation of ground heat exchanger that interacts thermally with the ground. The storage volume has the shape of cylinder with vertical symmetry axis. The layout of the borehole field is fixed hexagonally and uniformly within the storage volume in the simulation. The STTES and STCES have been modelled by means of the Type 534 with 10 isothermal temperature layers to better represent the stratification in the tank, where the top layer is 1 and the bottom layer is 10; the tank model has been calibrated based on manufacturer data [16]. The flat-plate solar thermal collectors have been modelled by using the Type 1b, where the collector efficiency has been modelled by a second-order equation and correction for off-normal solar incidence is applied by a second-order incidence angle modifier according to the manufacturer data [14]. The model of PVs is described in De Soto et al. [15]; it is a five-parameter model able to predict the power delivered to the load. The battery is modelled with the Type 47a, while the inverter/charge controller is modelled with the Type 48b. The Type 47a specifies how the battery state of charge varies over time, given the rate of charge or discharge (it has been calibrated based on manufacturer data [17]); the Type 48b is composed of two devices, where the first one is a regulator which distributes DC power from the solar cell array to and from a battery (in systems with energy storage), while the second component is the inverter. The Type 700 has been used for modeling the WPB; according to the manufacturer performance data [18], its efficiency has been evaluated as a function of the thermal power output. The Type 31 has been used to model the pipes and calculate the related heat losses by assuming a loss coefficient equal to 0.05 kJ/(hm²K). The heating/cooling pumps have been modelled by the Type 742, while the other pumps in the system have been modelled by means of the Type 656. The Type 909 is used for the ADHP; this component models an adsorption chiller, relying on user-provided performance data files containing normalized capacity and COP ratios as a function of the hot water inlet temperature, the cooling water inlet temperature and the chilled water inlet temperature (suggested by the manufacturer [19]). Five specific weather data files have been created for each location based on 1-year in-situ measurements of key-parameters (outside air temperature, relative humidity, wind velocity and global solar irradiation on a horizontal surface) every hour in order to model the climatic conditions; the weather data are one year long and, therefore, have to be the same each year. With reference to the CS, the Type 700 has been used for modelling the natural gas-fired boiler (assumed with a constant thermal efficiency of 90.0%), while the radiators have been simulated by means of the Type 1231. The Type 941 has been adopted to model the performance of the refrigeration unit; this Type is able to provide the cooling/heating outputs, the power absorbed as well as the temperature of both heat carrier fluid and air according to the performance map suggested by the manufacturer [22]. Table 5 reports the TRNSYS control strategies for the activation/deactivation of the main components of both CSHCPSS and reference systems, whatever the location is. The heating season ranges from November 15st up to March 31st for Acerra, while for the other cities it covers the period November 1st-April 15th); the cooling season relates to the remaining part of the year. The target of indoor air temperature T_room is set to 20.0°C and 26.0°C (with a deadband of 0.5°C), respectively, during the heating and cooling seasons only in the case of at least one occupant being inside the buildings.

Table 5. Control strategies of main components of plants

Table 6. Annual performance of the CSs

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Figure 2. PES, $\Delta$CO₂, $\Delta$OC, SF and RF associated to the 5^th year of simulation as a function of the city

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Figure 3. Main annual specific energy flows during the 5^th year of simulation upon varying the city

This study has been undertaken within the research project entitled “Solar smart Energy Networks integrated with borehole thermal Energy storages serving small-scale districts in the CAmpania region - S.E.N.E.CA.” funded by the “V:ALERE 2019 program” of the University of Campania Luigi Vanvitelli (Italy).