The thermal behavior of a novel wall radiator panel coupled with horizontal ground source heat pump heating system: improve indoor environment to reduce the airborne transmission of infectious diseases

Motivated by the rapid spread of the novel pandemic disease (COVID-19) that swept the most countries in the world, a new radiation heating system consists of wall radiator panel system connected to a reversible geothermal heat pump (GHP) coupled with horizontal ground heat exchanger (HGHX) was proposed as fast and permanent solution to the risks of the dispersion of airborne infectious diseases in air-conditioned enclosed spaces. An experimental system was installed and tested in the laboratory of thermal process of Research and Technology Center of Energy (CRTEn), Tunisia, in order to achieve the two main goals of this work: developing a new radiation heating system with quick and inexpensive implementation while ensuring high efficiency and environment-friendly performance for the entire system. The results obtained show that it is feasible to use the novel RPHs as heat rejecter of the horizontal ground source heat pump system (HGSHPs) for heating buildings with limited surface land areas epically those located in the Mediterranean regions such as Tunisia, the average performance coefficients of the geothermal heat pump COPhp and the overall system COPsys are found to be 6.3 and 3, respectively. The thermal comfort analysis indicates that there is only a small vertical temperature fluctuation in the test room that would not produce any negative effect on thermal comfort.


Introduction
In December 2019, an outbreak of respiratory illness was notified in China, it was later proven that the new disease which first appeared in Wuhan, a city in Hubei Province, was caused by a novel Coronavirus, officially called Coronavirus Disease 2019 (COVID-19) [1]. COVID-19 has rapidly spread in China and to multiple countries to reaching more than 21 million confirmed cases in less than 9 months [2].
The laboratory-confirmed cases count was markedly increased daily, which has led to conduct many studies focusing on the routes of transmission for this pandemic disease. Since the enclosed spaces are the main environments in which peoples and patients spend most of their time, the majority of research focuses on the routes of spreading Coronavirus in the indoor environments [3][4][5][6]. It is reported that the recirculation of indoor air by mechanical means such as heating, ventilating and airconditioning (HVAC) systems can be largely transport the novel Coronavirus from one space and distribute them to other spaces connected to the same system by increasing the rate of air flow that carrying the airborne particles of the virus [7,8].
To reduce the potential airborne transmission of this pandemic disease and any other airborne micro droplets containing viruses in enclosed spaces, especially public buildings which are used randomly (i.e. hospitals, restaurants, offices, schools, libraries, conference rooms and others), several engineering controls were recommended such as avoiding the air recirculation by air-conditioning (HVAC) systems and providing the outdoor air by natural ventilation process [8,9].
Note that many confined hospital spaces including some residential buildings cannot naturally ventilated through airflow passages (i.e. windows and doors), In addition, the natural ventilation is not appropriate for changing climate in several regions around the world where the air-conditioning is necessary especially in heating season where the most of transmission occurs. Therefore, it is critical to highlights the use of new technique that works permanently in all circumstances and suit with all technical and economic including the environment aspects, which can replace traditional HVAC systems, in order to prevent COVID-19 airborne transmission to healthcare workers, hospital patients, environmental services, residents and others.
The thermally activated building system (TABs) is one of the most efficient distribution systems that can be improve indoor climate with lower air speeds based on different energy sources [10,11], it consists of heat exchangers which meet the heating/cooling demands of the buildings by conducts heat to/from the thermal mass of building in form of radiation, which mean that the TABs reduces the dispersion of aerosol particles and gaseous pollutants to the minimum in the indoor environments [12].
These types of low enthalpy systems can be generally classified into two types: directly activated building system (DABs), such as coil unit, radiant wall panel and radiators panel that placed in building structures; and indirectly activated building system (IABs), such as radiant floor, chilled ceiling and wall active layer that embedded into building surfaces. As mentioned, since the public places and certain types of residential rooms are characterized by intermittent random usage, the DABs that allow fast conditioning are commonly the most appropriate option, and since we focus in this study on buildings with limited land surface areas such as rooms used for critical care and residential rooms, the radiators panel heating system (RPHs) which usually occupy small spaces is particularly more suitable by increasing the heat exchange surfaces through the pipe coils [13].
The RPHs provide the desired thermal comfort conditions in the indoor environment without the need for ventilation processes by the direct radiation or convection heat transfer with the indoor air, these characteristics gives a combination of technical, economic and comfort advantages. First, the RPHs can be installed anywhere on the roofs and walls, mounted more easily, occupied a smaller area, and its shape can be easily modified so that it does not affect the appearance of the building. Second, the RPHs is usually cheaper than other TABs heat exchangers and it has a long operation life with nearly no maintenance [13]. Third, the RPHs has short thermal response time [14,15]. Fourth, there is no fan or rotating part inside the RPHs which lead to improve indoor air quality with no noise [16]. Fifth, air-conditioning with RPHs could effectively ensure the indoor thermal comfort with a reasonable indoor vertical temperature gradient [17,18]. Finally, due to low supply water temperature going into the building (45/35°C), the RPHs has a relatively higher performance and require less energy than conventional air-conditioning systems. In fact, because the RPHs require low consumption costs compared with conventional heating systems, a combination of RPHs with heat pump systems is suitable [19][20][21].
The geothermal heat pump called also the ground source heat pump system (GSHPs) is one of the promising applications in the renewable energy that can be provide heating or cooling to the buildings by numerous clean and safe techniques such as the horizontal ground source heat pump (HGSHP) and the vertical ground source heat pump (VGSHP). Considering the first investment costs and the geological constraints for applying the VGSHPs, the HGSHPs are usually the best option in our case.
Despite many researches which deal with both RPHs and GSHP, there are few examples that integrate both RPHs and HGSHPs, and the most research has been done in simulations laboratory tests. From literature review, Experimental study on the radiator panels as heat rejecter of HGSHPs for buildings heating still does not exist. Consequently, testing of HGSHPs with RPHs under real outdoor conditions still required for further development to pre-investigate the effectiveness of the whole system for heating spaces with selected conditions.
In this context, the main aim of this paper was to evaluate the thermal performance of wall radiator panel heating system coupled with horizontal ground source heat pump used for heating test room with limited floor area in North Tunisia. The experiment was carried out at the Research and Technology Center of Energy (CRTEn), Bordj-sédria. The experimental set-up consists of ground source heat pump coupled with horizontal ground heat exchanger implanted in 1 m of depth used for heating building unit equipped with capillary mat system using as heat radiator. The experimental results obtained during relatively cold days prove that the novel wall radiator panel heating system matches well with the HGSHP system, and can effectively improve the indoor thermal environment. Energy consumption is lowered through the use of free energy from the ground, and the performance coefficient of the whole system simultaneously increases.

Climate of the test region
The experimental system were installed and tested in the Thermal Processes Laboratory (LPT) at Research and Technology Center of Energy (CRTEn), Bordj-sedria. The city of Bordj-sédria is situated in the northern Tunisia in the middle of North Africa's Mediterranean coast, it is located at 36°N latitude and 10°E longitude. The northern region has a Mediterranean climate which is characterised by mild, rainy winters and hot, dry summers. The average, minimum and maximum monthly variation of temperatures, including the average monthly variation of the wind speed and insulation measured with meteorological weather station installed in the CRTEn, Borj Cédria, were displayed in Figure 1 [22,23].
The study region in this work is one of the main geothermal areas in Tunisia, it has very important geothermal resources, as shown in Figure 2 [23], Bordj sédria is also characterized by a relatively low production depth (the depth in where the temperature is 40°C above the ground surface temperature) which is located between 1000 and 1500 m.
3 Basics of the design 3.1 Analysis of energy from the ground Using the first law of thermodynamics, the heat transferred from the ground (Q g ) can be computed from the following equation:

Analysis of energy to the building
The amount of heat injected into building (Q b ) was calculated from: Q b can be formulated also by the following equation: where LMTD represents the log mean temperature difference, written as: Thus, the required total area of the radiator, S RPHs (m 2 ), is obtained as follows: Where U RPHs is the overall heat transfer coefficient.

System energy efficiency
The coefficient of performance of the heat pump (COP hp ) can be estimated by the ratio between the heat injected into building and the compressor power input: The coefficient of performance of the overall heating system (COP sys ), is calculated from the following equation: Where _ W Comp and _ W pump is the electricity consumption of the compressor and the circulation pump, respectively.
_ W pump was defined as: where H pump is the total pumping head in the HGHX (Dp GHX ) and heat pump (Dp GHP ).

Methods and materials 4.1 Test details
As stated in Section 1, the experimental campaign in this work was focused on evaluating the possibility of application or replacement of conventional heating systems with low-temperature radiation heating system coupled with renewable energy source. The experiments were conducted in Mediterranean climate between 24th and 27th January 2020 in order to evaluate the thermal behavior of wall radiator panel system coupled with HGSHPs for heating load dominated building with limited floor area. The following assumptions have been made: -The required total area of the radiator has been calculated according to the building heating loads using equation (5), (see Sect. 3.2). -The GHX length was selected so that the installation does not excessively affect the soil thermal balance in the long run. An analysis of the influence of ground heat exchanger parameters in the studied conditions was performed in [24]. -The compressor switches on/off according to the controlled water tank temperature. -The internal circulation pump is continuously switched on during the heating period. -The external circulation pump switches on one minute before the compressor, and switches off 1 min later.

Experimental set-up
The experimental system consists of five components (Fig. 3): ground heat pump, horizontal ground heat exchanger, test office, radiator panel heating system and storage tank.
-North facing office room (Fig. 4), with a floor area of 12 m 2 and an internal height of about 3 m. -The geothermal heat pump unit is a reversible water-towater, Ageo CIAT (Fig. 5a). It is equipped with two circulating pumps to circulate water in the internal and external systems. The technical properties of GHP are shown in Table 1. -The horizontal ground heat exchanger which installed in 1 m of depth in the ground, is consists of a high-density polyethylene tube (HDPE) with 100 m of length (Fig. 5b), their technical properties are regrouped in Table 2. -The radiator panel heating system (Fig. 5d) consists of a multilayer heat exchanger (CM heat exchanger) installed vertically in the western wall of the test office. The techanical properties of the CM heat exchanger are shown in Table 3. -Insulated water tank of 0.1 m 3 was integrated between the heat pump unit and the radiator panel to increase the thermal inertia of the system.

Measurement equipment
During the test procedures all the measurement sensors were connected to a multichannel digital Agilent type HP (Fig. 5c), which was linked to a Microsoft software program that stock the results every 1 min during 3 days (24 January until 27 January 2020) to record any unexpected thermal behavior for the entire system. A K-type thermocouples (accuracy ±1°C) were used to measure the ambient temperature and the temperatures indoors at different levels in the center of the test office, 3 thermocouples (K-type) were also used to measure the surface temperature of radiator pipes at 0, 3 and 6 m of length. For the measurement of the GHX  water temperature two wires PT500 resistance thermometer sensors (accuracy ±5.71%) were placed at the inlet and outlet of the GHX.

The heating loads
In this work, the heating loads were calculated using a TRNSYS model of the climate test room, which is developed according to the construction specifications of the Thermal Process Laboratory (Fig. 4), as follows: The heating thermostat was set as 23°C according to the thermal comfort standards in the the test region, Figure 6 represents the annual heating loads of the test office which were calculated based on the weather data of north coast climate. It is clear that the heating period lasts about 7 months, from October to April. The heating loads reach their maximum in January of about 2.12 kW.

Results and discussion
This section analyses the performance and the feasibility of the wall radiator panel system coupled to a HGSHPs in heating mode. Experiments were conducted to find the ambient temperature, average indoor temperature, HGHX    5, 11 (2020) inlet and outlet water temperature and RPHs inlet and outlet water temperature. Figures 7 and 8 shows the hourly variation of the average indoor temperature of the test office (T amb ) compared with the outdoor temperature (T aver ) with and without air conditioning system, respectively. It can be seen from this figures that the radiator panel heating system has raised the average temperature inside the test office of about 6°C to reaches a comfortable levels at an average value of about 23°C, a slight fluctuations in the indoor temperature was noticed due to the decrease temperature in the storage tank when the compressor switch off.

The vertical difference of air temperature
As fluctuations in room temperature between head and ankle levels and the variations in air temperature with time affect the human thermal comfort [25], an assessment of the vertical difference of air temperature throughout the heating period was carried out. Figure 9 shows the variation of the indoor temperature at different altitudes (L1: floor, L2: 1.4 m above floor, L3: roof, as shown in Fig. 10). Results illustrate that after the activation of the GHP system at 9:00 am the radiator heating system raises the air temperatures inside the test office by 4-8°C compared with its values at the beginning of the heating period. The average indoor air temperature increases gradually to reaches 23.5, 23 and 22.4°C at the different levels L1, L2 and L3, respectively. Slight fluctuations in the indoor air temperature between head and ankle levels of about ±1°C from its desired value (7 in Tab. 4) were observed at midday due to variations in the outdoor air temperature and solar heat gains. The time duration to produce these temperature changes is about 4 h, which will not cause any thermal discomfort.

Floor surface temperature
Contact with floor surfaces that are too warm or too cool may cause thermal discomfort for foot. In general, people wear shoes or walk on carpets or floor covering, however this section is proposed for people wearing lightweight indoor shoes. The hourly variation of the floor surface temperature during the testing period was given also in Figure 9. After the thermal stabilization at 12:00 am, the floor surface temperature varies from 19 to 21.2°C and the mean value is 20.4°C during the testing period. According to the prescribed limits set in ASHRAE 55-2004 standards that depicted in Table 4, the floor surface temperature in the investigated room is within the normal range. Figure 11 shows the periodically variation of the inlet and outlet water temperature in the radiator panel heating system. We can note from this figure that the outlet and the inlet RPHs temperatures increases gradually at the beginning of the heating period to reaches its maximum at 12:40 am, after that the inlet water temperature ranged from 36 to 40°C and the outlet temperature varied from 32 to 34°C. The differences between the T outÀRPHs and T inÀRPHs was translated to an amount of heat rejected into the building (Q b ) using equation (2) (7 in Fig. 12). It was found that the amount of heat rejected into building reaches a maximum value of about 1.9 kW at the beginning of the GHP operation due to the high heating loads of the test office, and then the heat injected decreases gradually after 1 h of GHP operation, its values vary between 0.8 and 1.4 kW. This variation can be justified by the intermittent operation of the compressor.

The radiator heating system response
To determine the heating system time response, the surface temperature of the RPHs was collected at different distances (D1: 0 m, D2: 3 m, D3: 6 m) during the GHP operation period. Figure 13 shows that after the activation    has stabilized at these temperature levels in a short period of about 30 min, which indicates the rapidity response of the radiator panel heating system.

HGHX inlet and outlet water temperature
In order to evaluate the thermal capacity of the ground, the variation of the inlet and outlet ground heat exchanger temperatures were represented in Figure 14. The results shows that the outlet temperature reaches a maximum value of about 20°C when the compressor switches on and reaches a minimum value of about 12°C when the compressor switches off, this corresponds to the change in the inlet temperature varies between 4 and 18°C. Figure 15 presents the heat transferred from the ground calculated using equation (1), it is varies from 4.4 to 4.5 kW when the compressor switches on. This result can be explained by the high thermal response of the ground in the test region.

Overall performance
To understand the effect of the thermal behavior of the RPHs on the performance of the HGHP system, the     variation of the performance coefficient of the heat pump and of the whole system which were calculated using equations (6) and (7), respectively, was shown in Figure 16.
The average values of the COP hp and the COP sys in the beginning of the heating period, were found to be 7.4 and 4.1, respectively. After the stabilization of the system which was noticed at 14:00, these curves indicate a performance degradation of about 15% and 27% in COP hp and COP sys , respectively, the degradation can be explained by the decreases in the inlet temperature of the GHP evaporator, due to the decreases in soil temperature (T g ) around the outlet of the GHX. A performance results comparison of this study with the results of other research studies is summarised in Table 5.

Conclusion
In the present study, the thermal behavior of a novel wall radiator panel heating system connected with horizontal ground source heat pump was examined experimentally under the climatic conditions of the Mediterranean region.
The whole system was proposed to provide clean heating for confined enclosed spaces, in order to minimize the airborne transmission of the infectious disease caused by the traditional air-conditioning and heating systems. The following results are observed: -The combined radiator panel heating system is capable of maintaining the studied room within the required temperature level throughout the heating periods. -The use of 6 m of capillary mat as a radiator heating system raises the air temperature inside the building unit of about 6°C within 2 h maximum, which reflects the rapidity of the system response. -The thermal comfort analysis indicates a small vertical difference of air temperature inside the test building, which would not produce any significant thermal discomfort. -The coefficients of the performance of the heat pump (COP hp ) and the overall system (COP sys ) ranges between 6.2 to 6.4 and 2.9 to 3.1, respectively Finally, it can be concluded that the novel radiator panel heating system is an effective solution for improve indoor climate and the coefficient of performance of the geothermal heat pump simultaneously. Therefore, we emphasize that using such clean energy-efficient heating systems in hospitals and other residential and public spaces will reduce the rates of airborne infection not only for COVID-19 in the current novel pandemic, but also for other airborne infectious diseases if it is implemented on a large scale.

Nomenclature h pump
Electric efficiency of circulating pump (%) COP hp Coefficient of performance of the heat pump COP sys Coefficient of performance of whole system T g Ground temperature (°C) T inÀGHX GHX entering water temperature (°C) T inÀGHX GHX leaving water temperature (°C) _ m wÀGHX GHX water mass flow rate (kg/s) U Overall heat transfer coefficient (W /m°C) W Power consumption (kW) T inÀRPHs RPHs entering water temperature (°C) T outÀRPHs RPHs leaving water temperature (°C) _ m wÀRP Hs RPHs water mass flow rate (kg/s) T avr The average indoor air temperature T amb The ambient air temperature Q b The heat absorbed from the building (W) Q g The heat transferred to the ground (W) C pÀw The fluid specific heat of the water (kJ/kgK)