Efficiency evaluation of a solar water heating system applied to the greenhouse climate
Introduction
The Tunisian state invests more and more in scientific research specifically in the exploitation of solar energy. Actuality, Tunisia can meet its energy requirements by using the most of these natural resources. Solar thermal energy has been well investigated in the last decade by Tunisian researchers. Therefore, solar energy is used in different domains in Tunisia such as; the sanitary water heating (Hazami et al., 2013), air conditioning of building (Naili et al., 2015) and recently in agriculture (Attar et al., 2014). The main issue for the greenhouse is to provide an appropriate heating system which can ensure the good temperature condition and save energy outside the cultivation season (Joudi and Farhan, 2014). The heating system affects strongly the time of cultivation, quality and quantity of the products (Sethi and Sharma, 2008). Due to the relatively high cost of energy and big greenhouses surfaces, the auxiliary systems cannot meet alone the heating needs (Vadiee and Martin, 2013). Therefore, the use of an appropriate heating system, at low cost, is crucial to provide optimum indoor conditions during cold months. Rather than fossil fuels, different renewable energy sources can be used in a greenhouse heating systems such as; geothermal (Ghosal and Tiwari, 2004), solar, and biomass energy (Chau et al., 2009). The solar energy receives the most serious consideration for greenhouse heating. Two types of solar greenhouse systems are provided; it depends on using water or air as a transfer medium of energy (Jakhar et al., 2015). The solar heating technologies applied to the greenhouses include: hybrid solar heating (Kıyan et al., 2013), hybrid solar photovoltaic (Agrawal and Tiwari, 2015), photovoltaic heating (Cossu et al., 2014), north wall (Gupta and Tiwari, 2005), earth-to-air-heat exchanger system (EAHES) Nayak and Tiwari, 2010, Ozgener and Ozgener, 2010, Ozgener et al., 2011, integrated photovoltaic-geothermal heat pump (Nayak and Tiwari, 2010) and phase change material (PCM) storage (Kooli et al., 2015, Benlia and Durmuş, 2009).
The ground heat storage systems using solar energy could be very promising (Attar et al., 2013). However, the performance of a greenhouse coupled with any heating system is influenced by the greenhouse size and energy collected by the heating system. It is important to evaluate the heating needs and the system contribution to predict the effectiveness of the considered solar system. Therefore, different parameters must be taken into account like the greenhouse cover material (Tiwari and Dhiman, 1986), the type of cultivation, the greenhouse location, the weather conditions (Santamouris et al., 1994, Sethi et al., 2013), and the heat loses.
In this paper, we study the storing performance of a ground solar water heating system (GSWHS). A parametric study is conducted using Trnsys16 in order to estimate the effect of the exchanger length and the water flow rate on the system performance. An experimental study is used to establish the input parameters of the heating system in Trnsys16 such as: effectiveness of the collector and his loss coefficient, loss coefficient of the tank and local heat transfer coefficient of the buried exchanger.
The best exchanger length and inlet water flow rate are then considered for the economically evaluated system to estimate its rentability. Using Trnsys16, that rentability is investigated, as a function of the greenhouse size, by estimating the collected solar energy and the greenhouse heating needs during the period December to April. The simulation results are then validated by an experimental study made for a 10 m3 greenhouse.
Section snippets
Description of the Trnsys16 model
Transient systems simulation (Trnsys16) was used to develop the GSWHS model investigated in this work. In Table 1, the main components of this model are described and then schematically shown in Fig. 1.
The description of the building components is assumed by Type 56. This Type permits the specification of the walls surfaces, orientation and initial conditions of the studied area (indoor temperature and relative humidity).
Type 5 is a steady-state heat exchanger model that allows simulating
Balance equations for the components of the ground solar water heating system
In this section, we considered a chapel greenhouse divided into four layers, the inside air, the covering material, the crop and the soil. Fig. 2 illustrates all of the fluxes of energy exchanged. The single arrows represent individual fluxes and double arrows represent net radiative fluxes.
Heat and mass balance equations are written for the air layer in order to predict its temperature and its moisture;
- –
For air moisture:
For covering material
The experimental setup location
The experimental tests were done in the Laboratory of Energetic and the Thermal Processes (L.E.P.T) of the Center of Technology and Research Energy located at Borj Cedria, Tunisia (latitude 36°48′ N, longitude 10°10′ and altitude 3 m above mean sea level). The climate in this area is mediterranean with a relative humidity high rate, a good rate of sunshine in the summer and high frequency of bad weather days in the winter.
The GSWHS description
The experimental GSWHS is presented in Fig. 3. The heating system is
Inlet flow rate effect on the exchanger temperature difference
The GSWH system investigated in this paper is basically composed of; a plan collector, a circulation pump, a storage tank, capillary heat exchanger and a water source. The parametric numerical study was made during two typical days of January. Hourly variation of ambient temperature and solar radiation are shown in Fig. 5.
In this section, we modify the ground exchanger inlet flow. Table 5 represents the variation of the temperature difference as a function of time at different flow rates (200 L
Greenhouse heating needs
In order to estimate the greenhouse heating needs, one model (Fig. 8) was developed using Trnsys16.
The effectiveness of the heating system is necessary to provide the amount of energy required to heat the greenhouse according to its volume (Table 6). Therefore, a simulation is done using Trnsys16 to estimate the greenhouse heating needs in order to reach 20 °C, using Tunisia meteorological conditions. It is noted that the heating needs are proportional to the greenhouse volume and the difference
Validation of Trnsys16 simulation results
Fig. 10, Fig. 11 represent the variations of the inlet and outlet exchanger water temperature given by the experimental test and Trnsys16 simulation program during two typical days (01/01/2014) and (16/01/2014). It is noted that the Trnsys 16 model follows, with an acceptable accuracy, the measured values given by the experimental tests. In fact, the difference between the simulated and the measured values of the heat exchanger temperature did not exceed 4 °C. After a lap of time, no difference
Conclusion
This paper was devoted to study the efficiency of a greenhouse solar heating system in order to make it appropriate for agricultural purposes during coldest period of the year (December–April). The considered system is basically composed of: a flat plat collector, storage tank and circulation pumps. In order to found the best GSWHS, a parametric study was conducted. It was concluded that the inlet flow rate and the exchanger’s length affect strongly the performance of the heating system. It is
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2023, Applied EnergyCitation Excerpt :The north wall with good thermal preservation capacity has been applied, so that the thermal performance of the greenhouse depends on the active heating system. Using water as a medium, researchers have created wall-mounted solar heating systems with chemical stability, liquidity preference, and high specific heat capacity for heat transport and storage [14,15]. In these systems, heat was stored and exchanged by water tanks and transferred by water circulation [16].