Elsevier

Solar Energy

Volume 119, September 2015, Pages 212-224
Solar Energy

Efficiency evaluation of a solar water heating system applied to the greenhouse climate

https://doi.org/10.1016/j.solener.2015.06.040Get rights and content

Highlights

  • Use of Trnsys16 to predict and improve a GSWHS rentability.

  • Experimental validation of simulation results.

  • Important effect of GSWHS to reduce the greenhouses heating cost.

Abstract

Tunisia has a sunny climate; therefore, the use of solar energy to heat greenhouses can be a solution to reduce the heating cost. A thermal model has been developed to investigate the potential of using a solar water system for greenhouse heating. This system is mainly based on capillary heat exchangers integrated in the greenhouse and ensuring the ground heat storing. A numerical study is conducted using Trnsys16 in order to estimate the exchanger length and the water flow rate allowing the best performance of the system. The inputs of Trnsys16 software were evaluated by experimental tests. The cost and the efficiency of the system were estimated for different greenhouses size. It was concluded that the heating cost of a 1000 m3 greenhouse was reduced by 51.8% in April and the system can alone satisfy the heating needs for a 10 m3 greenhouse. The good efficiency of the studied system was validated by an experimental study. The simulated and experimental results were compared and error did not exceed 6% which makes Trnsys16 simulation program an efficient tool to reproduce the real behavior of the ground solar water heating system.

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;ρacpaVadTadt=-QCac-QCap-QCas-Qv

  • For air moisture:

ρaVadWadt=-Mc+Mp+Ms-Mv

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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