Elsevier

Applied Thermal Engineering

Volume 149, 25 February 2019, Pages 996-1007
Applied Thermal Engineering

Research Paper
Energy/exergy based-evaluation of heating/cooling potential of PV/T and earth-air heat exchanger integration into a solar greenhouse

https://doi.org/10.1016/j.applthermaleng.2018.12.109Get rights and content

Highlights

  • A solar greenhouse integrated with PV/T and earth-air heat exchanger was thermally modeled.

  • Only length of EAHE had an optimum value.

  • Heating/cooling potential of PV/T and EAHE integration into greenhouse were studied.

  • PV was recommended rather than PV/T integration while heating/cooling potential of EAHE was noticeable.

Abstract

Through the present paper, a solar greenhouse integrated with an Earth-Air Heat Exchanger (EAHE) and Photovoltaic/Thermal collectors (PV/Ts) was theoretically studied in terms of energy and exergy and validated against an experimental study presented in literature i.e. a solar greenhouse with floor area, buried pipe length and PV area of 24 m2, 39 m and 9.68 m2, respectively. The comparisons showed a fairly good agreement between the theoretical and experimental results with a relatively high coefficient of correlation around 95%. Afterwards, the given solar greenhouse was optimized in terms of energy and exergy efficiencies while the results indicated that only length of EAHE pipes showed an optimum value equal to 38 m on average. Moreover, the results showed that PV/Ts did not have a significant heating potential for raising the greenhouse air and plant temperatures and only the electricity generation potential of PVs was favorable. However, the EAHE integration seemed promising in raising and lowering the temperatures of greenhouse air by 9 °C and 8 °C in summer and winter, respectively. Moreover, the Temperature Load Leveling (TLL) due to integration of only EAHE was achieved 46% and 58% in summer and winter, respectively.

Introduction

High energy demand, mostly met by fossil fuels, for cooling/heating the greenhouses is a big problem which should be economically tackled. Consequently, a roughly cheap cooling/heating installation has high significance. Therefore, the eco-friendly substitute energy sources, found abundantly in nature, have been proposed by researchers [1]. Among the substitute energy sources, connecting earth and greenhouse through buried pipes showed to be a promising method for meeting the cooling/heating needs of a greenhouse while solar energy as a sustainable and pollution-free source of energy seems more applicable and reliable which can be exploited through PV/Ts [2]. Jain and Tiwari [3] developed a model for studying the thermal behavior of a greenhouse with effective floor area of 24 m2 heated via ground air collector. With the developed modeling, the plant and greenhouse air temperatures could be predicted while the proposed model was validated against extensive experiments in Delhi. Ghosal at al. [4] also developed a model providing the year round effectiveness of an EAHE including buried pipes integrated into a greenhouse through which the greenhouse air was recirculated. They reported that such a coupling caused the greenhouse temperature to be on average 6–7 °C and 3–4 °C more in winter and less in summer, respectively in comparison to the same greenhouse lacking EAHE. Tiwari et al. [5] performed an experiment through which an EAHE was coupled with a greenhouse. They tried to optimize the working hours of EAHE with the aim of gaining maximum heating/cooling potential. Their experiment showed that the maximum heating and cooling potential of the given EAHE were 11.55 and 18.87 MJ, respectively for a day in month of January and June through two twelve-hour intervals. Ghosal and Tiwari [6] also developed a thermal model describing the heating/cooling potential of an EAHE integrated into a greenhouse. Their parametric study showed that the increase and decrease of greenhouse air temperatures in winter and summer are along with increasing buried pipe length and depth of burial and also decreasing pipe diameter and mass flow rate of air passing through buried pipes. Nayak and Tiwari [7] theoretically studied the energy and exergy performance of the integration of a PV/T into a greenhouse in Delhi and performed an experiment to validate their theoretical results while the comparison exhibited fair agreement. Their exergy calculations also showed that such integration brought about exergy efficiency around 4%. Nayak and Tiwari [8] also developed a model for observing round the year effectiveness of both PV/T and EAHE integration into a greenhouse. They reported a 7–8 °C increase in greenhouse air temperature at night in winter season. They also mentioned that hourly useful thermal energy generated through the given coupling was 33 MJ and 24.5 MJ during day and night, respectively. Moreover, they reported the yearly thermal energy, net electrical energy and thermal exergy generated were 24728.8, 805.9 and 1006.2 kWh, respectively. Yildiz et al. [9] also performed an experiment to investigate the exergy performance of a PV system coupled with EAHE used for cooling a greenhouse. They studied the effect of climatic and operating conditions on the given system performance and concluded that the proposed system may be satisfactorily used for greenhouse cooling in Turkey. Boughanmi et al. [10] experimentally examined the performance of a novel geothermal heat exchanger in the shape of a cone for greenhouse cooling situated 3 m deep. They reported the maximum average temperature between the inlet and outlet of the given heat exchanger reaching 30 °C along with mass flow rate of 0.08 kgs-1. They also reported the greenhouse air temperature of about 12 °C which seems so promising. Hussain et al. [11] applied concentrated photovoltaic thermal systems, with an without a glass reinforced plastic enclosure, for heating greenhouse replacing the electricity and other fossil fuel types already used and developed a model validated against experimental work. They found that the given system having an enclosure was more efficient. Romantchick et al. [12] also calculated the required energy for cooling process in greenhouses via fan-pad systems fed by photovoltaic system. So, they developed a mathematical model predicting the greenhouse temperature and ventilation rates. They concluded that model calibrated against experimental data had an acceptable accuracy prognosticating the required energy via fans for cooling the greenhouse. Awani et al. [13] numerically and experimentally studied the exploitation of horizontal heat exchanger and a solar collector of 8 m2 in surface area coupled with a heat pump for heating a greenhouse of 14.8 m2 in Tunisia. Boughanmi et al. [14] also proposed a heat exchanger of conic shape for heating greenhouse. They also mentioned the priority of this geometry over the horizontal and vertical heat exchangers. Their experiments showed that the given exchanger could provide 692.208 kW heat into greenhouse increasing the greenhouse temperature by 3 °C corresponding to 0.6 kg s−1 water flow rate. To the best knowledge of authors only few researches have studied the integration of PV/Ts and EAHEs into greenhouse in terms of energy and exergy. As mentioned in the literature, only Nayak and Tiwari [2], [8] studied the energy and exergy of EAHE and PV/T integration into a greenhouse while their researches did not include the presence of plants in their mathematical modeling. Besides, Nayak and Tiwari [7] also investigated the energy and exergy of a PV/T integrated into a greenhouse while their study did not comprise the effect of EAHE. Hence, through the present study there is an attempt to not only consider the PV/Ts along with EAHE integration into a greenhouse but also include the existence of plants in formulation of the problem. Therefore, presenting a comprehensive thermal modeling of a greenhouse including PV/Ts, EAHE with plants along with investigation of PV/T and EAHE heating/cooling potentials can be considered the novelty of this study.

Section snippets

System description

The system includes three main elements i.e. greenhouse, EAHE and PV/Ts shown schematically in Fig. 1. The given greenhouse has a plastic cover with a floor area of 24 m2 while it is east-west orientated. The north side of the greenhouse is constructed of a brick wall being 0.25 m wide while the wall interior side is also blackened for more solar absorption. The height of greenhouse is 3 m in the middle and two exhaust fans are installed on east side of greenhouse. The EAHE comprises of polymer

Thermal analysis

The detailed energy analysis of the given solar greenhouse entitles the thermal analysis of EAHE, thermal and electrical analyses of PV/Ts and thermal analysis of solar greenhouse while the main objective is to find the temperatures of solar cells, greenhouse air and plant based on design, operating and climatic conditions i.e. solar radiation, ambient temperature, plant mass, mass flow rate of air, number of PV modules, the length of buried pipes etc.

Electrical analysis of PV/T

Presence of PV electrical efficiency, ηel, in Eq. (1) depicts the thermal analysis dependence on electrical analysis of the PV/T system. In previous studies, the electrical efficiency of a PV/T system was calculated by an empirical equation as follows Evans [17]:ηel=ηel,ref[1-0.0045Tc-Tamb,ref]where the subscript ‘ref’ indicates the value of parameters at the reference conditions. The above equation (Eq. (24)) had a big deficiency i.e. at low solar radiation intensity, the PV module electrical

Energy efficiency

To find the energy efficiency of the given greenhouse (ηen) the whole greenhouse including PV/Ts and EAHE is considered a control volume, shown in Fig. 2

Hence, the energy balance, based on Fig. 2, gives:Q̇i-Q̇o=dQdtCV

While the rate of inlet energy into the control volume is given as follows:Q̇i=Q̇solar,greenhouse+Q̇solar,PV/T+Q̇h-earthwhereQ̇solar,greenhouse=AiIi=A1I1+A2I2+A3I3+A4I4+A5I5Q̇solar,PV/T=AcI(t)Q̇h-earth=FR'·ṁpcrTs-Ti,p

While Ai, Ii and Ac are the area of each side of greenhouse,

Exergy efficiency

The useful energy inside the given control volume (Fig. 3) includes heat and electricity generated while these two have different qualities from the exergy point of view. Hence, finding the exergy efficiency has high significance.

The exergy balance leads to:iEẋ-oEẋ=Eẋdes+ddt(Ex)c.v.where iEẋ, iEẋ and Eẋdes represent the total input exergy, output exergy and destroyed one inside the control volume, respectively.iEẋ=Eẋsolar,greenhouse+Eẋsolar,PV/T+Eẋh-earthoEẋ=Eẋh-loss+Eẋloss+Ex

Validation and optimization

To measure the degree of validity of the current modeling against real situation, the theoretical results of the current study are validated versus the experimental data introduced by Nayak and Tiwari [7] with the exception that through the given experimental study, the effect of EAHE is not included. Therefore, FR' should be assumed zero through given theoretical modeling to exclude the effect of EAHE for having the same problem as Nayak and Tiwari [7]. The design, operating and climatic data

Validation

The trends of experimental and simulation-based results for solar cell, back surface of Tedlar and greenhouse air temperatures along with ambient temperature versus time are brought in Fig. 4.

As observed, the simulation data could acceptably follow the trends while the calculated RMSDs for solar cell, Tedlar back surface and greenhouse air temperatures were gained 12.55%, 9.24% and 6.13% being lower than those of Nayak and Tiwari simulating the same experiment with reported RMSDs as 13%,

Conclusion

In this study, an attempt was made to thermally model a greenhouse integrated with PV/T and EAHE for finding energy and exergy efficiencies. These equations were then optimized to find the optimum values of independent operating/design parameters. By optimization it was concluded that only length of buried pipes could show an optimum value of 38.11 m, on average. Moreover, the heating and cooling potential of PV/Ts and EAHE were separately evaluated for typical days in summer and winter and it

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