Brief NoteOn the addition of heat to solar pond from external sourcesSayantan Ganguly, Ravi Jain ⇑ , Abhijit Date, Aliakbar AkbarzadehEnergy Conservation and Renewable Energy Group, School of Engineering, RMIT University, PO Box 71, Bundoora, Victoria 3083, Australiaa r t i c l e i n f oArticle history:Received 8 September 2016Received in revised form 4 January 2017Accepted 5 January 2017Available online 16 January 2017Keywords:Solar pondEvacuated tube solar collectorsHeat additionThermal storagea b s t r a c tThis brief note addresses the method of adding heat to a solar pond from an external source which is usedto enhance the performance of a solar pond. Heat energy collected by Evacuated Tube Solar Collectors(ETSC) is transferred by circulating f l uid from the Lower Convective Zone (LCZ) of a solar pond. Whileadding the heat, a strange phenomenon is observed here which is against the intuition that higher theamount of heat added to the pond higher will be the temperature of the pond always. It is noticed thatin the case of no or low heat extraction from the pond the heat is actually lost from the pond to thecirculating heat adding f l uid in specif i c periods of time which causes a drastic downfall of solar pond tem-perature. Moreover, it is observed that larger the capture area of the ETSC, larger is the heat loss from thepond. A strategy to control the heat loss from the solar pond by installing temperature sensors and f l owcontroller in LCZ and the circulating f l uid exiting ETSC is discussed here.? 2017 Elsevier Ltd. All rights reserved.1. IntroductionSolar ponds are solar thermal collectors with inbuilt thermalstorage. Solar ponds have not been widely used in the industrydue to their low thermal eff i ciency as compared to other types ofsolar thermal collectors. A solar pond is a body of water withincreasing density gradient with depth. The higher density of thef l uid at the bottom helps suppress convective heat loss and trapsthe solar energy that reaches the bottom of the pond. Heat lossfrom a solar pond can occur in multiple modes. Heat is lost by con-duction to the ground from the walls and the f l oor. Heat is lost byconvection and evaporation to the atmospheric air from the topsurface of the upper convective zone (UCZ). Some extremely smallamount of heat is also lost by radiation. Solar ponds have beenresearched for over 5 decades and researchers around the worldhave tried to improve the eff i ciency of the solar ponds (Yaakobet al., 2011; Andrews and Akbarzadeh, 2005; Leblanc et al., 2011;Bozkurt and Karakilcik, 2012; Rubin and Bemporad, 1989). Oneof the methods proposed to improve the eff i ciency of the solarpond is by simultaneously extracting heat from LCZ and Non-Convective Zone (NCZ) of the solar pond (Yaakob et al., 2011;Andrews and Akbarzadeh, 2005; Leblanc et al., 2011; Date et al.,2013). This method was proposed to extract the solar energyabsorbed in the upper layers of the solar pond and by doing soimprove the eff i ciency of the solar pond. Studies show that themain limit to the solar pond energy collection eff i ciency is therapid attenuation of sunlight in water and this presents a practicallimit to the sunlight that reaches the storage zone. The mainadvantage of the solar ponds is their long term thermal energystorage capability. The idea of using solar ponds as thermal energystorage device has been proposed in recent years by someresearchers (Singh et al., 2014, 2012, 2013). ETSC have higheff i ciency as compared to solar pond collectors to capture solarthermal energy, but they need a separate thermal energy storagedevice (i.e. insulated tank). Commercially available non pressurisedinsulated tanks can store hot water at around 90 ?C for a very lim-ited period of time (a couple of days) with over 8 ?C of temperaturedrop in 2 days (Cynthia and Stephen, 2010; Hasnain, 1998). Whilesolar ponds as storage device can store heat for much longerperiods (weeks) without a signif i cant drop in temperature(Cynthia and Stephen, 2010). Combining ETSC with solar pondscan help improve their performance as a hybrid system. Thepresent note numerically investigates the coupling of ETSC withsolar pond, discusses some problems of this hybrid system andf i nds out practical solutions to that.2. Heat addition to the solar pondHere the total energy added by the ETSC to the solar pond is cal-culated using Eq. (1), where g et is the thermal eff i ciency of the ETSCand is a function of incident solar radiation and the temperaturedifference between average hot water temperature and ambienttemperature as discussed by Budihardjo and Morrison (2009).The heat added to the solar pond q a is proportional to the ratio(R) of aperture area of ETSC to the solar pond f l oor/surface.http://dx.doi.org/10.1016/j.solener.2017.01.0120038-092X/? 2017 Elsevier Ltd. All rights reserved.⇑ Corresponding author.E-mail address: ravi.jain@rmit.edu.au (R. Jain).Solar Energy 144 (2017) 111–116Contents lists available at ScienceDirectSolar Energyjournal homepage: www.elsevier.com/locate/solener
