The operation of the SGSP in Melbourne starts on 1st October which is early spring season. The removal of heat starts 60 days after the start of operation. The SGSP under consideration is 3 m deep in which the thicknesses of the UCZ, NCZ, and LCZ are assumed to be equal to 0.3 m, 1.2 m, and 1.5 m, respectively [1–3]. The data of monthly average daily temperature and the monthly solar radiation in Melbourne are adequately approximated here by sinusoidal expressions and presented in Fig. 3. The numerical model similar to that discussed in Refs. [1] and [2] is further developed for this study with a layer of polystyrene insulation of different thicknesses at the bottom of the SGSP. The numerical model here is one-dimensional finite difference unsteady heat conduction model, as done by many researchers in the past [1,11,12]. The heat loss from the sides of the SGSP is assumed negligible compared to that from the bottom since the bottom area is large enough compared to the sides. Solar insolation is absorbed by different layers of the SGSP and is converted to heat. A part of that is lost to the atmosphere and the ground, and the rest is available for extraction. The target of insulating the bottom is to minimize the part of heat lost to the ground such that more heat is available for recovery. The ground temperature beyond 5 m below a SGSP can be assumed to be equal to the yearly average ambient temperature [1]. Hence, the heat which is lost from the SGSP to the ground is considered to be stored in the depth of 5 m from SGSP bottom [3]. The 5 m thickness of ground is divided into 20 sublayers for numerical simulations to evaluate the temperature at the nodes which are at the center of each layer, while the gradient layer NCZ is divided into eight sublayers. UCZ and LCZ are assumed to be single layers with uniform temperatures due to convective mixing. The temperature of the UCZ and the initial temperature of the heat transfer fluid (HTF) are assumed to be equal to the monthly average local daily temperature [1,2] of Melbourne. Heat is removed from the storage zone of LCZ by flux of HTF through in-pond heat exchangers in the LCZ. Hence, the easiest way to control the heat removal is to control the HTF flux through the heat exchangers. Effectiveness of heat transfer of the heat exchangers has been assumed to be equal to one. The thermal performance of the SGSP without insulation and with polystyrene insulation of different thicknesses is evaluated here in terms of temperature development in LCZ, heat removal from SGSP, and thermal efficiency of the SGSP. The density, thermal conductivity, and specific heat of polystyrene used for insulation are 35 kg/m^{3}, 0.03 W/m·K and 1400 J/kg·K, respectively [5].