The axial and radial thermal responses of a field-scale energy pile installed in dense sand and subjected to monotonic and cyclic temperatures are examined. It is found that the axial thermal strains in the pile are more restricted to thermal expansion/contraction compared to radial thermal strains. The radial thermal strains are close to that of a pile expanding/contracting freely, indicating minimal resistance from the surrounding soil in the radial direction. As a result, very low magnitudes of radial thermal stresses developed in the pile compared to axial thermal stresses. The pile-soil radial contact stresses estimated from cavity expansion analysis are up to 12 kPa for a pile temperature change of 22.5 o C and are likely negligible for the range of commonly-encountered operating temperatures of energy piles installed in dense sand. During cyclic heating and cooling, unstable changes in axial and radial thermal strains were observed initially during initial cycles indicating a ratcheting response. The changes in strains became more stable over further cycles, without significant changes in side friction, pile-soil contact stresses, or strength of the dense sand.

Solar thermal energy conversion is gaining more attention among researchers due to the recent development in nanofluids and molten salt technology. Among various solar collectors, parabolic trough collector has received significant attention from researchers due to their operating temperature range (60-240 °C) feasible for power generation. Parabolic trough collector is currently having a higher number of installations compared to other concentrated solar power technology around the globe. Most of the conventional heat transfer fluid used in PTC has poor heat transfer and light to heat conversion properties. Therefore, it is advantageous to enhance the thermophysical properties of heat transfer fluid to improve the overall efficiency of the system. Well-engineered nano-enhanced heat transfer fluid is advantageous because a very low mass fraction of nanoparticles bring considerable enhancement in thermophysical properties. This paper focuses on the most recent advancement in heat transfer fluids, their preparation, and stability issues when doped with nanoparticles. Various heat transfer fluids currently used in parabolic trough collectors and the nano-enhanced heat transfer fluids having the properties better than conventional heat transfer fluids are compared and their preparation methods and properties are discussed.Enhancement of thermophysical properties of molten salts by doping nanoparticles and their enhancement in thermal stability at high temperature, the possibility of using mono and hybrid nanofluid, ionic liquids, gaseous heat transfer fluid, and vegetable oil as the heat transfer fluid in parabolic trough collectors are the key highlights of this review.

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SUMMARYOcean water covers a vast portion of the Earth's surface and is also the world's largest solar energy collector. It plays an important role in maintaining the global energy balance as well as in preventing the Earth's surface from continually heating up because of solar radiation. The ocean also plays an important role in driving the atmospheric processes. The heat exchange processes across the ocean surface are represented in an ocean thermal energy budget, which is important because the ocean stores and releases thermal energy. The solar energy absorbed by the ocean heats up the surface water, despite the loss of heat energy from the surface due to back-radiation, evaporation, conduction, and convection, and the seasonal change in the surface water temperature is less in the tropics. The cold water from the higher latitudes is carried by ocean currents along the ocean bottom from the poles towards the equator, displacing the lower-density water above and creating a thermal structure with a large reservoir of warm water at the ocean surface and a large reservoir of cold water at the bottom, with a temperature difference of 22 C to 25 C between them. The available thermal energy, which is the almost constant temperature water at the beginning and end of the thermocline, in some areas of the oceans, is suitable to drive ocean thermal energy conversion (OTEC) plants. These plants are basically heat engines that use the temperature difference between the surface and deep ocean water to drive turbines to generate electricity. A detailed heat energy budget of the ocean is presented in the paper taking into consideration all the major heat inputs and outputs. The basic OTEC systems are also presented and analyzed in this paper.

Heat pumps connected to geothermal energy piles have variable operating hours. These variable hours can affect the thermal behaviour of these piles. Some key findings are presented for intermittent (16 hour) and continuous (24 hour) cooling modes in this abstract. It is found that the 16 hour cooling mode is more beneficial in terms of energy extraction, ground recovery, pile temperature and strain recovery. Introduction and backgroundMany studies conducted on variable operating hours of heat pumps have mostly focused on the variation of heat exchange and its effect on ground temperatures. Li et al.[1] simulated the effect of intermittent and continuous operation on the underground temperature fields of boreholes and piles. The continuous mode always had higher impact on the ground temperature compared to intermittent mode. You et al. [2] conducted experimental studies on piles for continuous and 24 hour intermittent modes and found that heat transfer was higher in intermittent mode compared to continuous mode. In another study, Park et al. [3] had concluded that intermittent operation is good for ground recovery as well as for better heat exchange. Jalaluddin et al. [4] experimentally studied the heat exchange in a steel pile, for continuous (24 hour) and discontinuous (2 hour) modes. They found that heat exchange rate for 2 hour operation was always higher.While it is clear from previous studies that intermittent modes improve heat exchange due, studies on the reaction of internal pile temperatures are limited. Also, the response of thermal strains has not been studied. The studies of thermal strains will show how elastic the pile response is for stop-run operations compared to continuous operations. Also, continuous operation may have continuous thermal stresses at the concrete/soil interface due to induced thermal strains in the pile. The main objective is thus to investigate the thermal behaviour of a full scale geothermal energy pile under intermittent (16 hours per day) and continuous (24 hours per day) modes of operations. Some key findings are presented in this abstract.