A lake is one of the geothermal energy sources to meet increasing demands for renewable energy use. In this study, a series of numerical modeling was performed to evaluate the applicability of a close-loop lake water heat pump (LWHP) system in Lake Soyang, Korea. A non-isothermal pipe flow model was used to simulate the flow and heat transfer processes occurring in the LWHP system with the main pipe and several helical tubes for heat exchange. Based on the temperature data measured in the Lake Soyang for 4 years, the installation depth and the number of helical tubes were determined sequentially, and the sensitivities of additional installation and operation factors on the system performance were analyzed. Assuming a mild current in the lake, the installation and operation conditions for the efficient operation of the system were suggested as follows: The installation of 16 helical tubes at 50 m deep, the circulation rates of heat-carrier fluid of 189.3 L/min, the inner diameter of tubes of 32 mm, and the wall thickness and thermal conductivity of 2.9 mm and 0.4 W/mK, respectively. Considering many lakes and reservoirs in Korea, the closed-loop LWHP system would be a viable renewable energy application.Energies 2020, 13, 762 2 of 16 at a planning stage [7][8][9][10][11][12]. Above all, because the performance of the closed-loop system depends on the heat transfer efficiency of the system itself [5], it may not be necessary to characterize the dynamics of lake water in detail. If an antifreeze solution can be used as the heat-carrier fluid, the closed-loop LWHP system enables heating operation in cold climates [8].There have been several studies about the operation of the closed-loop LWHP system. Actually, the LWHP system is a type of surface water heat pump (SWHP) system, and, therefore, most of its features and operation are similar to those of the SWHP systems using various water bodies, such as pond, lagoon, river, seawater, and even wastewater. Chiasson et al. [13] demonstrated a model to simulate the heat transfer performance of a shallow pond considered as a supplemental heat reject of the closed-loop ground source heat pump system. Büyükalaca et al. [14] investigated the potential of the Seyhan River and the dam lake as the heat source and sink for the application of the LWHP system in Turkey. Morton [3] investigated the feasibility of a reservoir as a heat source for the LWHP system application on a university building in the UK. Schibuola and Scarpa [15] showed the case study on the SWHP system using lagoon water as an alternative for space heating and cooling of a historic building in Venice, Italy, where groundwater source cooling and heating is restricted. Zheng et al. [16,17] presented numerical and experimental investigations of the thermal characteristics of a helical coil exchanger for a seawater heat pump system under icy and non-icy conditions in winter. Kuyuk et al.[18] suggested a cooling system using cold water from the hypolimnion of a deep lake for air conditioning of an underground mine i...