Performance analysis of cascade multi-functional heat pump in summer season Samuel Boahen a , Selorm Kwaku Anka b , Kwang Ho Lee c , Jong Min Choi d, * a Department of Mechanical Engineering, Kuame Nkrumah University of Science and Technology, Kumasi, Ghana b Graduate School of Department of Mechanical Engineering, Hanbat National University, Daejeon, 34158, South Korea c Department of Architecture, Korea University, Seoul, 02841, South Korea d Department of Mechanical Engineering, Hanbat National University, Daejeon, 34158, South Korea article info Article history: Received 18 May 2020 Received in revised form 11 August 2020 Accepted 7 September 2020 Available online 9 September 2020 Keywords: Cascade multi-functional heat pump Entering water temperature Capacity COP Summer season abstract The use of cascade multi-functional heat pumps to provide simultaneous cooling or heating, and hot water has become essential in recent times. This study analyzed the performance of a water-to-water cascade multi-functional heat pump according to the variation of heat source and heat sink tempera- tures in cooling mode, hot water mode and cooling hot water mode. The cooling-hot water cycle was optimized to produce the designed capacities of the cooling and hot water cycles by maintaining the compressor speed of the low stage (LS) cycle as that in the cooling mode and increasing compressor speed of the high stage (HS) cycle above that in the hot water mode. In cooling-hot water mode, capacity of the hot water cycle and hot water temperature increased by 14.8% and 1.1  C respectively, as the standard outdoor entering water temperature (OD EWT) increased by 15  C. Variation of OD EWT and indoor entering water temperature had similar effects on capacity of the cooling cycle in cooling mode and cooling-hot water mode. Moreover, capacity of the hot water cycle was more stable in cooling-hot water mode than in hot water mode according to variation of hot water heat exchanger entering wa- ter temperature (HW EWT). © 2020 Elsevier Ltd. All rights reserved. 1. Introduction Heating, cooling, and the provision of hot water are essential for human comfort and living. Heat pumps are mostly used to meet the heating and cooling demands of buildings in an efficient, environ- mentally friendly, and cost-effective manner, while boilers are mostly used for generating hot water [1]. However, with the recent emergence of small-spaced cluster houses and apartments, an efficient single heat pump system that can provide simultaneous cooling and hot water during summer seasons and simultaneous heating and hot water during winter seasons promises to be a great technological breakthrough [2]. Multi-functional heat pumps pro- vide simultaneous space cooling and/or space heating with hot water generation, and have therefore been of great interest to re- searchers due to the cost reduction, energy-saving potentials and minimum space use they provide. Over the years, single-stage multi-functional heat pumps have been used to provide cooling, heating, and hot water in residential buildings [2,3]. Joo et al. [4] analyzed the performance of a single- stage variable capacity multi-heat pump unit which had one out- door unit and four indoor units. The four indoor units provided heating or cooling for four different rooms at the same time depending on the need of the building occupants. The multi- functional heat pump had no hot water generation option. Mod- era et al. [5] performed field test on a heat pump desuperheater that operated as a multi-functional heat pump. The multi- functional heat pump was found to have high energy and space saving potentials with limitations in the aspect of cost and opera- tion because the system was not optimized. Busnardo et al. [6] conducted a field study in Milan, Italy to compare the performance of a multi-functional heat pump for simultaneous cooling and heating with a conventional chiller and boiler. The multi-functional heat pump was found to have higher energy savings. Aiguo and Xiaozhong [7] experimentally investigated a single-stage multi- functional solar assisted heat pump that had an air conditioning unit for space cooling and heat pump water heater for hot water generation. A solar water heater was also used to assist in the hot water generation. The solar-assisted multi-functional heat pump * Corresponding author. E-mail address: jmchoi@hanbat.ac.kr (J.M. Choi). Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene https://doi.org/10.1016/j.renene.2020.09.036 0960-1481/© 2020 Elsevier Ltd. All rights reserved. Renewable Energy 163 (2021) 1001e1011