Techno-economical evaluation of domestic water heating technologies

Abstract

Water heating accounts for up to 40% of the total energy consumption in an average residential household. With the drastic rise in electricity prices over the last decade, the use of renewable and alternative methods of water heating has become all the more attractive and cost effective over the years. There is a number of different renewable and alternative domestic water heating technologies available to consumers, but it is unclear which technology is the most cost effective. In this study, four different domestic water heating technologies were evaluated, namely: (a) solar photovoltaic, (b) solar thermal, (c) heat pump and (d) gas water heating. These water heating technologies were compared against the baseline electric resistance water heater. The technologies were compared in terms of their ability to supply hot water when required and the cost effectiveness in doing so. In order to determine the cost effectiveness of the four technologies, a system level mathematical model was developed for each water heating technology. The models made use of data from a climatic design year and a normalised hot water consumption profile. The solar photovoltaic model includes an experimentally validated simulation model. The photovoltaic simulation model uses solar radiation data to determine the power output of a PV module, based on its characteristics. The solar thermal and heat pump water heating mathematical models respectively incorporates system specifications provided by reputable suppliers. The gas water heating model was verified against experimental work which determined the water heater efficiency. In order to ensure the supply of hot water, a conventional hot water cylinder with an electric resistance heater was included in each model. A retrofit approach was assumed from a cost perspective. In order to calculate the installation cost, it was thus assumed that the hot water cylinder was already installed and only the cost of adding the specific technology was considered. Quotations were requested from reputable South African suppliers in order to determine the capital cost of each retrofitted water heating system. Significant differences were noted in the capital cost given by various suppliers for the same technology. For each water heating technology, the yearly amount of electrical energy required from the utility grid was determined. By comparing the different technologies to a conventional hot water cylinder with a resistive element, supplying the same amount of hot water, the following was calculated: (i) simple payback period, (ii) annual operational costs, (iii) annual savings and (iv) the percentage grid input saved by each technology. For the case study it was found that the flat plate solar thermal water heater has the shortest payback period of 3.8 years, followed by the heat pump at 4.7 years. The solar photovoltaic system also had a payback period of 4.7 years and the evacuated tube solar thermal water heater has the longest payback period of 4.8 years. Gas water heating had no payback period. The annual operational costs of the heat pump water heater were the lowest at R2879 per household, followed by solar photovoltaic at R2922, evacuated tube solar thermal at R3327, flat plate solar thermal at R3357 and gas water heating at R8674. Gas water heating was found to be more expensive than the baseline electric resistance water heater at R7933. It was also found that in terms of the amount of grid input saved, gas water heating as applied in this study, outperformed the other water heating technologies with a total yearly grid input saved of 99.7%. This is followed by the heat pump at 63.7%, solar photovoltaic at 63.2% and the evacuated tube and flat plate solar thermal respectively at 58.1% and 57.7% grid input saved.