OPTIMAL ENERGY MANAGEMENT OF A HYBRID SOLAR WATER HEATING SYSTEM WITH GRID CONNECTION UNDER TIME-BASED PRICING

Abstract

In South Africa, 40 to 60% of the total energy of a normal residential building can be allocated to the heating of water. Traditionally, a standard electric storage tank-water heater (ESTWH) has been the main device for residential water heating within the country. However, as a result of the increase in the South African population, economy and living standards have led to an energy shortage, which has resulted in a steadily increasing electricity price. As an attempt to solve this electricity crisis, Eskom, the main electricity supplier, has recently introduced energy management activities such as energy efficiency (EE) and the use of renewable energy (RE) systems. On the one hand, the EE activities consist of reducing the total (overall) energy consumption during all the time periods, while load management (LM) activities aim to reduce the energy consumption during given time periods, such as peak times, when the Eskom grid cannot meet the demand. During peak times, the electricity consumption is charged at higher rates to encourage customers to shift their loads to off-peak and standard periods when the electricity is at a lower cost. This type of tariff is referred to as time-of-use (TOU) electricity tariff. With TOU, customers can therefore reduce their electricity bills by shifting load demands away from the peak time periods. On the other hand, in order to reduce the larger amount of residential peak load demand, renewable energy systems, such as the solar water heater (SWH), was recently introduced and implemented in South Africa as a replacement to the ESTWH. However, it has been observed that SWHs was not continuously meeting the thermal comfort of the users, under certain weather conditions. During winter, for instance, the amount of thermal energy required is greater than that of summer due to the temperature difference of the water that needs to be heated, while the solar radiation in winter is considerably less due to shorter days and the position of the sun with reference to the earth’s location. As a solution to this, the coupling of the SWH with the ESTWH, referred to as hybrid solar water heating (HSWH) system, is nowadays seen as technical and economic feasible option for water heating in South Africa. The system is composed of a solar collector that uses solar radiation to increase the temperature of water and the ESTWH, which stores the hot water. In the case of poor solar radiation, the SWH fails to increase the temperature of water to the comfortable level; therefore, the required temperature is maintained by the ESTWH. However, implementing optimal energy management of the HSWH can help to meet the required thermal comfort level while reducing the electricity cost, even more so when the TOU tariff is implemented. With this in mind, the aim of this work is to develop an optimal energy management model that will improve the operation efficiency of the HSWH. The main objective is to minimize the water heating energy cost from the grid by taking advantage of the TOU electricity tariff, meanwhile maximizing the thermal comfort level of hot water users. Simulations are performed using Matlab software, and the results demonstrate that operating the proposed hybrid system under the developed optimal energy management model reduced the operation cost when compared to a traditional ESTWH. In addition, the comparisons made in lifecycle costs of these systems shows that in the long run, the hybrid system will be the less costly option with a 49 % saving over a project lifetime of 20 years.