Scientists suggest the cheapest, the best option for solar water heating in Zimbabwe

As Zimbabwe struggles with electricity demand, the government has recommended using solar water heating systems to offset electricity demand. This study  identified the cheapest solar water heating system that is best suited for Zimbabwe, using a model that can be applied to any other country.

A solar water heating system uses radiation from the sun to heat water for home use, and in Zimbabwe, this system is becoming an attractive alternative as the country experiences loadshedding. However, these systems can be expensive and different types and brands have different shapes and sizes, and different abilities to heat water effectively.

The researchers used a mathematical model to find the perfect size of the collector component of the heating system that would catch the most heat from the sun for the lowest cost.

To find this “sweet spot” the researchers determined the heat energy that each system would collect from the sun for the whole year, and divided it by its cost in dollars to find the energy-per-dollar measurement. The study used the Net Present Value of Solar Savings model for 10 brand models of solar water heaters and ranked them from the highest energy-per-dollar to the lowest.

The researchers also used the volume, or amount of water each system can heat up to determine the energy-per-dollar measurement. This is because the bigger the solar heat collector, the less the hot water that can be contained in the system, and this study aimed to find the “sweet spot” between the volume of water heated and the collector size as well.

The study tested 10 solar water heaters at 50 C, the typical temperature for central Zimbabwe situated at latitude 19° South and longitude 30° East. Researchers  found that the best, and cheapest solar water heater option was a flat-type collector that had the highest energy-per-dollar score of 26.1 kiloWatt hours per dollar (kWh/$).

With this specific model of solar water heater, the researchers calculated that it can heat 900 litres of water by using 91% of the solar radiation it receives from the sun.

These results apply to solar water heaters in Zimbabwe, but the researchers recommend that the model they used in this study can be used for any other solar water heating systems at any other location.

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Abstract

The choice of solar collector type to employ and the number of chosen collectors to subsequently deploy, are important planning decisions, which can greatly influence the economic attractiveness of solar water heating systems. In this paper, a thermo-economic model is developed for the computation of a suitable metric that can aid in choosing the most cost-effective collector to use in a solar water heating system and to determine the optimal sizing of the solar water heater components once the choice collector has been picked. The energy-per-dollar comparison metric, calculated as the annual heat energy output of the collector in an average year, at the so-called “sweet-spot” size of the collector array, divided by the annualized life-cycle cost, based on warranty life and collector initial cost, was recommended as instructive for comparing cost-effectiveness of different solar collectors. For the determination of the sweet-spot size of collector to use in a particular solar water heating system, at which the energy-per-dollar is calculated, the Net Present Value of Solar Savings was used as the objective function to maximize. Ten (10) different models of liquid solar thermal collectors (5 flat plate and 5 evacuated tube type), which are rated by the Solar Ratings & Certification Corporation (SRCC), were ranked according to the energy-per-dollar criterion through the thermo-economic model described in this study. At the sweet-spot collector area for the solar water heating system, the corresponding volume of hot water storage tank and the optimal solar fraction are also simultaneously determined. The required hot water storage volume decreases as the deployed collector area increases while the solar fraction increases, with diminishing marginal increase, until it saturates at a value of unity. For the present case study where the required load temperature is 50 °C and the solar water heating system is located in central Zimbabwe (latitude 19° S and longitude 30° E), the selected collector model happened to be a flat-plate type, which achieved the highest energy-per-dollar score of 26.1 kWh/$. The optimal size of this collector model to deploy in the solar water heating system at the case-study location is 18 m2 per m3 of daily hot water demand; with a hot water storage volume of 900 l/m3; at an optimal solar fraction of 91%. Although the method of this paper was applied only for a solar water heating application of specified operating temperature, at a specified location, it can be applied equally well for any other solar water heating application and at any other location.

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