Should I buy a solar water heater or a heat pump?

With the price of electricity and the awareness to sustainable living sharply increasing, the popularity of renewable energy technologies for heating water has also sharply increased. But, with many products on the market, one can find considerably contradicting claims with regards to the different technologies used for water heating.

Many of these claims and advertisements are nothing more than marketing “blah” and have no facts behind them.

We have therefore decided to try and shed some light on the saving that a homeowner will get from a solar water heater specifically and a domestic hot water heat pump.

A BACKGROUND TO THE TECHNOLOGIES:

Let’s first touch on some basics about solar water heaters and heat pumps. Solar water heaters use the radiation from the sun to generate heat. The size of the solar panel will determine how much energy can be collected from the sun. So if we, for example, have a 3m2 solar panel connected to a 150L geyser, this might give us 150L of 60°C water at the end of a warm, sunny day, but during cooler days with less sunshine, it might only be able to heat the 150L to 35°C. In this case we would need an electrical element to heat the water further. If we have a solar panel that is only half the size (1.5m2) we would only get out half the energy and an electrical element will need to do the rest. Also if we assume we have 150L of 60°C water at the end of a warm, sunny day and we use hot water in the evening, the water will more than likely be cold in the morning and so if someone would like to take, for example, a shower in the morning, an electrical element again will need to heat the water to a useable temperature. From the above it should always be clear that solar water heaters do rely on electrical elements to provide hot water. In our experience a high efficiency, properly sized solar system will typically provide up to a 50% saving on the energy required for hot water. The sad truth however, is that most solar systems in South-­‐Africa are undersized and therefore will provide much less than a 50% saving on the water heating bill. We see so many houses where families of 3 or more are living having just a 2m2 solar panel on the roof. Most likely the company that sold it to them promised them big savings, but it is simply physically impossible.

Please also note that if a high efficiency properly sized solar water heater gives a saving of 50% on your water heating bill, this will not result in a 50% saving on your total electrical bill, unless the geyser is the only electrical device in your house.

Domestic hot water heat pumps work slightly different. The heat pump uses a small amount of electricity to extract a lot of energy from the surrounding air. A heat pump is also using the energy from the sun, but only indirectly and it can work day and night, winter and summer. The efficiency of a heat pump is called the COP. A COP value of 4 means that the heat pump produces four times as much thermal energy as what it uses electrically – in other words a 75% saving on the water heating bill. Unfortunately, the COP of a heat pump is dependent on the ambient temperature and the water temperature and so, in a practical, domestic hot water system using a high efficiency heat pump, a more realistic annual COP value is 3. A high efficiency heat pump takes about 1.5 hours to re-heat a 150L geyser, which is used in most households in South Africa. This enables you to always have hot water at a fraction of the cost no matter when or how much water you use.

CONCLUSION

Even for a family that uses water conservatively the heat pump will pay for itself in just over 2 years and provide a much bigger long term saving than a solar system. The solar panel savings calculated, assumes that the solar collector panels are mounted Solar-North with the optimal inclination. Variances in the facing and inclination of the solar collector panels will decrease the systems output. The life expectancy of both the solar system and the heat pump is very similar and is estimated at 10 years, but we know of systems that are running for more than 25 years and are still going strong. Both solar systems and heat pumps need to be serviced annually to ensure optimal performance. Servicing consists of simply cleaning the system and making sure everything is working correctly. A solar system can provide a bigger saving than a heat pump, but for that the solar system needs to be the correct size and water usage patterns need to be adjusted. Typically, you need double the volume of hot water than what you would need for a normal electrical geyser or a heat pump system. The overall saving, not to mention having power and hot water during load shedding, makes this the perfect solution for the South African market.