What is the best option – 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 technology for heating water has also sharply increased.
But, as with many products on the market, one can find considerably contradicting claims with regards to the different technologies used for water heating.
Let’s try and shed some light on specifically the saving that is possible with a solar water heater and a domestic hot water heat pump.
Solar Water Heaters
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 half the heating capacity and an electrical element will need to do the rest.
Also if for instance 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. If someone would like to take a shower in the morning, an electrical element again will need to heat the water to a useable temperature.
Bear in mind that if all the hot water heated by the sun is consumed in the evening it may take anything between 3 to 5 hours for the electrical element to recover the hot water to an acceptable temperature; this is based on the size of storage tank and temperature of the water.
Solar water heaters rely on electrical elements to provide hot water at all times. A highly efficient properly sized solar system will typically provide up to a 50% saving on the energy required for hot water.
The 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. You see many houses where families of 3 or more are living with 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 impossible.
Domestic hot water consumption:
Statistically, it is estimated that the average family’s consumption of hot water is 35 to 50 litres per person per day. If we add the consumption of other hot water consuming devices in and around the home (e.g. washing machine, dishwasher etc), then these add an additional load to the hot water requirement depending on the usage, it therefore becomes imperative to select the correct size solar solution to meet your requirement.
As an example, a family of four, with an average hot water consumption of 40 litres per person per day will require a 160 litre solar water heater, but if household appliances are taken into account then it would be wiser to rather select a 200 litre solar solution. This is commonly known as over-sizing and is the wiser choice.
In order to take full advantage of a solar water heater system maximum use of hot water should be made during daytime hours so that the system can recover during the later part of the day and continue to produce hot water during the evening, therefore maintaining its maximum efficiency.
Roof should be North facing.
The position chosen for the solar water heater installation should not be shaded by any obstacles such as trees, buildings etc (all year round), so as to ensure at least 4 hours of sun daily.
Roof truss support and bracing needs to be done where required.
Hot Water Heat Pumps
Domestic hot water heat pumps work slightly different. A heat pump uses a small amount of electricity to extract a lot of energy from the surrounding air. Therefore, a heat pump is also using the energy from the sun but indirectly. 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. 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 one to always have hot water at a fraction of the cost no matter when or how much water is used.
Not enclosed area – outside as close as possible to the geyser.
Could be against a wall or on a stand.
At least 1.5 meter space in front for free airflow.
The life expectancy of both a solar system and heat pump is very similar and is estimated at 10 years. In both cases however there are systems that are running for more than 25 years and still going strong.
Both solar systems and heat pumps need to be serviced annually to ensure optimal performance. Servicing is basically just 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 oversized and water usage patterns needs to be adjusted. Typically you need double the volume of hot water that you would need for a normal electrical geyser or a heat pump system.
A solar system relies on sunny weather and needs electricity as a backup at night or on cloudy days.
A heat pump is not weather dependant but needs electricity.
For areas like the Western Cape (Cape Town) a heat pump is a better option due to the winter rainy season.
Saving Comparisons between Solar Geysers and Heat Pumps