An instantaneous gas water heater in our view makes sense in places where there is no electricity available. The completed installed cost of a proper unit is only a few thousand less than a flat plate with geyser and the saving from the solar system is almost double for 200L hot water usage. Gas water heating is not influenced by Eskom load-shedding, but keep in mind that we have had gas supply shortages and that will leave you with cold water.
With storage water heaters load-shedding is normally not a problem unless the outage last for more than a day, since the water can stay warm in the tank for long periods, and re-heating using an electrical element or heat pump takes only a few hours. If you use 200L of 60°C water per day, 9kg of LPG will last you only 10 days and so you will have to think about the gas supply and logistics. Instantaneous gas water heaters are also great backup options for solar water heaters
installed in locations with no electrical grid connection. The gas heater in such an installation will only cover the shortfall of the solar water heating system.
A solar PV water heating element in our view makes sense only when the geyser is situated far from the solar panels, like in a multi-story building. The thermal losses in the piping of the normal solar thermal system in such an installation will reduce the output to such a level that the PV heating could be more cost effective. For standard installations where the geyser is within 10m or so from the solar panel, the flat plate system will provide a much better return on investment.
A 3 x 330W installation isslightly more expensive than a 2.4m2 flat plate installation, but the flat plate willsave R14k more over 5 years, in our example. Most solar PV element systems we see installed are completely undersized and is done by people that are simply trying to get away with the cheapest possible option. A PV thermalsystem that will give you almost the same output power as a 2.4m2 flat plate will need 5 x 330W PV panels, resulting in an expensive system that requires 10m2 of north facing, non-shaded roof space. Should the client at some stage want to install a PV electrical system to become more independent from Eskom, he might not have enough roof space left.
Another great concern with PV element systems is the current lack of regulation of the installations and equipment. PV element systems operate at lethal DC voltages, and installing it cheap and nasty could potentially be deadly.
A solar thermal water heater system will provide a great return on investment but it is important that it is installed correctly. Unfortunately, we are also seeing installers that are pushed for price going for undersized systems. If we for example look at the 200L system, using an ITS 2m2 flat plate collector, your saving will be R26758 over 5 years. By going for the slightly bigger 2.4m2 flat plate, your installation cost will increase by about R400, but your saving over 5 years will increase by R5797 to R32556. A 2.4m2 flat plate is also by no means too big for a 150L system and so a 2m2 flat plate is only really making sense for bigger systems where you require multiple panels
A domestic hot water heat pump will provide the best saving in this example. An ITS 4.7kW heat pump installation will cost you about R5k more than a 2.4m2 flat plate installation, but will result in a R9k better saving after 5 years.
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 waterthat what you would need for a normal electrical geyser or a heat pump system. In coastal areas like Cape Town however the winter irradiation is much lower than summer irradiation. This means that even if your solarsystem was sized for 100% of your solar usage in
summer you will have only about half the thermal output you need in the winter, and the electrical element would need to do the rest. Winter is also when people use the most hot water, so a heat pump would almost always be a better solution in areas like Cape Town.
Solar PV and Thermal systems can only “harvest” energy while the sun is shining. The system sizing is done to give you a certain percentage of your energy requirement per day, which is based on how many liters of hot water you use per day. If some friends visit and stay over the extra hot water required will need to be heated using Eskom. With a heat pump system the heat pump will simply continue heating as you use more, while providing around 66% saving on whatever you use.
To get the most out of solar PV and solar thermal heating systems, it needs to be used correctly. If you use most of the hot water in the evening, you will need Eskom to have hot water in the morning. If you are not using a lot of water in the morning, the solar system is heating a tank that is already almost on temperature and so you will not use all the energy available from yoursolarsystem during the day. With a heat pump it makes no real difference when and how you use the water.