Written by:
5/11/2010 

So you’ve been thinking about buying a solar water heating system. With hot water heating being a large part of many people’s energy needs, the payback periods certainly do stack up well for most. If you own your building and intend stay there for the next 8 years, it’s likely that your system will pay itself off in electricity savings over that time. After then you’ll virtually be getting free hot water for the next 12 years, because your system will hopefully be built to last 20 - 30 years.

You need to be aware that solar water heating is different to solar PV (photovoltaics), which convert the energy from the sunlight to electricity rather than directly heating water. With the PV, you have to have either batteries to store the electricity, and usually an inverter to feed the electricity back to the grid, or into a form useable by household appliances. These add substantially to the set up cost. The technology required for solar water heating is much cheaper than for PV, so while solar water heating is financially viable, this is only true for PV where the installation is a long way from the power grid. That said, I recently saw roofs of almost whole villages and sometimes large fields in southern Germany covered in PV panels, but they do have encouraging tax incentives to do this, which don’t exist here.

Are you sure using heat from the sun to warm your water works in Southland? Yes, I have my own Thermocell solar water panels on my roof right here in Invercargill. Even Invercargill gets more sunshine that the solar city capital of Freiburg in Germany. It certainly makes you wonder why we don’t see more solar panels around. Perhaps it’s because some who have headed down the solar water heating path in Invercargill have run into challenges. So, let’s outline some of the challenges now, because arming yourself with a little bit of knowledge up front will help ensure the rest is plain sailing.

Is it going to get hot enough to heat all my water? Of course with any system you will have a back up such as a wetback, or some other form of additional heat generation for winter and wet, sunless days. You can reasonably expect that during summer the sun will heat all your water, provided that the total surface area of the panels is appropriate for the amount of water you have in your cylinder, which should be matched with your demand for hot water.

Which panels? Good question, as there are various kinds all with their own benefits. Solar water collector panels are normally either evacuated tubes, or flat plate, while Thermocell panels are evacuated plate. Anything painted black, with glass over the top and water running through it in an efficient manner is likely to heat up about as well as anything else. So, what about those manufacturers’ claims about the percentage efficiencies of their model? Having been involved in the establishment solar water heating panel production in a business, there are some insights I’d like to share. I don’t believe there is deliberate misleading going on, but there is a lot of ignorance. The performance of any solar hot water panel is characterized by how well it deals with the three environmental influences of (1) insolation, i.e. how much sunlight is falling on the panel, (2) the temperature of the water being heated and (3) the ambient air temperature. The Hottel-Whillier-Bliss (HWB) equation provides the industry accepted measure of solar collector thermal performance. When these variables are graphed, it shows that for NZ conditions, any well designed and constructed panel will have similar performance – so don’t get bogged down with manufacturers’ claims around their percentage efficiencies! The most important factor is the aperture, that is the area of the collecting part of the panel that intercepts the sun. An average household will require at least 4m2 collection area. As long as it’s a well constructed model and installed correctly, the only thing to watch for in Southland is that it can handle the frosts and the installer knows what he is doing.

Do you have a good place to site the panels? If you don’t have an area to mount the collector panel which faces in a general northern direction, you are not going to get the efficiencies promised by the manufacturer. Further, if you have trees or buildings shading the panel, they will also reduce the energy capture. This could affect your potential energy gains, and hence your savings, so it might take longer for the energy savings to pay back the costs of the set up. Additionally, the angle at which the panel is placed will affect it’s performance. Ideally in Southland it should be 46˚ for best energy collection over the year. However, you can get away with a much flatter angle should you have a wetback, because the reduced winter performance can be compensated by heat from your fire anyway during the winter.

Am I going to need a new hot water cylinder? Not necessarily, for the system on offer may be able to use your existing cyclinder. Be aware that you will need a good, knowledgeable plumber. Finding someone with experience, or training in solar installations might be hard, but it is essential.

Do I need building consent? Yes! Because your panels are likely to be mounted on your roof, they will likely penetrate the roof fabric in order to ensure they stay put. Your consent will also have to cover any changes you may be making to the location of your water cylinder – for example if you are moving the cylinder to the roof cavity (as some solar water heating systems are designed to be set up) you need to be sure it can handle the weight. Competent solar water heating suppliers will have what is known as ‘producer statements’ for their equipment, which means they have had structural engineer’s sign-off all ready. If your solar panel supplier/installer is not willing to prepare the building consent application for you, you should stop wondering why and save yourself a lot of misery and cost by finding someone more competent who will. Not all systems will even pass muster for a building consent. For this reason, a bargain purchase found on TradeMe could easily end up as an expensive white elephant.

In Nelson, their council managed to get around many of these difficulties by picking some good suppliers of panels and streamlining the consent process. Their smartest move was to offer loans which can be paid back through a small surcharge to your rates, so that you don’t carry the burden of loan repayments if you sell the property. It looks like it’s working well there, so perhaps it’s time to start asking why we can’t do it all around the country!

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Tags: solar water heating , solar energy , energy from sun , energy

Categories: Energy

Location: Blogs Thoughts, Opinions and Advice About Sustianability

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