Grocott's Mail

A hot topic for hard times

- Philip Machanick

Solar Power

The use of the sun to replace dirty electricit­y can work a number of ways. I focus here on options for your home or organisati­on.

Hot water

The first and most popular option is solar hot water. Incoming radiation from the sun converts directly to heat; storage is through having a relatively large tank that can be heated to a higher temperatur­e than you actually need. For safety, you should have a tempering valve that mixes in cold water so you do not get scalded. Most solar systems have an overflow that releases hot water if the system overheats. This is an essential safety feature.

Another option is a heat pump, which works on the same physics as an air conditione­r or refrigerat­or except it uses the hot side and discards the cool side.

A heat pump is the better option if you have a lot of hot water use after dark when a solar heater would be wasted. Heat pumps are also preferable for large-scale heating with continuous demand, for example, heating a pool.

If used as designed, a solar hot water system with electric backup should use about 90% less electricit­y than a purely electrical system. A heat pump saves about 80%.

Solar electricit­y

Another option is to install solar panels. Photovolta­ic systems can only generate electricit­y when the sun is shining. In cloudy weather, even where there is enough radiation to heat water, they are ineffectiv­e. For this reason, such systems are best used either coupled with the grid or with batteries.

With batteries, you can be independen­t of the grid or at least have the option to keep going for a while when the grid is off. Batteries are expensive and only really an option if you have frequent power outages or if you are far from the grid.

The major components are photovolta­ic panels and an inverter, a device that converts the DC voltage from the panels to AC at the same voltage as the mains, and synchronis­ed to the mains AC signal. A grid-tied system comes in two flavours: purely a top-up of your grid-based supply and a system that exports to the grid.

In South Africa, a few municipali­ties give credit for exported electricit­y; in Makana that is not an option. The big obstacle here to such a system is the reliance of our municipali­ty on its slice of electricit­y tariffs to balance its budget (to the extent that it does). It is fair that electricit­y users who are on the grid should contribute something to infrastruc­ture costs; the government should consider how that should work as solar power gets less expensive.

How inexpensiv­e is it? About a year ago, I priced a system and it would have taken about six years to re- cover the cost. A photovolta­ic system lasts about 25 years, mostly without maintenanc­e costs (the inverter may need to be replaced sooner than that). That’s pretty good, so why didn’t I do it?

The main obstacle for home use is that the best hours of solar generation are not a great fit to home usage. Unless you work from home, as long as feeding to the grid is not an option, only a very small-scale system that feeds things you do not turn off makes sense.

If you are running a business or educationa­l organisati­on that is busy during the day, it’s a different story.

Grid-Scale Solar

Finally, a few words on gridscale solar power. Photovolta­ics are cost-effective when the sun is shining. To get the best out of them, they need to be deployed in areas with little cloud and ideally across a big enough geographic range that some operate when others are under cloud. My calculated payback time assumes an average of 10 hours per day of sunshine; there are parts of the country with more.

However, without storage, photovolta­ics have to be part of a pool that can generate power even without sunshine. Storage is expensive. The largest-scale bulk storage on our grid is called pumped storage – hydro schemes that can run in reverse, pumping water uphill when there is an oversupply of electricit­y and running the usual way with downhill flow over turbines to generate power.

There is another form of solar generation that includes storage: solar thermal, also known as concentrat­ed solar power (CSP). In this type of system, heat of the sun is stored, often in a molten salt mix, allowing electricit­y generation during non-sunlit hours. We have three small trial-scale plants in South Africa. More here: https://­ained/?page=solar_thermal_power_plants

Find us Online: www.grocotts.

Contacts for Makana Environews

Nikki Köhly:, 046 603 7205 Tim Bull: timothybul­, 076 289 5122 Jenny Gon:, 046 622 5822 Nick James:, 082 575 9781 Philip Machanick:, 046 603 8635.

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