Solar Water Heating and Plumbing, Western Cape, South Africa

If a client currently pays R400.00/month on electricity, in 2011 that same electricity will cost R539.08. In 2012 that same electricity will cost them R727.51, nearly double the amount in just over 2 years time! This increase pattern will continue every year as Eskom cannot keep up with the electricity demand of South Africa.

There is no better time to start harnessing the free energy of the sun.

Electricity consumption pie chart of an average home

1. Is solar water heating a viable alternative to gas or electric heating?

Yes. Solar water heating can provide for the majority of your water heating requirements. Although a solar thermal system could be sized to replace the dependency on electricity completely this would require a substantial system, not normally used in domestic situations, and mean a sizable capital investment. Given current electricity prices, it is prudent to size your system so that it provides approximately 70% of your water heating needs. The remaining heating would be done through the electrical backup that makes up any shortfall during periods of prolonged poor weather. In this way the payback period of the system is optimized, leaving a longer period of useful life with which to maximize your return on investment.

2. How long will it take to recoup my investment?

This depends on a number of variables including, location, orientation, hot water usage patterns, etc. The annual savings will vary from house to house. However, in an average household that spends 40% to 50% of its annual energy bill on water heating, the full cost of the purchase may be recouped as quickly as 5 years. A quality solar system has a life expectancy in excess of 20 years, which will result in significant returns. The payback period will drop dramatically as electricity prices increase at above inflationary rates, as expected. Payback period is greatly reduced if you exclude the cost of the geyser, as in the case of a new build or geyser replacement, as the geyser would be purchased regardless. Beware of promises of shorter payback periods, except in these situations, as this usually means that a simple payback method has been used rather than factoring in the time value of money.

3.What size system do I require?

For domestic purposes the system size is determined by allowing between 50 and 70 litres of hot water per person per day.

4. Will the system heat the water on a cloudy day?

Good heat output is still achieved in overcast conditions as the system relies on UV light from the sun, which still penetrates cloud cover. Although the output of the collector will be reduced on cloudy days, it is still able to provide a significant amount of heating. On overcast days the backup electric element will be used to maintain the temperature. For this reason, a quality solar system should save approximately 70% of your electricity used in heating water, as opposed to 100%. Assuming the average house uses 50% of their electricity to heat water, this translates to a 35% savings on your total electricity bill.

5. How does the backup element operate?

We prefer to install all our solar systems fitted with a time switch, which controls the hours of operation of the electrical element. The reason one would not have the backup element active on a constant basis is that it would heat water in the tank quicker than the sun. In order to reduce the extent of electrical usage, the time switch deactivates the element and allows the sun an opportunity to perform its work during the day. The time switch can be set to activate the thermostat and element at different times during the day to check the water temperature and boost heating if necessary. For example, the time switch would deactivate the electricity from 06h00 to 15h00 during the day and from 17h00 to 04h00 through the night. This type of automated system ensures that you never run out of hot water, regardless of weather conditions.

6. Can I use a solar water heater with my current geyser?

Yes, it is possible. Retrofit valves can be used to allow solar panels to connect to your existing conventional geysers cold water inlet.

Retrofit systems are the most cost effective way to go solar.  These systems have also improved dramatically over the last few years and has showed big savings compared to the same geysers without a solar retrofit kit

If you have the funds to install a full new solar thermal system with a solar geyser, do it.  If you want to save water with solar at a much lower input cost, install a solar retrofit system.

7. Can the collectors heat water to a high-enough temperature?

Solar systems can heat water to more than the required temperatures. If the collector to geyser ratio (size) is incorrect, a system will either be under or over efficient. It is important to spec a system right.

8. What is the difference between a direct and indirect system?

Direct systems: With direct systems, the potable water stored in the cylinder circulates through the solar collector and is heated. Indirect systems: With indirect systems a heat transfer fluid (usually an antifreeze glycol solution) is circulated from the hot water cylinder through a heat exchanger. Some heat exchangers go through the center of the cylinder; some go around the cylinder as a jacketed system. These systems are ideal for areas which are prone to frost and/or hard water.

We suggest an indirect system for any area that experience ambient temperatures of 5degrees or less.

9. What heating alternatives are there for solar water heating?

If you do not want solar, a heat-pump is the best alternative.
Heat-pumps uses one third of the energy needed to produce the same heat generated by a normal geyser heating element. For example: A heat-pump producing 3kWof heat will use only 1kW of electricity to produce that heat.

Aesthetically heat-pumps can be more pleasing to the eye than a solar on a roof.
It is important to service heat-pumps annually to extend the lifetime of a heat-pump.  Manufacturers will also not honor warranties on heat-pumps that has not been serviced within 12 months.  Heat-pump services cost around R750.00.

There are two types of collectors used in a solar hot water system:

  1. Flat plate collectors
  2. Evacuated tube collectors

Flat plate solar collectors

Flat-plate collectors are the most common solar collectors for use in solar water-heating systems in homes. A flat-plate collector consists basically of an insulated metal box with a glass or plastic cover (the glazing) and a dark-coloured absorber plate that consists of a manifold in the bottom and the top of the collector interconnected by thinner copper risers. The risers have copper fins (covered with a special low reflective paint) that are ultrasonically welded and the collectors are insulated to maximise heat transfer to the circulation fluid. Solar radiation is absorbed by the absorber plate and transferred to a fluid (water or an antifreeze heat transfer fluid) that circulates through the collector in tubes. Flat-plate collectors heat the circulating fluid to a temperature considerably less than that of the boiling point of water (when circulating) and are best suited to applications where the demand temperature is 30-70°C. These panels can be installed in a close coupled, split thermo siphon or split pumped arrangement.

Tests have proven that flat plate collectors heat more efficiently than evacuated tube collectors in perfect sunny conditions. Tube collectors perform better in partly cloudy conditions.

Evacuated tube solar collectors

An evacuated-tube collector comprises a closed glass tube, inside which is a metal absorber sheet with a heat pipe (normally copper pipes) in the middle, containing a temperature-sensitive medium such as methanol. The sun heats up and vaporizes this heat pipe fluid, and the vapour then rises to the condenser and heat exchanger (common manifold) at the end of the pipe. There, the steam condenses, and transfers heat to the heat carrier of the solar cycle, water with antifreeze agent. The condensed fluid flows back to the bottom of the heat pipe where the sun begins heating it up again. To work properly, the pipes must have a minimum angle of inclination (ideally 20degrees or more), in order for the steam to rise and the fluid to flow back. These panels can be installed in close coupled (thermo siphon), split thermo siphon or split pumped arrangement.

The evacuated tube systems can extract the heat out of the air better on a humid day and needs less intense light to heat efficiently. Due to the vacuum inside the glass tube, the total efficiency in all areas is higher and there is better performance when the sun is not at an optimum angle - such as when it is early in the morning or in the late afternoon.

Evacuated tube vs. flat panel - which is more efficient?

The evacuated tube is more efficient overall and cheaper to repair should there be accidental damage

Flat plate vs. evacuated tube solar hot water collectors

The latest is certainly not always the greatest, so, which is actually better?

A flat plate solar hot water system or one that uses evacuated tube collectors? While evacuated tube technology is more of an investment, the benefits certainly outweigh the cost!

Evacuated tubes are a fairly new technology, where the flat panel collectors have been around for over 20 years and is a tried and tested system and still proves to be very popular amongst clients.

Both flat panel and evacuated tube collectors are great products. The debate as to which is better will always be there.

  1. Evacuated tube system. These tube collectors are easily maintained and the tubes are cheap to replace in case of breakage. Also tubes can individually be removed in the summer season if the system is over efficient. However, the collectors need to be mounted at least at a 20degree angle for the collector to be fully efficient.
  2. Flat panel system. The flat panels are very durable and require very little or no maintenance. If the panel is in proportion to the geyser, there will hardly ever be overheating of the system. Overheated potable water can be managed by a tempering valve that gets set at the client’s desired temperature and will therefore mix cold water with the hot to get the desired hot water temperature. These panels do not necessarily need to be installed at an angle. In this regard they are more versatile than evacuated tube collectors as they can be installed literally flat if a pump does the circulation where the tube collectors must always be installed at least at a 20degree gradient.

It is recommended to have all panels installed at an angle, this increases the systems efficiency in the winter months when the sun’s arc is lower.

It is highly recommended to always have a solar collector installed facing north. Collectors can be installed up to 30degrees east or west off north but not more. After that the systems efficiency gets compromised.

The water tank and thermal mixing

As heat rises, the hot water delivered to the tank from the solar collector is drawn from the top for use in the house

As the temperature of the water may exceed 60 degrees Celsius (a requirement to kill bacteria), in order to ensure a safe temperature at the taps, a tempering valve will reduce the temperature to a desired temperature, normally between 45 and 60 degrees Celsius.

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Thanks Richard and Chris for the wonderful work you did on our new bathroom , it's the showpiece of our home!!!

- Brett Gay, Pinelands, Cape Town