Sizing the solar system.

alt It is exciting be to in the 21st century with advanced technologies at our disposal. Today solar energy is taking on a new image combining it with computer and energy optimisers. With this in mind, some basic concepts must be understood when deciding on a solar system.

It is not necessary to bore you with the same old facts that you have read a thousand times before, like the fact that the sun gives five thousand times the total energy that the earth needs, but a few useful facts are not out of place. For instance,

the term ‘solar technology’.

Solar technology consists of two basic branches:

1. Solar Thermal. By this we mean the transformation of incident solar rays into heat by using solar collectors. This captured heat is used to heat directly or indirectly our hot water requirements in the house, hotel and so on.

2. Solar Photo Voltaic. Here we are concerned, not with heat, but the light we obtain from the sun which we convert directly into electricity. This is generated in the form of direct current which can be stored in batteries and ultimately fed into an electronic inverter which then produces the alternating current at a voltage and frequency suitable for household use. The challenge today is to make the system large enough and cheap enough, so that the surplus energy over and above your own requirement can be fed back into the grid. Thereby putting cash into your pocket.

Sizing the solar system.

First off, if you have been using a 150 liter conventional geyser you cannot just replace it with a 150 liter solar system. If you do exactly that, your new system will save you practically nothing. Remember, your old geyser that you used before, had 24 hours per day to do the job of heating your household water. Your new solar system only has 8 hours per day to do the same job. It is easy to see that the backup element that comes with the solar system will be more than active. With the back up heater doing most of the work, all you have achieved, is to shift your geyser from inside the roof to the top of the roof. You have however, done two good things – i.e.

1. You have made the salesman happy because he sold you a system –

2. You have impressed your neighbours because they will believe you are doing a green job

To improve this basic system, consider a practical example.

The manufacturers put back up elements into the solar tanks, not only for the two or three days of bad weather, but also to be used regularly, even when the sun does shine. Assume your tank is filled with hot water at midday. You now take a bath before you go shopping. You fill the bath and drain half the water out of the tank. This hot water is replaced with cold water, lowering the tank temperature. When the temperature goes below the temperature where the thermostat cuts in, the element will switch on. You might be advised to install a timer to prevent the element from switching on during the daytime. The problem with this is that the timer doesn’t know if there is more sunshine in the afternoon to reheat the water? In addition to the timer, which overrides the element, you will need a heat sensing device which will in turn override the timer. By now you have spent more money than what a 150 liter system is worth and your repayment period goes to 10 years plus. Remember timers and other electrical equipment may only be installed by qualified, registered electricians.

Companies like Stiebel Eltron in Germany have been heating water (in various forms) since 1924. They have more experience in solar heating than any company in South Africa. KVA Control has been in solar heating since 1976 at which time the company’s CEO, Harold Escherich patented a solar collector that electronically tracked the sun to give a substantial better overall efficiency than stationary collectors. This newly patented tracking system did not get very far in production since at the same time; Germany produced the first domestic heat pumps that were suitable good looking and compact in size to place in the most modern of homes. By the time that the patent registration was completed, a comparison was done between the new ultra efficient solar system to the heat pumps and the solar collector used a lot more electricity over a period of a full year. The heat pump uses a very small amount of electricity and works in all weather conditions and supplies you with hot water, cheaper than the equivalent size solar system.

To calculate the correct size solar system for your family’s needs… read on.

The first step to determining the size of the solar system needed for your household is to determine your families bathing habits and then by using table 1, the litres needed for the family.

Tabel 1.Liters of hot water (at 55 degrees C) used per day

Persons in the family	1	2	3	4	5	6	7	8
Economical family	50	100	200	250	300	340	360	390
Average Family	75	150	250	320	360	425	460	490
Family using lots of hot water	100	200	300	420	460	550	580	600

Once you have established your family’s requirements for hot water, you can size your solar system by studying the following:-

Now: To be cost efficient, remember that the aim is to use the solar heater without the element as far as possible to improve the payback period.

Example:- Select a town nearest to you from the above.

If you happen to live in East London and have a family of four people, using an average amount of water, then from table 1, you would require 320 liters as shown. If you live in East London area, you will heat 60 liters of water for every square meter of panel that you erect. Therefore, we divide 320 liter by 60 and obtain 5.3 sq meters of panel. From this explanation, you should select a solar system with at least 300 liters of storage capacity and at least 5 square meters of solar collector area.