Solar panel area to heat accumulator tank

[greenway]

I have a thermal store (accumulator) heated at the moment by  burning wood, but it also has a solar coil fitted (rated at 45 ltr per min). The accumulator does domestic hot water and heating. It is a very large store (1000ltr) but it usually only gets fired up every 2-3 days and this provides all our needs.

I would like to add in solar panels. Any idea how big an area would be required. I've been told by the Finnnish suppliers of the tank that it needs 10-15 metres sq. but this depends on the the type of panel and the efficiency I guess. We are also quite far north (Inverness) - lots of wood but less sunshine.

Paul

[Ivan]

Hi Paul,

For standard sized solar cylinders, we recommend around 6.5-8.5 litres of store volume  for each solar tube. On the basis of your store volume, this means 120-160tubes (6 - 8 standard 20tube 47mm panels). This is a rule of thumb.

However, it will depend to a certain extent on water usage (or heat usage, if you are extracting heat from the store for other purposes), and the quality of insulation on the store (usually good for large stores). So, you may be able to underrate the solar panels on this basis. As the panels are modular, you can always add more at a later date if you feel you require more heat.

Ivan

[greenway]

Thats a lot of panel - would it be cheaper to use the 30 tube panels rather than the 20 tubed ones?

[Ivan]

If the system is in its design stage, I would recommend separating the hot water from the heating requirements. For the majority of households, you should not need more than 170litre cylinder, which means you can get away with 1x 20tube or 1x30tube solar panel.

If you are heating 500litres of water, then your requirements basically equal your hot water usage plus your heatlosses from the cylinder. The heatlosses from a 500W cylinder will be around 3 or 4 times higher than for a standard sized cylinder, insulation values being equal. If the insulation is good, you may be able to get away with far less than the 'rule of thumb' - as the rule of thumb works on the basis of water consumption proportional to cylinder size. If you use the same amount of hot water as an average family, then it is only your cylinder heat losses that will be higher.

The price of 30tube panels is similar to the 20tube panels on a 'per tube' basis

Ivan

[greenway]

Ivan
The system I have installed is an 'all year round cylinder' allowing me to draw off hot water and feed the central heating from a single tank. With the option of burning wood, solar and wind generation through the immersion heaters I felt this offered the best compromise. The cylinder is very well insulated and will hold its heat for 2/3 days. It can also be taken up to 90 degrees if required. At the moment I'm burning wood every copuple of days to bring the temp up and provide all the water we need.

The job for the solar panels is to reduce my dependence on burning wood at times when I can capitalise on solar/wind.  Thermometers on the tank allow me to keep a close eye on the temperature in the cylinder. The tank is also designed to encourage thermal layering so that there is no cold water entering the tank and reducing the temp of the cylinder dramatically.  A heat exchanger high up on the tank feeds the domestic hot water. My best approach is perhaps to experiment with a couple of panels and see how that impacts on the recover time for the tank temp.

Paul