What would the dimensions need to be for a demonstration domestic solar thermal water heater heater?
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We call this the Rube Goldberg wall in the utility room. Eventually we may find some form of Victorian modesty to hide all the working parts, but for now we’re proud to display our solar water heating equipment and tankless water heater.
Question by Jason: What would the dimensions need to be for a demonstration domestic solar thermal water heater heater?
What would the dimensions of the boiler be and the power of the pump to create the right pressure. And what would the size of the solar panel need to be. The whole rig needs be within 400mm (.4m), 500mm (0.5m) and 456 mm (0.456m) in total.
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One Response to What would the dimensions need to be for a demonstration domestic solar thermal water heater heater?
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I assume this is for domestic hot water. A typical house has about 2 square meters of panel. The power from the sun is around 1000W per square meter. If you convert 70% of this to output heat flow in watts you are doing well. The power in watts converts to joules (watt seconds) for a period of time. Joules heat water, about 4.18 joules raises the temperature of one gram (or one cc) by one degree celsius. Your panel seems to be 0.2 square meters, so could provide about 150W with full sun. This would provide 140W * 3600s = 504 thousand joules in one hour (0.5MJ), and depending where you are, about 2 (temperate) to 5 (tropics) times that in a 24 hour period (2 to 5 hours equivalent full sun).
Maximum heating in one hour full sun with 0.5MJ of heat:
Consider 1 liter of water requires 1000g * 4.18J = 4180J for each degree of temperature increase. The temperature increase might be from 5C to 60C in colder weather, so 55 degrees. Thus 4180J * 55C = 230KJ or 0.23MJ. It seems around 2 liters will be heated fully in an hour with full sun. You have the method here to work out tank size yourself for a given temperature rise and volume or mass of water. It is simpler if no water is drawn off for use like making a cup of tea.
This is a heat storage tank, not a boiler. It must be open to the atmosphere, not sealed, as it is possible to boil the water when you look at these figures under different circumstances. If the tank is sealed, there must be a fool proof pressure relief mechanism (a safety valve and a rupture disk), otherwise it could build up pressure and explode catastrophically. You will be liable. You could use a saucepan with a loose fitting lid as the tank, and add insulation, making sure the lid can just lift off easily.
There will be losses due to leakage of heat, depending how well it is constructed.
The type where the tank is above the panel can work by convection alone (thermosiphon), no pump. Look this up.
If there is a pump, it is only to circulate water from the panel to the tank and back to the panel, so the tank temperature rises over time. It needs to be recirculating, or it may be difficult to find a suitable small inexpensive pump with sufficient head. The pump inlet is located at the bottom outlet of the tank and if it is not closed circulation (e.g. water on the panel is open to atmosphere), the discharge pressure has to exceed the height to the top of the solar panel, allowing for head losses (pressure drop) in the pipes. If it is 3m higher, then the pump must provide that discharge head with the flow required.
If it is truly closed and recirculating, the pump inlet and circulation outlet pipes are in the water in the tank (which doesn’t have to be sealed), and the pipes, pump, panels etc are all fully primed beforehand. The panel must be sealed. Any pressure drop across the panel is added to the pressure drop due to length of pipes, but not required to lift against gravity as it is primed and pipe up = pipe down. Only a few liters a minute flow is needed for this small system. It seems around 13mm diameter tubing will suit, adding about 0.5m of head for 6 liters per minute flow and circulation 8m length. Lower flows and shorter lengths could allow smaller tubing. Note that most very small pumps only have a discharge head (lift) of a meter or so. Look into this carefully. The second link might help for pipe pressure drop.
It is usual to have a control system for the pump so it does not pump hot water around when there is no sun, but this might not matter for your demonstration. You do need a thermometer in the tank to show its temperature, and measure the rate of temnperature rise. The interconnecting pipes and the tank need to be thermally insulated.
Ecko
August 31, 2013 at 1:39 am