12v DC pump from photovoltaic panels

[frogeyepete]

I live part of the year in Spain and am keen to minimise my electricity bills. I have a panel about 4ft square with half inch stainless steel tubing attached to a large aluminium sheet. The whole thing is painted matt black and in a galvanzed steel box with slab insulation behind the panel and glass in front. It can be pointed due South and tilted to any angle. The intention is to mount the panel above a 300 litre tank and use a 12v DC circulation pump to push the water from the tank through the panel and back down to the tank. The pump is rated at 1.5 amps which I assume equates to 18 watts. It is not very powerful, but then again, it is only circulating. I am proposing to use two 20 watt photovoltaic panels to directly drive the pump. I believe that the start up current for a pump is about double its rated current. If two panels are not sufficient I can add a third. I would like to avoid having a battery in the system.

My concern is that when there is sufficient light to produce electricity from the panels, but not enough to run the motor, it might overheat the motor and damage it. Does anyone know if this would be the case. I was thinking also of fitting a thermstat to the panel that would shut of the electric supply to the motor if the panel temperture falls below say 70 deg C.

Rather than using the water in the tank directly. I am intending to put a copper coil in the tank and run mains water through that. If anyone has experience of this and the sort of surface are that would be requred, I would be very grateful.

[inetdog]

...The intention is to mount the panel above a 300 litre tank and use a 12v DC circulation pump to push the water from the tank through the panel and back down to the tank. The pump is rated at 1.5 amps which I assume equates to 18 watts. It is not very powerful, but then again, it is only circulating. I am proposing to use two 20 watt photovoltaic panels to directly drive the pump. I believe that the start up current for a pump is about double its rated current. If two panels are not sufficient I can add a third. I would like to avoid having a battery in the system.

The standard solution to both starting the pump and running at the maximum speed that can be obtained without crashing the panel output voltage is to use a "linear current booster" as described in several other threads.
It acts much like an MPPT voltage/current converter for the motor.

PS: Welcome to Solar Panel Talk, Pete!

[frogeyepete]

Linear current booster

Thanks for the information. I'll do some research into linear current boosters.
Regards Pete

[frogeyepete]

Linear Current Booster

Having read a little about these devices, it seems that it would be a good idea to add one to my system in order to help protect the pump motor. I do not however want a slow running pump when there is no direct sunlight. I need the pump not to run at all if the solar panel has a lower temperature than the tank. I am thinking of putting a central heating thermostat in the box with the panel and setting it to a high temperature, so that if the panel is cool, the thermostat will be closed. This in turn would send a 12 volt supply to a coil on a relay which would open circuit the connection from the pv panels to the pump. When the temperture of the panel increases the thermostat would turn off the supply to the relay coil and re-instate the supply to the pump. Has anybody got experience with this configuation? I am fairly confident from what I have read, that having the pv panels open circuit will not do them any harm even if there is direct sunlight on them.

[mapmaker]

? I am fairly confident from what I have read, that having the pv panels open circuit will not do them any harm even if there is direct sunlight on them.

You're correct... if you have silicon based solar panels. There are some cdte (cadmium telluride) panels that will be harmed by sitting at Voc with direct sun.

--mapmaker

[inetdog]

What you really want is a differential thermostat which senses the difference in temperature between the thermal panel output temp and the pool temp. These are widely available as parts for solar thermal systems.
Your idea to just use a single thermostat for panel temp will also work, just not as precisely.