Terry,
You don't have all the information, this is why you can't understand the context. Water is heated by a heat pump up to 47°C, overheated by heat pump up to 55°C, than overheating with an electrical heater, originally up to 75°C.
Overheating occurs only when PV power is available, but PV power is not free because in Luxembourg we have to pay taxes and network costs on it (network was available to provide the PV power we used), around 5 cents per kWh all included. Heating with network electricity and overheating with PV electricity has a similar price, but when we overheat and loose the heat at night because the boiler is not enough insulated, there is a financial loss.
Sounds crazy. Taxes and network costs are only on AC current, so there are solutions to heat your water directly from the DC current coming from the PV panels.
If water in the boiler has 47°C, it's like 25°C more than room temperature, if it is at 75°C, it's a 53°C temperature difference, more than twice the original difference, so heat loss are more than twice the values in standard configuration, so you better have a good insulation. If you have solar thermal, it's not so much an issue, but when heated with electricity, it's problematic.
Well, the only thing I wanted to say at the begining is that it is not so easy to develop smart grid technologies because the existing infrastructure is not compatible with concepts like overheating, overcooling... that there are many hidden costs on the infrastructure.
Etienne
