I am a final year b-eng building services student.
I am doing some research into PV system incorporation into building services and I have read some of your articles with interest. If possible I would would like to clarify some thoughts.
1) Would it be practical to run a PV array directly without storing the energy in batteries or inverting it to AC.
2) Would it be possible to use a system on a roof above the a toilet block where point of use water heaters are located. The system would drive a DC heating element, rated at 1.5 – 2 kW 12v.
3) At worst the system could reduce the difference between incoming and outgoing water temperature or at best provide all the required power to heat the water from ±15C° to 70C° through an instant heater. The power from one 20m2 array could power several point of use water heaters because diversity of use would ensure power demand would remain below peak.
4) Alternatively, the energy could be used to heat water in a hot water cylinder in affect a heat store. With minimal losses because power from the array goes directly to the heater element (of course circuit protection and control is necessary)
5) If my calculations are correct I should be able to heat water in a hot water cylinder with a capacity of 110 liters via a 2kW 12v DC element in just over 3.5 hours. I expect the array would span 20m2 of roof area. Higher operating voltage arrays could reduce losses further.
1) No inverter, reduced cost
2) No batteries, reduced cost and more environmentally friendly.
3) Water storage could be boosted via conventional system where necessary.
1) System usage reduced to on demand (possible life expectancy increase)
2) Payback period increased
Your calculations on energy usage may indicate to you that this will work, but you have not considered the economics. A 4 ft. X 8 ft flat plate solar hot water collector will easily heat this much water and cost around $900. A solar photovoltaic module this same size would cost over $3,000 for the same area of collection. The ONLY time we use solar pv power to heat water is with a diversion controller on a battery based solar power system for those times when the solar energy would have been lost. Otherwise, it is much cheaper to use solar thermal panels for heating and solar pv modules for non-heating type electric loads.
Anytime you cannot use solar power directly as it is generated, you will lose about 15 to 20% in the conversion process from electric energy to chemical energy, then back from chemical energy to electric energy. When you add another 5 to 10% conversion efficiency loss through the inverter and a charge controller, you end up with about 70% of the original solar energy back after storing in the batteries. This loss cannot be helped if you want to save collected solar electric energy for use later. However, heating a well insulated hot water storage tank with solar thermal heated water has much less standby losses and does not have the added energy conversion losses of a chemical battery storage system. Therefore, a direct grid tie solar power system is more efficient and lower cost than a battery based solar system to generate electrical power, but many systems still need battery storage. However, I think you will find there is a good reason solar thermal panels are still best for heating applications.
Hope this helps,