I want to build a small off-grid PV system (<1kWp, pure DC) with simple components. The majority of the generated power during the day should go to a DC heating element, the systems main purpose. I also want the system battery to be (priority) charged to drive various small things like a fan and some LED Lights over night.
I’m targeting a heat output of 200W (at maximum of 70°C) for ~24h which is 4.8kWh. Using 200l hot water as “battery” to cycle between 95° and 70° gives me 5.8kWh – such a high temperature is not easy to achieve with direct solar heat. A PV installation with 1kWp in northern europe gives me 2.25kWh/day in october, so I can run this every three days. Heat lost to the environment with 30cm of straw isolation between 95°C water and 10°C air is around 2.5kWh per day… Maybe I need more insulation here.
I do not want to drive the heater with “precious” energy stored in the battery – just what is available during the day after charging it. I know that SmarSolar controllers can work without a battery with pure resistive loads, but I still need a source of power during the night.
I figure something like an “Ideal Diode” (ArgoFet?) would allow the SmartSolar to sense the battery voltage while no current is allowed to drain back at night.
When the heater is active all current will flow through it since it has a very low resistance compared to the battery. To allow priority charging I could enable the heater only after the battery is fully charged.
Would that work? Am I missing something easier, more obvious?