Since solar Panels reached 5cents per Watt and Retail price is almost everywhere 10cents per Watt it is very cheap and undemanding energy source.
It seems to be but… when you want simplest system for water heating/space heating you see that controller can be most expensive piece of this system and could make investment not so attractive
but sometimes you do not even need controller at all. Because of Solar Panel Power to Voltage Characteristics it is quite easy to calculate output power of our system and choose appropriate heaters
All you need is resistant heater and energy source like Photovoltaics and/or Wind Turbine and a little bit of calculation.
I made calculator for you just below
This is Based on Ohm Law because contrary to popular belief resistor(and all cheap heaters are resistors) doesnt have Power rating per se only one parameter it does have is RESISTANCE
But how to buy Heater when you dont know the resistance of it? and what resistance do I need?
It is quite straight forward but lets calculate it for simple 2 solar panel system
We have 2 400W cheapest solar panels.
Vmpp of this panel is 30V
Vmpp of 2 panel string is 60V
Power of string is 800W
Power of any electrical electric device is Voltage multiplied by Current
And from Ohm Law we know that resistance R= Voltage(drop) divided by current
So Current(I) is Voltage divided by Resistance
and we can use it in power equation
power transferred to the heater can not be greater than power produced by solar panels
therefore 800W=< 60V^2/R
Which means 800W=3600/R so R=3600/800= 6 Ohm and we need Heater with resistance at least 6 Ohm
But how to buy such a heater? who sells 60V heaters?
Resistor -heater can work at any voltage up to declared/nominal
which means we can use any cheap resistor on the market with same resistance and use it here, or few of them just like when we build resistor ladder
I am from europe so We are using 220-230V elements but in calculator you can use 110V too
for example in our case we will need 6 Ohm heater . in 220V heaters this means we will need heater which is rated with power of P= 220^2/6= 48400/6= 8000W =8kW
but how can I buy such a big heater element?
you dont have to just buy 2pcs 4kW connect them in parallel and its done
But… not exactly
this calculation is done for ideal conditions of 1000W solar irradiation on each 1m2 of solar panels. It is not very likely scenario for Heating just about anything.
We can of course calculate it for 50% irradiation where power of our string is 400W but voltage remains the same
400W=60V^2/R1 =3600/R1 so R1 =3600/4 which is 9
we need 9 ohm resistor heater to obtain maximum energy from our solar heating system so if we are to buy againg 220V Heaters this would need to be: 220^2/9=48400/9= 5,3kW heater which could be problematic to buy but hey we can use 2 pcs 2kW 220V heaters and be just fine
Of course this system will work in full sun too but with limited power
Wind Turbine
it is even easier with wind turbine since voltage and Power are correlated with this kind of energy source
so we need Voltage of our turbine when it reaches maximum power
in this case we will need simple Voltage driven controller(you can use crowbar circuit as a controller circuit)
Lets say we have 2kW 48V wind turbine and we want to calculate heater/resistor which will not brake turbine but will take maximum energy from it.
For our 2kW wind turbine with 3,2m diameter 2kW will mean 90V working voltage so it will need …
2kW=2000W=90V^2/R R=90^2/2000 = 8100/2000= 4 Ohm
which is equivalent of more than 11kW heater when we are buying 220V equipment but hey if you’d buy 3x3kW heaters and connect them parallel you’ll receive 9kW heater for 220V which will be also 2kW for abt 100V from our turbine!
this is not best possible solution since it will put big stress on thermal disconnectors if you use any(this could be avoided if you just use signal from thermal switch used to control the dc contactor .
As for switch-on voltage for crowbar circuit which will connect load to wind turbine I would propose abt 40VDC for 48V systems and 20V for 24V systems
Better use some Solid state relays because at lower end of voltage this will behave very much as rudimentary PWM
Of course all Voltages here mentioned are voltages after rectifier. Because of ease of use and future proofing our system I propose using only DC or HFDC(High Frequency Direct Curren) in all Wind Turbine energy systems up to grid Inverter