unique solar application

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  • dwh
    dwh Solar Expert Posts: 1,341 ✭✭✭
    #32
    Re: unique solar application
    C Keith wrote: »
    The fact that switching DC is a problem is very true , but I have solutions for that. Useing the thermostat to switch an oversized DC rated contactor.

    Latching contactor or does the magnet draw power continuously? How much does it draw?
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  • raydias
    raydias Solar Expert Posts: 68 ✭✭✭✭
    #33
    Re: unique solar application

    keith - Here is another idea

    I was searching google for more information on Solar, heating etc and came across a thread on a motor home forum on DIY Instant hot water plans.

    http://repairingyesterdaystrailers.yuku.com/topic/1022/DIY-Instant-Hot-Water-Heater-Plans?page=1

    Instead of using AC elements what about using DC elements connected to the dump load. Connect the device to a second pre-heater tank and have a slow DC pump cycle the water through the heating elements while the excess solar energy is heating the water and driving the pump.

    4534810709_9410afb143.jpg

    or this expanded version depending on how much excess you have
    4535444582_bf6fa8a0d6_o.png

    added to
    http://www.teladaq.com/heliatos/ayb/connectionSchemes.htm

    I found an inexpensive high temp low flow pump on lightobject.com and there are others there.

    Specification

    Input: DC 5~6V
    Output: 1.0L/min or 0.264 gallon/min
    Range: 1M (3ft horizon) @6V
    Current: 300mA@6V
    Intake: 9mm Dia.
    Outlet: 7mm Dia.
    Life span: > 20,000hrs @ 1600rpm~4200rpm
    Noise: << 20dB
    Working Temp: 100'C (non-submersed)
    Size(L*W*D): 32mmx22mmx36mm (1.26" X 0.87" X 1.42")
    Weight (net): 2.0oz

    Just a thought from a rookie
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #34
    Re: unique solar application

    Technical point: there's really no such thing as a DC heating element. It's just a piece of resistance wire. It doesn't care if the current flows in one direction or both. The same can't be said for controls such as thermostats, however. Any element will have a Watt rating which will translate into the amount of heat you can get out of it. That is based on a given supply Voltage and current draw (reflective of the resistance value). Lower the Voltage (is in a DC system) and you get less Watts and less heat.

    Beware of the use of a term like "DC water heating element" as an excuse to charge high prices for the same thing.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #35
    Re: unique solar application
    Cariboocoot wrote: »
    Technical point: there's really no such thing as a DC heating element. It's just a piece of resistance wire. It doesn't care if the current flows in one direction or both. The same can't be said for controls such as thermostats, however. Any element will have a Watt rating which will translate into the amount of heat you can get out of it. That is based on a given supply Voltage and current draw (reflective of the resistance value). Lower the Voltage (is in a DC system) and you get less Watts and less heat.

    Beware of the use of a term like "DC water heating element" as an excuse to charge high prices for the same thing.

    coot,
    you are correct in your analogy, but would you put a 240vac heating element to use with say a 24v source? of course not as it wouldn't heat. now using a 24vac heating element with 24vdc would work. ac or dc as you said does not matter, but being a resistance the voltage does matter.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #36
    Re: unique solar application

    Basically, to estimate the power of a higher voltage element at low voltage:
    • Power = (Element Watts at Vac rating) * (Vbatt)*2 / (Vac rating)*2
    Say you have a 4,800 watt element rated at 240 VAC, then running the same element at 24 VDC will be roughly:
    • Power = 4,800 watts * 24v^2 / 240v^2 = 48 watts
    -Bill

    Note: Nichrome wire resistance is sensitive to temperature... As the wire cools, its resistance drops--So it is very possible that the above "48 watt" DC element will draw significantly more power because the element runs cooler on 24 VDC... Do your own measurements to confirm.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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