Sand batteries and solar heating q

[ThePog]

So I want to experiment with sand batteries, ie fill a stainless tank with copper coils and ducting, pack it out with sand and get it nice and hot with various means.

Ultimately I would like to see if I can even out the temperature in the lined section of my shed by blowing air through the sand battery automatically when it drops to a certain temp.

One of the potential heating methods is an element powered by a solar array. I know I probably need a 12v or 24v element, but the question is do I need a controller or can I just dump straight into the element and it will just get hotter or cooler depending on the input amps...?

I know that is a pretty inefficient way of doing it but it would be an interesting experiment.

Any advice appreciated.

[MaxPower]

used to install underfloor heating (water) back in the day , boss used to say your solar collector size would need to be the same size as your floor area to be efficient. Which no-one ever went ahead with. Not sure if this is relevant 

[ThePog]

Yea I am not expecting much, i have other heat input methods that will be better in this case. Its more to test if it is worth hooking up something like this to a serious solar array if/when I install one. There is a certain amount of excess solar power on sunny days that may was well go somewhere useful.

[h4nd]

Depending on how much solar you're wanting to chuck in, these controllers are pretty cheap and will PWM convert from what the panel gives you to 12/24V https://www.trademe.co.nz/a/motors/caravans-motorhomes/parts-accessories/solar-panels-accessories/listing/4139869517   ...  Actually on 2nd thoughts, although MPPT will help get the best from the solar, most controllers are set up to deliver best amps at 12/24V, whereas you're wanting to get best energy at whatever heater resistance.

Oh, here we go, MPPT water heater controller should do it: https://www.aliexpress.com/w/wholesale-mppt-solar-water-heater.html

 

 

 

[ThePog]

23 minutes ago, h4nd said:

Depending on how much solar you're wanting to chuck in, these controllers are pretty cheap and will PWM convert from what the panel gives you to 12/24V https://www.trademe.co.nz/a/motors/caravans-motorhomes/parts-accessories/solar-panels-accessories/listing/4139869517   ...  Actually on 2nd thoughts, although MPPT will help get the best from the solar, most controllers are set up to deliver best amps at 12/24V, whereas you're wanting to get best energy at whatever heater resistance.

Oh, here we go, MPPT water heater controller should do it: https://www.aliexpress.com/w/wholesale-mppt-solar-water-heater.html

 

So this works specifically for an resistive element situation rather than a load/battery? But a dirty old mppt solar controller would do it but inefficiently...?

Also can you eli5 why not just hook the solar up to the element for ol thicky here?

[smokin'joe]

i made a heap of 12vDc heating pads to go under propagation trays, and fitted a basic ' variable resistor' type controller as getting too hot and frying the seedlings.

i have a nuggetty 12v coil heater here if you wanted to see if that will work. 12v heating wire wrapped around steel/copper core, incased in concrete inside 100mm PVC. yours if you want it

 

[ThePog]

28 minutes ago, smokin'joe said:

i made a heap of 12vDc heating pads to go under propagation trays, and fitted a basic ' variable resistor' type controller as getting too hot and frying the seedlings.

i have a nuggetty 12v coil heater here if you wanted to see if that will work. 12v heating wire wrapped around steel/copper core, incased in concrete inside 100mm PVC. yours if you want it

 

Keen.

[cubastreet]

I've got a 24v element in a 300l hot water cylinder and I tried hooking up some pv panels directly in different configurations but I think i only managed to squeeze about 25-30% of the panel wattage into the element.

A normal charge controller needs a battery to turn on, you can't put it directly into a load. I ended up using batteries and a voltage switch so it doesn't drain them.

@h4nd's link doesn't work for me but something that works directly would be sweet. I think a regular dc-dc converter would work but haven't tried one.

[h4nd]

^ this.

The best power output for a solar looks like this: 

Made up numbers example to show a point:

So e.g. a 144W cell (for instance) might make  12V at 12A = 144W. This means you have a R=V/i = 1 Ohm load (heater)

If your load is too low resistance, the voltage starts collapsing as you pull more current e.g. at 13A draw, it falls to 4V =52W

If your load is too high resistance, the voltage climbs just a little, but you aren't pulling best current from it, e.g. 13V, 4A (3.25 Ohm load) = 52W

So far so good, but, the point on the curve where that peak power is, moves as the amount of light on the panel changes.

An MPPT controller (Maximum Power Point Tracking) controller diddles the draw to keep the panel at best output.

(stolen/summarised from https://www.pveducation.org/pvcdrom/solar-cell-operation/iv-curve)

 

Now, if you have couple of devices on the same panels, (e.g. also grid-tie inverter) they may (probably will) interfere with each other and get septic.

 

You could run a heater off your 230, but only draw the amount extra that the grid tie would otherwise expert - i.e. Power Point Tracking  - but flying in formation with other peoples gear is a bit of a PiTA, and prone to weird interactions and failures - and you'd have to have a variable Ohm heater (yeah, nah).

WAter heater has MPPT, and diddles the output voltage to get the best(-ish) efficiancy.

OR, just set the heater Ohms for mid/hot day, and have a go! (cheap)

 

 

[ThePog]

This might be fun;

https://projecthub.arduino.cc/stevetearle/loadmaster-xp-a-smart-pv-mppt-solar-hot-water-controller-5264c7

 

[nzstato]

Any particular reason why you want to use sand?  It has lower heat capacity and poorer thermal conductivity than water, even when accounting for density.

[ThePog]

27 minutes ago, nzstato said:

Any particular reason why you want to use sand?  It has lower heat capacity and poorer thermal conductivity than water, even when accounting for density.

 

Water is limited to 100 deg, sand can run up to 600deg which in real terms means it can store more energy. If you want to heat water then a delta T of 500+degrees works in your favour. Plus it will maintain the heat for way way longer.