Dumping PV into PTC heating elements

[moosemaster]

Thinking of a simple way of cooking direct with PV in the summer - ie, DC, no batteries or inverters. This concept seems simple and scalable

I am envisioning something pretty scaled up - using around 3kW of PV.


I am considering using PTC elements.Ask me about a cataloque with some basic information on them. They negate the need for a thermostat which is handy.


I am struggling to wrap my head around how PTC performs when hooked up to PV. I am aware that traditional resistive elements require impedance matching - and ideally MPPT controllers to increase efficiency. With PTC elements the resistance increases as the temperature rises.

I've tested this on a small scale with a 60w panel and 12v PTC element. Gets up to the set temperature pretty quickly but I've no real means of ascertaining the efficiency.



I can show you a little cooker that got me thinking - according the manufacturer it uses around 85kWh/week which I can match in Summer with PV.


Construction something a bit like this perhaps -







- http://heatkit.com/html/bakeov11.htm


Here's a small example of the same idea...




MOD NOTE: Please do not include catalogue or weblinks to solar equipment without first getting approval from the Admin

[PNPmacnab]

I've no real means of ascertaining the efficiency.

You have a voltmeter don't you? Panel should be at its power point voltage based on resistance. It may not matter if you have any electronics to keep it at power point al long as you consistently have the same sun and cook at noon. As long as your heating resistance is ideal for the panel. One temperature cooking if that is what you want. I've made small thermally stable ovens with a PTC fuse and resistor. I can only ask why anyine would want to use PTC in an oven, if they even make PTC at those temperatures. I suppose they do, they are called oven igniters for gas. To me it just seems like a bad idea. PTC for water heaters isn't bad if you are in a habit of letting them run dry. PTC have to have a higher failure rate than just a resistance. If you are in the middle of nowhere, I'd prefer an element that could be made from a spool of steel wire.

[bob-n]

To start with, solar panels are roughly 20% efficient (17% to 24% range). In other words, for each kW of sunlight, you get roughly 200W of electricity, assuming that the load is perfectly matched to the panel (MPPT). So you start off this battle with 80% of your armor gone. Without MPPT (electronics that adapts the load condition to match the panel output for Maximum Power Point Tracking), your efficiency will be even lower.

Wouldn't it be smarter to directly use the sun to heat the food? There are many ways to do this. For example:

https://www.youtube.com/watch?v=xXxrX0JvKa8

Because of the PV 20% efficiency, any solution which directly uses the sun will need less light, so be smaller. The above idea uses aluminum foil as a reflector. That's light and cheap. I've also seen people use mirrored tiles glued to stiff wires. The wire allows you to angle the tile appropriately. You may be able to adapt an old satellite TV dish as a reflector. High-tech isn't always the best solution.

[bcroe]

Wouldn't it be smarter to directly use the sun to heat the food? There are many ways to do this.
Because of the PV 20% efficiency, any solution which directly uses the sun will need less light, so be smaller. The above idea uses aluminum foil as a reflector. That's light and cheap. I've also seen people use mirrored tiles glued to stiff wires. The wire allows you to angle the tile appropriately. You may be able to adapt an old satellite TV dish as a reflector.

I have seen a solar cooker do an impressive amount of heating. Trouble is, it only
works with negligible clouds near solar noon, and even then needs constant tending
to be well aligned with the sun. Where I live that would be nearly useless.

I agree that a submersion heater in a well insulated chamber is the way to go. First
problem is matching up the MPPT of the solar panels with the heating element. That
is not critical, and may be made less so by using more of the now rather cheap solar
panels. Given a certain power requirement, the resistance can be matched for an
operating voltage of near or somewhat above the panel system MPPT voltage. MPPT
voltage will vary little over a modest temp and power range.

From there use enough panels (or strings of panels) in parallel to generate the needed
current and power near mid day. To extend the practicality of this to 8 hours of constant
power, figure on doubling up the number of panels with some facing east, some west,
and some south (in the norther hemisphere). This concept has been used by others
and produces this power curve here at 42 deg Lat. good luck, Bruce Roe


NScurve.jpg

[moosemaster]

I've no real means of ascertaining the efficiency.

You have a voltmeter don't you? Panel should be at its power point voltage based on resistance. It may not matter if you have any electronics to keep it at power point al long as you consistently have the same sun and cook at noon. As long as your heating resistance is ideal for the panel. One temperature cooking if that is what you want. I've made small thermally stable ovens with a PTC fuse and resistor. I can only ask why anyine would want to use PTC in an oven, if they even make PTC at those temperatures. I suppose they do, they are called oven igniters for gas. To me it just seems like a bad idea. PTC for water heaters isn't bad if you are in a habit of letting them run dry. PTC have to have a higher failure rate than just a resistance. If you are in the middle of nowhere, I'd prefer an element that could be made from a spool of steel wire.

Appreciate your comment! Re the PTC element, for some reason I had in my head the idea they are durable. I'm with you completely on reducing complexity. Would you be able to share any details on how you use a PTC fuse to manage the temp in your ovens and what range this was at?

I love the idea of using a simple direct sun to cooking set-up, but we're in the same boat as bcroe. Infact, we're at 58 deg lat. But I have access to cheap second hand PV units - upwards of 12kW in total which will kick out loads in the summer (covering cooking and hot water) - and enough in the winter for lights and small devices (when wood/peat would cover our cooking and heating).

On another note, this is a new build - turns out the cost of cladding the roof entirely in used PV (with an approved watertight mounting system) is cheaper than most other roofing materials...

Thanks for the power curve bcroe - good to learn folk are having success with this approach. Completely without MPPT controllers eh? As it stands I will have 12kW worth of panels split across both sides of a SE facing roof which isn't ideal.

[bcroe]

Completely without MPPT controllers eh? As it stands I will have 12kW
worth of panels split across both sides of a SE facing roof which isn't ideal.

As I described it, you are going a relatively constant voltage system, to operate somewhere
between the MPP and the open circuit voltage. To get near constant power all day and even
under modest clouds, you take the power the heater needs, not necessarily what the
panels have available. Bruce Roe

[inetdog]

As I described it, you are going a relatively constant voltage system, to operate somewhere
between the MPP and the open circuit voltage. To get near constant power all day and even
under modest clouds, you take the power the heater needs, not necessarily what the
panels have available. Bruce Roe

Actually, to get as much power as possible from the panels you will have to have the capability to manually or automatically cut in the right number of parallel connected heating elements to keep the voltage near Vmp. Keeping the voltage at MPP while letting a thermostat regulate the heating only works when you have quite a bit more panel power than you need, i.e. if the thermostat turns the elements on for only part of the time.
If the heater resistance is low enough that it drops the voltage below Vmpp when the incident solar is limited for any reason, you will lose too much power. As an example, if the heater draws full-sun Imp at Vmp, then at half insolation you will get 1/2 maximum power IF you deliberately double the load resistance, but only 1/4 maximum power or less if you keep the same load resistance.
Drawing only the power the heater needs by keeping a constant resistance and cycling a thermostat is only an option if the panel produces far more power than you need,

[bcroe]

. Drawing only the power the heater needs by keeping a constant resistance and
cycling a thermostat is only an option if the panel produces far more power than you need,

Right that is the price of eliminating a complex control, and being able to use the system over more
than a couple hours around solar noon. Also helps for a minimal level of clouds. Bruce Roe

[inetdog]

One other comment: Although a resistance element with high positive temperature coefficient will provide temperature self-regulation when supplied with a constant voltage with low source impedance, it is exactly the wrong thing to use with a current source like PV. When the heater is cold it will try to draw enough current to deliver a lot of power to heat the oven but will instead collapse the output voltage of the PV and deliver even lower power to the oven.

[moosemaster]

Right that is the price of eliminating a complex control, and being able to use the system over more
than a couple hours around solar noon. Also helps for a minimal level of clouds. Bruce Roe

Thanks again Bruce. I can either maximise panel efficiency using complex MPPT controllers or similar - or, oversize the array and do without this component and added complexity. Does this sound about right?

Looking at designing a system that first charges a small battery bank for powering lighting and small devices. Once charged, dumping the PV into elements - heating a thermal store for water and a cooker as described in my original post.

[bcroe]

I will just show this curve that a Sun Danzer system produced, to make useful
power over much of the day, a good thing for cooking. Bruce Roe.

SunDazer10Oct17.png

[PNPmacnab]

Looking at designing a system that first charges a small battery bank for powering lighting and small devices. Once charged, dumping the PV into elements - heating a thermal store for water and a cooker as described in my original post.

I do exactly that, heating water and quite efficiently. While I believe building a control to do this is on the level of building a crystal radio, it is a moon shot for most. You won't find the answer in something off the shelf.

[moosemaster]

I will just show this curve that a Sun Danzer system produced, to make useful
power over much of the day, a good thing for cooking. Bruce Roe.

SunDazer10Oct17.png

Fantastic, thanks! I had been eyeing up a Sun Danzer...

I do exactly that, heating water and quite efficiently. While I believe building a control to do this is on the level of building a crystal radio, it is a moon shot for most. You won't find the answer in something off the shelf.

Glad to hear you're having success with this approach, you've built your own controller for this then? I've been eyeing up NIFE battery banks, seems they're more forgiving with regards to overcharging and discharging. Lots of folk building DIY charge controllers for them, in some cases charging directly from PV. Still plenty to wrap my head around.

[moosemaster]

One other comment: Although a resistance element with high positive temperature coefficient will provide temperature self-regulation when supplied with a constant voltage with low source impedance, it is exactly the wrong thing to use with a current source like PV. When the heater is cold it will try to draw enough current to deliver a lot of power to heat the oven but will instead collapse the output voltage of the PV and deliver even lower power to the oven.

Only just caught this message, thanks inetdog!

[moosemaster]

Apologies for digging this post up back up, but there is another approach I'm exploring to achieve the same goal. Stuck on a few basic points.

Now considering using a "all in on" inverter/charge controller. 2x of these MPP Solar 8000W units in parallel, to manage a 16kW array. Providing I don't get a knock-off, these inverters tend to get pretty good reviews. They can even run without batteries.

We're at a 58° northerly latitude - 16kW will see us through the short winter days for our basic needs - but kicks out a huge amount in summer.

The plan is to dump summer excess into heating elements - a hot water thermal store - and a Rayburn range I'm converting; it has a bank of heat storage bricks that can hit 700°C.

Using AC from the inverter makes a few things simpler - in terms of selecting standard elements and off the shelf thermostats.

My head gets fuzzy when trying to figure out what size elements to install - and how I might program an inverter to dump to these when the batteries are charged.

For instance, if I attempt to power 16kW of elements (3.3Ω at 230v) - the theoretical maximum of the system - but the PV array is only kicking out 8kW... what happens? I imagine this all depends on the specific inverter.

Any thoughts appreciated!