Help please with DC to DC step up

[Roger Gunkel]

I have been looking at various ways to convert my waste stove heat into electricity and also the heat from the warming of the roof during the hotter months. I have got as far as producing power from the heat, but as the voltage is variable, I need to keep it at a constant 13.5 volts or there abouts, to use it effectively. Step up converters that I have found so far, seem to require an input of at least 6 volts. Not being someone with component and electrical theory knowledge I could do with some help.

 

There appear to be loads of chips on the market that will allow variable input voltages and fixed or switchable output voltages, but how to use them with additional components to make a working transformer, I would't have a clue.

There may well be manufactured units already, but what they are called or how to find them I don't know, or perhaps I don't recognise them as such. The components to build one would seem to be very inexpensive.

 

The required input voltage would be anywhere between 0 and 16volts although a cut in point of around 2 volts would be fine.

I am working on 2 systems, both with the above input limits, the smaller unit has a maximum potential current of 3.9 amps and wattage 33w. The larger unit has a maximum potential current of 15.6 amps and wattage 135w. The idea is to use the stabilised voltage to charge a 12v lead acid battery, probably eventually linking into the main domestic battery bank if it works OK. It is unlikely that I would be able to achieve the maximum potential of the system, but even 25% would be a useful amount.

 

Any ideas would be greatly apreciated

Roger

[smileypete]

I have been looking at various ways to convert my waste stove heat into electricity and also the heat from the warming of the roof during the hotter months. I have got as far as producing power from the heat, but as the voltage is variable, I need to keep it at a constant 13.5 volts or there abouts, to use it effectively. Step up converters that I have found so far, seem to require an input of at least 6 volts. Not being someone with component and electrical theory knowledge I could do with some help.

 

There appear to be loads of chips on the market that will allow variable input voltages and fixed or switchable output voltages, but how to use them with additional components to make a working transformer, I would't have a clue.

There may well be manufactured units already, but what they are called or how to find them I don't know, or perhaps I don't recognise them as such. The components to build one would seem to be very inexpensive.

 

The required input voltage would be anywhere between 0 and 16volts although a cut in point of around 2 volts would be fine.

I am working on 2 systems, both with the above input limits, the smaller unit has a maximum potential current of 3.9 amps and wattage 33w. The larger unit has a maximum potential current of 15.6 amps and wattage 135w. The idea is to use the stabilised voltage to charge a 12v lead acid battery, probably eventually linking into the main domestic battery bank if it works OK. It is unlikely that I would be able to achieve the maximum potential of the system, but even 25% would be a useful amount.

 

Any ideas would be greatly apreciated

Roger

 

Hi,

 

A good start would be to measure the voltage and current for different loads for a given heat input, and work out the available power.

 

It may be that there is very little power available when your reading 2V with nothing connected, and it's just not economical to convert it.

 

cheers,

Pete.

[Roger Gunkel]

Hi,

 

A good start would be to measure the voltage and current for different loads for a given heat input, and work out the available power.

 

It may be that there is very little power available when your reading 2V with nothing connected, and it's just not economical to convert it.

 

cheers,

Pete.

 

Hi Pete

Thanks for the reply. The reason I mentioned the low starting power, is that the system takes a few minutes to start generating reasonable output voltages, but I don't want to damage any powered accessories due to undervoltage or overvoltage. It seems to me to make sense to have the correct voltage even if the current may be extremely low at times. Also some of the subsystems require very low power drain, but are essential to keep the whole thing operating effectively.

I anticipate useable voltage and current for most of the time when the stove is at normal temperatures, but it will never be constant.

Roger

[chris w]

Hi,

 

A good start would be to measure the voltage and current for different loads for a given heat input, and work out the available power.

 

It may be that there is very little power available when your reading 2V with nothing connected, and it's just not economical to convert it.

 

cheers,

Pete.

 

I think this would be an excellent idea as one very important parameter is the "source impedance" of the system that produces the conversion from heat to electricity. If this parameter is high, then it will be extremely difficult to extract any useful power from the overall system. Efficiency would be another key parameter, viz: how much heat produces how much electricity? Voltage on its own is not enough. It's the combination of the available volts and current (ie: the power developed) that matters.

 

Chris

 

How are you achieving the conversion; ie: how is the heat converted to electricity

[Roger Gunkel]

How are you achieving the conversion; ie: how is the heat converted to electricity

 

Hi Chris,

Heat to electricity is from Seebeck effect from TEG modules

Roger

[chris w]

Measure the open circuit voltage from the TEG modules and then divide that by the short circuit current. This will give you the source impedance of your set-up.

 

Now suppose you get 14v open-circuit output and 1.5 amps short circuit current (I suspect the current will be a lot less than this actually). Your source impedance is therefore approx 10 ohms.

 

Now suppose that you can tolerate a drop of 2v from 14v to 12v. The drop of 2v will appear across the source impedance and will therefore limit the available load current to just 0.2 amps which isn't very useful as the power output will be just 2.4W. Even if the source resistance turned out to be just 1 ohm, the output power will be limited to just 24W.

 

Chris