Micro-CHP (μCHP)

It’s μCHP’s turn. So the prime reason to use μCHP is not to generate electricity, but to generate heat. That’s right, it’s a boiler. But it’s a boiler that generates electricity, how cool! So what happens when it’s not cold, you turn the boiler off, and off goes the electricity as well. So you need a grid, and when you turn your boiler off, you are as efficient as the grid.

The fact that you are connected to the grid however is quite useful, because it means that if in course of generating enough heat for your house you generated too much electricity, it’s won’t go to waste. You just pump it into the grid and maybe make some money out of it.

So the efficiency here really comes from generating electricity as you generate heat and not the other way round, as in normal large CHPs. So μCHPs are useless in the summer (just like your boiler). Again the massive efficiency goes down the drain. It’s still better than those old central massive power plants though, so don’t get me wrong.

There are two problems I can think of right now about μCHP. First the poor people at the massive power plant. Not only are they being attacked for something that’s not their fault, (the fact that they have to dump 2/3 of the energy into the sea to give us 1/3, in reality the numbers are better, but still) but now they have to be ready to offset any electricity coming out of our μCHPs in case we feel like switching it off. Because they can’t control us, and we don’t have to even tell them when we are switching off our boilers, no one knows how much μCHP electricity there will be at any one time to count on. So they should always have backup capacity installed, and they have to be very responsive to our whims. Sounds very expensive and difficult.

This is not a big deal if there are a few units installed here and there, but it could be an issue if you have them everywhere.

Second problem, how much money do you give the people for their electricity. Give me too much money and I will have my heating on all the time. Hey you’re the one who’s paying me to heat my house. In summer, I’ll just open all the windows. The cash is worth it. Pay me too little and I won’t care. Why would I pay more for a fancy boiler and then donate my electricity to you. No thank you, I think I will stick with the old system.

So frankly, if you have the right policy in place you will find that indeed μCHPs are useful. But of course they are no where near as useful as some (Greenpeace) want us to believe. No they won’t save the world, but they’ll help bring us one step closer.

Sustainable Energy

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January 15th, 2009

5 responses

5 responses


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tio cael

I’m not sure that having a large number of micro CHP units would cause much of a problem. If there are enough of them, you should be able to model the micro CHP demand loss statistically to come up with the capacity you should remove from the grid. A similar analogy would be that if everyone turned on their kettles at once in the country (which power plants are also unable to control), the national grid would panic and there would be brownouts and blackouts, but since that is so unlikely we don’t install enough capacity to cope with the eventuality.

In fact I was thinking that micro CHP might even make the grid more efficient since they will be used more during winter (since they are boilers) when the average load on the grid is also at a maximum. This would reduce the peak to average load ratio, thereby reducing the capacity required to ensure the system can meet peak winter demand, making the grid run more efficiently.

I agree the grid buyback price needs to be fixed carefully though to avoid people running their micro CHP units to make money!

January 15, 2009 10:33 pm

admin

Interesting comment. So if there are loads of units, then turning off one randomly is not an issue, and if there are very few then they all go in the noise anyway. So I wonder if there is a point in the middle when the number of units does affect the grid, but statistical modeling is difficult because there are not enough units. Once all units are installed I imagine it would be easier to model. But as CHP is being rolled out this could me more difficult as people with different habits start getting into the game.

Also not all micro CHP units will be replacing electric boilers, they could be replacing gas ones that don’t affect the grid, so in that case they wouldn’t be removing any load off the grid at all, although they would be helping the generation part. So again depending on whether CHPs start replacing which boilers first, this could be difficult to model. And I imagine it is likely that they will replace gas boilers first as the gas connection is already there.

January 15, 2009 10:48 pm

tio cael

Yeah there could be a transitional wobble period I suppose as people start to buy micro CHP.

Regarding the displacement of grid power during winter – I was making the suggestion that if people replaced their gas (not electric) boilers with micro CHP units, this would reduce the load on the grid. The demand for grid electricity is highest during winter months, which coincides with when people want to heat their homes. Doing this with a CHP unit instead of a boiler would reduce demand for grid electricity more in winter months than in summer months, thus flattening out the seasonal demand curve.

January 20, 2009 12:24 am

Warner Efaw

Great read! I have been looking for a way to save money on my energy costs since money is lower these days. I found this post very helpful. Thank you

January 24, 2010 6:06 am

M Albertson

Hi guys, Tio is quite right. It would not be that difficult to show, using stochastic calculus (http://en.wikipedia.org/wiki/Stochastic_calculus), that mCHPs feeding the net will not pose any special problems. On the contrary, they should ease the load off the big “plants” and distribution network during peaks, as Tio mentioned, and therefore increase efficiency in energy distribution too. It’s 10 years ago since I studied stochastic theory now and I’m not sure who would benefit from it so I will refrain from trying to show the calculations.. Hope you believe me anyways. The important aspect is that the Gaussian spread of units going on/off is very wide compared to the time it takes to power up a power plant.

A much bigger problem then is the “famous surge problem”, quite specific to UK power plants, that all the kettles are turned on after the game, royal weddings etc on Saturday/Sunday afternoons (see for example http://news.bbc.co.uk/1/hi/uk/5059904.stm), again mentioned by Tio. This problem is not so common in other countries that don’t use kettles as much. It can also be modeled stochastically, but with other parameters and with a much worse result for the power plants since the spread will be much smaller (all go on within a time frame of a few minutes so the Gaussian curve will be spiky) and the power peaks are very high (typically 1800W per kettle). Thus, the power plants already have to be prepared to start to increase production before they can actually see it happening.

Now to the main issue – admin is absolutely right concerning the “summer problem” for mCHPs. Actually, that’s how I found this post, by googling for “micro chp summer”. I also predicted that this will be a problem and it’s actually one of the biggest challenges when calculating the total ROI of investing in such a system. If the winter is really long and cold, you will save more on electricity bills, since you have to warm your home more (and vice versa of course). Further aspects are that efficiency drops during the summer months although gas costs much less compared to the winter moths. So, I concluded that it would be a fantastic investment to buy a huge gas tank, filling it up during the summer and emptying it during the winter (if it were allowed of course)!

Thanks for some great posts and an interesting discussion!

February 2, 2011 8:00 pm

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