CHP in DSM

[Chris A]

Hiya... again!

I am doing an assessment on a development where there is a central plant with CHP. The part I am assessing is only a small section of the whole site.

In DSM we are asked for the thermal output of the CHP. I have this, but as it is such a large site, and therefore such a large CHP, if I enter the actual CHP thermal output, then the heat demand from assessed part of the site never actually reaches the output of the CHP. Therefore IES assumes the CHP is never running and I therefore have no benefit from the CHP.

I was wondering if anyone else has come across this problem and how they solved it. I think my options are:

- use a district heating option and adjust the fuel factor to suit. Here I am worried that the fuel factor is carried through to the notional, and therefore it wont make compliance any easier.
- only take a percentage of the CHP output - Maybe the as a percentage of the floor area?

Many thanks in advance

[Complex Potential]

Hello again

You have a development with a central plant.... sounds like a district heating system to me. Of course you are correct that the direct benefit you see of a central CHP is drastically reduced compared to a building specific CHP under the NCM methodology. This is completely unfair and stupid and requires fixing but for now we are lumbered with it.

Having said that, I have seen instances where a BCO approved the method of modelling the CHP as if it were part of a local plant room and "proportioned it down" even though working out the adjusted CHP parameters to input is effectively unregulated guesswork and open to horrendous abuse. Generally you can get a BCO to agree to anything on the modelling side as long as you throw enough complex words at them (however, the EPC is a different matter and is down to the LCEA who may not be so easily convinced).

If you do model it as a district system you will need to determine the site carbon factor which again leads into grey waters with very little guidance. You have to somehow approximate the total site energy consumption and then work through the detail of the central plant as well as distribution losses yada yada... Once that's done, you hopefully have a nice low carbon factor that the notional building goes and matches but at least the total carbon is reduced which means it's good for your EPC rating and boosts the effects of any other static renewables local to your building such as PV.

I think there are a few efficiency breaks the notional building gives you if your carbon factor is low enough, but it's still nowhere near as good as modelling the CHP in a local plant room.

[bootsam]

Hi, we always size the CHP to the direct hot water base load. We carry out a calculation (spreadsheet) over a year and profile the hot water demand including the effects of diversity etc. We then select the CHP to run all year servicing the base hot water demand. Not the heating as this is seasonal. There is a year round hot water demand, thus you get the benefit. A CHP in practice needs to run for approx 4500hrs/annum (CIBSE). Only a good hot water base load or an electrical base load and systems to export the heat provide this. Sizing the CHP for the peak heating and hot water demand will grossly oversize the CHP and render it uneconomical.

Any hot water above the base load and of course the heating load is carried out by conventional boilers.