Over the summer there was a debate between some big names in engineering over whether combined heat, cooling, and power (CHCP) using absorption chillers actually saves carbon. The theory goes that because engine size is usually dictated by the base summer heat load, the additional heat load from the chillers allows you to upsize your engine and generate more low-carbon electricity throughout the year.
However on a current project, we’re looking at the feasibility of installing a district heating and cooling network, including installation costs. And one thing is clear: regardless of whether CHCP saves carbon, the capital cost of cooling is unaffordable.
The project is a mixed use urban development of several thousand homes, plus offices, retail, school, and so on. It’s a good dense site mostly consisting of medium/high rise blocks, so there are no excessively long pipe runs.
We looked initially at locating the absorption chillers alongside the chp engines, pumping the chilled water to those units with cooling demand. But despite the fact that our peak cooling load for the site was roughly equal to the peak heating load, the cross sectional area of the cooling pipes was coming out more than 200% larger due to the lower deltaT in the cooling system. Even though it was much less extensive than the heating network, installing insulated cooling pipes would have added 60% to the total cost of distribution pipework.
We then considered using uninsulated pipes buried deeper in the ground as Utilicom have done in Southampton - the theory being that the steady ground temperature at depth would lead to minimal losses. But a brief discussion with the Danish district heating engineer and some back-of-envelope figures killed that idea. For our network it would bring prohibitive gains from surrounding soil.
For this project the cooling and heating peaks are predicted to be similar; however total annual cooling consumption is predicted to be only 12% of heating consumption. Furthermore, the value of delivered cooling is only equivalent to the baseline cost to the consumer, i.e. the cost of whatever system they’d have put in place without district cooling - in this case we assumed electric chillers with an average COP of 4. Even considering the additional electricity output from the CHP engines (an increase of around 10%), income from cooling and electricity was overwhelmed by the massive expense of the cooling pipework, not to mention the absorption chillers.
Next: using distributed absorption chillers on the heat network.
5 comments
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October 30, 2007 at 7:41 pm
Jonny Marrion
How come the peak cooling load=peak heating load. Is this in the UK? Surely simple good design measures to over over heating (solar shading, exposed mass etc) would reduce the cooling load and hence the pipe work and therefore the cost.
October 30, 2007 at 9:30 pm
Casey
The cooling load is lower but not by much. The efficiency measures bring the heat loads way down but there’s less we can do about the cooling peaks apart from, as you say, shading to reduce the solar gain and thermal mass to shave the peaks. Even so, it’s easy to imagine an efficient office where both heating and cooling peaks are in the neighbourhood of 40W/m2.
November 1, 2007 at 11:33 am
IF Kite
In examples like this to avoid cooling you really need to revisit the architecture and the brief. The form of the building: it’s ceiling heights, floor plan depths, occupational density and acceptable comfort levels will all help you design out as much cooling as you want.
If any of these items are constricted by the architect, client or tenant then get on the phone to Airedale, York, Carrier or Powermaster.
November 1, 2007 at 1:52 pm
Casey
@IF Kite-
Is it your view that cooling can be designed out of buildings within an urban heat island like London? It seems the key question in what you’re proposing is: what is “acceptable” in terms of comfort levels.
November 2, 2007 at 12:23 pm
IF Kite
oooh, now there’s a question.
What I’d say is that there are barriers in large cities like London which are all there to stop you even trying to design out cooling; the heat island effect being one of them.
I think it can be done but dismantling the barriers poses major economic and social questions.
One key question in city centres being: what business is going to accept 40% less people on a floor plate to conform to the nat. vent. strategy?
We need to accept, on some projects, we have a client/brief/site that can have low emissions associated with cooling, but can’t rely on natural techniques to give them comfort.