ENERGY - DISTRICT HEATING
DISTRICT HEATING: INEFFICIENCIES ARE BEING IGNORED
W
hen designing the RHI (Renewable Heat Incentive scheme), DECC were persuaded that bigger is better. Hence the decision to exclude individual domestic properties from the first phase of the RHI but to include “community schemes”
where two or more domestic properties shared a boiler. The latter option was perceived to be more cost-effective. But that is only true if you assume minimal losses in (and installation costs for) the piping that is necessary when a boiler is shared between properties. Fortunately, DECC were wise enough to require metering on all installations in the RHI Phase 1, and to pay RHI only on the heat delivered, not on the heat produced. Ofgem (who administer the RHI) have been gathering data that could reveal the distribution efficiencies on “complex installation” (including those with district heating networks), but so far DECC and Ofgem have refused to publish the data or analyse it to show the efficiencies. Consumers will notice the inefficiency in their pockets, because there will be a discrepancy between the RHI that people receive and the amount of fuel that they have to buy. So long as their meters are properly installed, they will be able to work out where the problem lies.
In 2011 we installed a 3 property district heating system. The ETA pellet boiler appears to be roughly as efficient as expected. Even with a Delox fuel store monitor it is impossible to be absolutely precise about the quantity of fuel used, but I am confident that the error in estimation of this figure would not account for a discrepancy of more than around one or two percentage points. From the first meter reading the boiler efficiency has been around 90%, but the network efficiency has been around 61.5% on average. The combined efficiency is just over 55%. Even without meter readings, it wouldn’t be difficult to work out that we are losing a lot of heat to the ground.
These are district heating pipes that met the efficiency requirements of the RHI at the time they were installed. And this is in winter, when the systems are more efficient. The reduction in distribution efficiency over the summer is predictable. The network is much less efficient than the theoretical rating of the insulated pipework because the theoretical figures assume that only as much heat is delivered as is required. The theoretical figures are therefore very optimistic about the amount of heat that is recirculated. Even in winter, heat demand is not continuous and heat is therefore circulated and lost, to a greater extent than assumed. In summer, the losses are very much greater because there is minimal space heating demand. The water circulates at a temperature high enough for domestic hot water use, but is called on only occasionally and unpredictably. It is possible to incorporate intelligent controls (“weather compensation”) to reduce this effect by reducing the speed and temperature of circulation when hot water is not required, but it cannot eliminate it, and it is very expensive. One supplier has quoted an amount that would add 25% to the capital costs, to add weather compensation to our system. We will nevertheless look for some way to implement weather compensation, to test its effectiveness. That would be another illustration of the importance of metering. The data is necessary both to reveal the need for it and to measure how effective it is.
This is not an isolated case. One of our customers is experiencing losses of over one-third between his boiler and the properties served by the boiler. Another customer was so disappointed by the amount of heat delivered that they queried whether the CV of the wood pellets is 40% lower than claimed (it isn’t - the meter on the boiler and the analysis on the pellets can both prove that). Discussions with contacts in Austria and Sweden confirm that this scale of loss should be anticipated in district heating systems.
As a wood pellet fuel supplier we have seen the same effect at different sites installed by different companies. District heating advocates may try to blame the equipment or the installer, but the fact that this is a common experience in places like Austria and Sweden (as well as the UK) indicates that this is a fundamental issue and not just the result of bad installation. Likewise, the fact that it is worse in summer than winter indicates that this is an issue with recirculation, not with poor insulation (which would leak more heat in winter than in summer).
This is going to have a significant effect on running costs. Imagine a system that demands 100 MWh of heat per year. Let’s say the boiler is 90% efficient. If district heating losses were 2%, the system would consume 23.6 tonnes of wood pellets per year. If the district heating losses were 38.5%, the same system will consume 37.6 tonnes per year. The running costs are nearly 60% higher. If your fuel costs 4.5 p/kWh (around £215/tonne) and your boiler is 90% efficient, your fuel cost per unit of heat consumed (as opposed to fuel supplied) will be 5.1 p/kWh with 2% district heating losses, but 8.1 p/kWh with 38.5% losses.
There will be many circumstances where it would be better to install several smaller boilers for multiple properties, rather than one larger boiler and a district heating network. The higher cost per kW of several small boilers will have to be set against the capital cost of the district heating network and the lower efficiency. From a sustainability perspective, several discrete boilers are clearly better than a larger system with higher losses, because the losses will reduce the carbon avoided by the renewable fuel, and will increase the strain on the biomass resource for a given level of delivered heat and fossil-fuel
displacement.
www.forever-fuels.com
PUBLIC SECTOR ESTATES MANAGEMENT • JULY/AUGUST 2014
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