The Last Piece Falls into Place for the Domestic RHI
MCS recently unveiled its new calculation methodology for
predicting the energy benefits of solar heating, and its great news for the
industry. The announcement was eagerly awaited not least because it will be the
way payments are calculated under the domestic Renewable Heat Incentive.
What's Changed?
The previous version was based upon the government's Standard Assessment Procedure (SAP), and so is the new version, but with some
rather important changes:
1. The Number of Occupants
Arguably the single biggest factor impacting the solar heating energy yield
is the amount of hot water used, and in domestic properties this is almost
entirely down to how many people live in the house.
The previous version of the calculation followed SAP,
which was originally developed for new build where the number of people using
hot water in the building is not normally known. The SAP calculation predicts the occupancy
from the floor area of the property, with an increasing number of occupants as
the property size increases.
Because this SAP calculated occupancy peaks
at around three people for homes that are not abnormally high, it is underscoring
the potential for solar energy greatly for the many households with more people
than this. If this were to have been used for the RHI payments it would
incentivise a solar installation in a mansion with a retired couple living in
it over a smaller house with a family of five living there.
The new calculation requires the solar installer to enter
the actual occupancy of the property at the time of the installation, taking
into account any part time residents (for example children that visit regularly
but who don't count the house as their primary residence). Installations will be subject to random audit by OFGEM, discouraging potential cheating, but for RHI deeming purposes, it was also decided to limit the
occupancy to six full time equivalent.
This change is broadly neutral for smaller households of
one or two people, and increasingly positive as the number of occupants
increases. The new calculation will significantly boost the energy yield for
larger households, rewarding those solar installations that deliver the highest
energy savings.
2. Electric
Showers
The SAP calculation assumes a certain hot water use per
person. In the previous (2009) version, the hot water use was based on an
average for all homes in the UK including those with an electric shower. Electric showers are fed with cold water so
don't use water from the hot water cylinder and this has the effect of lowering
the hot water use in the UK average.
By separating out homes without electric showers, in the
2012 version, hot water use is boosted by around 30 per cent and the benefits from solar
heating further increased.
Incidentally, this welcome change should see some of the
daft (predominantly new build) installations disappear where solar water
heating has been installed alongside electric showers.
3. Backup Heater
Efficiency
The previous calculation worked out the solar heat input
to the hot water cylinder and this was the figure presented to the
customer. The new calculation recognises
that the customer is more interested in their saving on energy bills, and this
is not the same thing. Why? Because the boiler that would have heated the
water if the solar system wasn't there works with an efficiency, so in most
cases the fuel energy bought is higher than
the energy input to the cylinder.
For example, a boiler operating with 80% efficiency would
need to burn 1,250 kWh of gas to produce 1,000 kWh of heat. (1,000/0.8 =1,250)
It turns out that backup heaters have a summer and winter
efficiency. Most are less efficient in summer than in winter - energy overheads
from starting up are a larger proportion of energy delivered when you're only
heating water and not heating the whole house too. Of course, most solar energy is
delivered in summer when the back-up heater is least efficient.
The calculation takes this into account and a "solar
efficiency" figure is used to calculate the energy saving as well as the
renewable energy. The figure is expressed as an "additional fuel
saving" so should be added to the deemed renewable heat to arrive at the
total energy saving.
4. Irradiation
Where the previous version used a single irradiation for
the whole of the UK, the new version divides the country into 21 regions (see my earlier article on SAP 2012). The
UK average remains the same, but some regions are lower than the average and
some higher.
In addition, the irradiation is calculated for the
absolute angle and orientation of the panel instead of using a look up
table.
5. Cylinder standing losses
A standing loss for the cylinder is added to the hot
water demand increasing the total heat load that the solar system can contribute
towards.
6. Other Minor Changes
The second order heat loss coefficient for the solar
panel performance is taken into account.
Homes designated water efficient under Approved document
G have their hot water demand reduced by 5%
Believe it or not, there are solar thermal panels out
there with efficiency lower than the SAP default. A loophole where these could
use the SAP default values has been closed.
The Effect of the Changes
Let's run a few examples through the previous and new
calculations. To keep things simple, we'll use the same high quality solar thermal panel for each situation, as well as keeping the shading (none or very
little) orientation (south) and pitch (30 degrees) the same for all options. We’ll also locate the panels in Sheffield
(which receives the UK average irradiation) and remove electric showers from
the house. We'll assume the house has a modern condensing gas boiler.
For the previous calculation, I have used a house size of
85 m2, an average sized town house with three bedrooms. For the new calculation, a range of occupants
between two and six is considered, with a panel area and cylinder size chosen
appropriately.
The previous calculation ignores the actual number of people
who live in the house and uses 2.7 residents in all cases, a figure derived
from this floor area. The results
highlight the way that the previous calculation under-sold the benefits of
solar in situations where more people live in the property.
In addition, the energy saving presented to the customer was the solar
heat added to the cylinder, ignoring the efficiency of the boiler. The new calculation takes the boiler
efficiency into account.
The new calculation gives energy savings between 50% higher
and 142% higher than the old calculation.
The deemed renewable heat is calculated to be between 14% and 85%
higher.
Many solar installers would have been disappointed by the
tariff level of 19.2p set for solar heating under the forthcoming domestic
Renewable Heat Incentive. This improved
calculation method provides just the boost that solar heating needs, and radically
changes the economic attractiveness of solar heating within the domestic RHI.
The announcement of the RHI deeming calculation for solar heating is the final piece in the jigsaw. Finally, solar installers can get out there and start presenting the financial benefits of the domestic RHI to potential customers. In fact with a £600 upfront grant available for installations before the start of the scheme, there's every reason to install solar now.
