The Future Homes Standard consultation has proposed that a new requirement based on Primary Energy use should be brought into the next building regulations . In this article we look at how the Primary Energy use of a house might be calculated, and what is fair or desirable for solar.
Primary Energy is energy found in nature that has not undergone an artificial (man-made) transformation process. Electricity generated from gas, oil, coal, nuclear or biomass is secondary energy - the original fuel found in nature has been extracted, transported and converted to electricity. All the way along the process, energy is used or lost. So to deliver one unit of electricity to your home, a greater number of units of primary energy is consumed.
In an earlier blog I explained the concept of Primary Energy and Primary Energy Factors, see What is Primary Energy.
The Primary Energy Factor (PEF) is a measure of how many units of primary energy are needed to get the unit of final energy to your house.
So, for example in SAP 10.1 (the draft calculation method for the next building regulations) the primary energy of natural gas from the gas grid is given as 1.13, meaning that for every kWh of gas delivered to your house, gas of energy content 1.13kWh needs to be taken out of the ground. This figure is a weighted average of the PEF for all the different sources of natural gas that make up the UK supply - for example gas extracted from wells in the North Sea, Russia, USA and Qatar.
Primary Energy of Electricity
Electricity is even more complex.
The generation mix includes power stations that use gas, oil, coal, plutonium and wood as their feedstock, each with different Primary Energy Factors once they have been converted in a thermal power station and transmitted across the power distribution network to your consumer unit.
In addition to these thermally generated electricity sources you can add direct conversion renewables such as wind turbines and solar PV panels. The convention is that, since the natural energy they convert is limitless, the PEF for energy generated this way is 1.0 at the point of generation.
So the electricity generation mix results in an average Primary Energy Factor that depends upon which types of electricity generation are in use at any time. SAP 10.1 makes assumptions about what the UK's electricity generation fleet will look like in 2020-25 and estimates what the average combination will be in each month of the year. The average figure through the year for grid electricity at the point of use is a PEF of 1.51.
A home fitted with solar PV panels will generate solar electricity. At some times the solar electricity will exceed the electricity use of the house and electricity will flow back onto the distribution network where other buildings will use it, so called export.
Increasingly, homes with solar are also fitted with other technology that allows the building to retain more of the solar generated electricity and minimise amount exported to the grid. Devices include PV power diverters that send excess generation to an immersion heater to heat water in a hot water cylinder, battery storage to keep the electricity generated during the day for use in the evening and smart car chargers that optimise car charging to use self-generated renewable power to the max.
Of these three technologies, SAP 10.1 includes provision for PV diverters and battery storage, but does not give credit when both are used in the same house.
The Primary Energy Use of a Dwelling
The Primary Energy used by a house is calculated by adding up the total of the different energy types used by the house, each multiplied by the Primary Energy Factor (PEF) for that energy type.
The building regulations is rightly focused on the Net Primary Energy use over the year - which takes into account both the flows of energy into and out of the building and allows for energy generation in the building. Since the solar PV system is part of the building the solar energy flows that cross the boundary consist only of the solar generation exported to the grid. (See image)
Solar energy generated by the building and used in the building reduces the electricity needed by the building. So the first benefit of putting a solar system on a house for the net Primary Energy used is the solar energy kept in the building multiplied by the Primary Energy Factor of the electricity use that was avoided - the PEF of grid electricity.
The second benefit of a solar system on a house is that the solar energy exported from the house is a negative flow of energy and should reduce the net Primary Energy use of the house. The government has suggested in the consultation that the PEF for this exported electricity should be 0.51.
Why not a PEF of 1.0? This is the primary energy factor for solar generated electricity at the point of generation (before transmission losses). A query to the team in charge of SAP received this explanation:
Since grid electricity has a PEF of 1.51, and solar electricity has a PEF of 1.0, the net benefit of the exported electricity is 1.51 - 1.0 = 0.51 per unit of electricity exported.
In effect what they're saying is that this unit of solar generated electricity with PEF 1.0 flows into a nearby building and saves that building from using electricity from the grid generation mix with PEF 1.51, so the net benefit (to the grid) of the exported electricity is 0.51
I can see that there is a logic to this, but on balance I think the net Primary Energy of the building can and should consider the building and not the grid - this means that the flows of energy that cross the building boundary are the ones that matter. Unlike carbon emissions where you cannot apply a carbon saving to the exported electricity without considering the carbon emissions avoided where that energy is used, Primary Energy for solar energy has an agreed value for the PEF, and that value is 1.0.
Grid connected PV feeds into the grid at 1.0, why should microgeneration be treated any differently?
The government is keen to promote the uptake of smart technologies such as PV diverters, smart hot water tanks and battery storage, and so is the solar industry.
Does a lower PEF for exported solar energy not create a stronger driver for the uptake of these technologies in new build? Surely the lower the PEF of exported electricity the greater the value of installing equipment to use the solar electricity in the building?
The answer to this question is yes, but only up to a point. If the calculation is set up in such a way that solar starts to look less appealing as a technology to achieve building regulations, housebuilders may not use it at all, and then there will be zero incentive to add in smart technologies that increase solar energy utilisation.
Giving solar export a PEF of 1.0 still creates an incentive to use smart technologies, but without so significantly diminishing the benefits of solar PV. In addition, there will be a new affordability criteria in building regulations and this will create an incentive to keep energy in the building (saving 16p/unit) rather than exporting it (yielding only 5p/unit).
BRE and DCLG should reconsider the logic behind the treatment of solar PV in the net Primary Energy calculation in building regulations as the approach being consulted upon runs the risk of unfairly under-reporting the benefits of solar electricity.
