Building regulations - new domestic buildings: identification and assessment of energy standard improvements

Appendix E HEM energy flexibility and time of use considerations

Opportunity

The HEM half hourly modelling allows for energy flexibility and time-of-use to be better understood in the context of metrics such as regulated fuel cost, CO₂ emissions, Primary Energy consumption and Delivered Energy Rate. In addition, the HEMv0.33 FHS v0.24 introduces a variable grid CO₂ and primary energy profile (produced by DESNZ) which demonstrates the variability of the National Grid energy mix due to renewable energy generation and national consumption profile. This variability has both seasonal and daily effects. SG will need to decide whether this HEM functionality will be adopted in the final wrapper for Building Regulations assessment and if specific metrics are required to demonstrate this sensitivity.

This appendix item outlines some of the potential energy flexibility measures that can be captured by the modelling. The impact on sensitive metrics is also discussed. It should be noted that the impact on regulated fuel cost is not discussed here; however, generally, if the CO₂ and PE consumption reduces then this could result in lower bills if a dwelling were on a tariff that reflected this variability.

Accounting for seasonal and daily variations in grid carbon and primary energy intensity has been found to significantly impact certain output metrics. For CO₂ emissions, it is observed that a variable profile decreases the reported emissions. The opposite was observed for PE. This appears to happen for 3 key reasons:

• Space heating demands typically occur during the winter where the CO₂ and PE factors are lower than the summer, typically down to high wind yields in the winter months.
• A higher proportion of cylinder recharging is scheduled overnight, which again benefits from lower CO₂ due to higher wind output at night. Conversely PE is higher overnight as the lower demand means that fixed losses of the transmission and distribution networks are proportionally higher.
• PV export occurs during the daytime and is higher in the summer. Both times typically have higher CO₂ and PE factors due to low wind yields and the fact that the UK solar generation capacity is not as significant as wind technology. As PV generation is reported as a negative value in the HEM, outputting at times when CO₂ and PE factors are lower result in a lower overall saving.

These relationships apply to the UK grid. Regional data indicates that different relationships are present for Scottish electricity grids. The following technologies/occupant scheduling decisions will have an impact on modelled outputs, should the effects of grid variability be captured by a Scottish Homes Wrapper.

Appliance load shifting

The current new build dwellings wrapper (produced by DESNZ) now supports the option to set (unregulated) appliance usage patterns rather than follow the national average behaviour profile, which was derived from Energy Follow Up Survey (EFUS) 2017 data for the previous version of SAP. The control schedule for large energy consuming appliances, such as washing machines and dishwashers, can be linked to an electrical tariff schedule (an economy 5 tariff for example). The impact of such a change is a reduction in the amount of useful internal gains within the model which displaces heating demand.

HW cylinder charging

The current new build dwellings wrapper (produced by DESNZ) assumes that the HW cylinder charging programme consist of a thermal disinfection cycle for 2 hours between 00:000 and 02:00 each day at which the temperature of the tank is raised to 60oC. It is in line with Health and Safety Executive (HSE) guidance on legionella prevention. The HW cylinder is allowed to be recharge back to the tank set point for the rest of the day when the thermostat detects the tank temperature is below the set back of 52oC.

There is likely to be an efficiency benefit from avoiding HW cylinder charging during the heating hours as the heating system can operate more continuously, benefiting efficiencies. There may be a negative impact on efficiency associated with running technologies such as heat pumps overnight when the dry bulb temperature is lower. When considering the differing levels of grid electricity CO₂ and PE overnight there is a potential benefit associated with predominantly charging the cylinder overnight provided that the additional volume needed to support this does not result in higher standing losses.

Battery energy storage

Electrical storage technologies used as battery storage can be applied using three different core approaches:

i. Charge using local excess generation and discharge to the dwelling when there is demand greater than the generation output; or

ii. Charge using cheap rate electricity (based on a schedule) and discharge to the dwelling when there is demand greater than the generation output; or

iii. Charge using cheap rate electricity (based on a schedule) and discharge back to the grid at high export rate periods (typically 4-7pm).

It is possible to combine aspects of the above to suit different dwelling and occupancy patterns; however, this presents a near endless possibility of schedules. Of the core three, each offers different benefits for the three metrics proposed by SG.

For i, improved solar energy self-use will reduce the imported electrical energy. Given how Delivered Energy Rate is calculated this could provide a benefit. Some benefit would be expected for CO₂ and PE; however, the relationship would be more complex if variable grid factors were applied.

For ii and iii, load shift using the battery storage system would offer a negative Delivered Energy impact as the same amount of energy is being consumed by the dwelling but with the additional ‘round trip’ losses associated with the battery. Both CO₂ and PE could significantly reduce through battery load shifting