To add the heating load to the current electric load profile, and to model the current and potential electricity demand of the Island before and after transformation from heating with biomass, fossil fuels and direct electric to heating with heat pumps exclusively, the current heating load profile is needed. To simplify the model, the focus will be only on domestic properties and assumption made that poorly insulated (9.2% from total), high heat loss per area properties would be improved at least up to current building regulation standards. The approach to calculate the heating load follows methodology described in Chartered Institution of Building Services Engineers (CIBSE) "The Domestic Heating Design Guide" [6], which is based on British standards (BS EN 12828) [7] and is MCS compliant [8], therefore allowing for the owners of the individual property, apply for RHI funding [9]. All calculations were done manually. MS Excel was used to speed up the process and allow to adjust different inputs with ease.
Limitations, simplifications and assumptions
It was not possible to acquire detailed floor plans and building fabric details for all domestic properties of the Island, nor complete the required measurements in real life. Therefore, it was decided to categorise all of the 66 domestic properties of the island in groups and then apply the same dimension and fabric data to each group. With the help of the energy audit and information from the Island’s representative, 4 house types were identified:
It was not possible to acquire detailed floor plans and building fabric details for all domestic properties of the Island, nor complete the required measurements in real life. Therefore, it was decided to categorise all of the 66 domestic properties of the island in groups and then apply the same dimension and fabric data to each group. With the help of the energy audit and information from the Island’s representative, 4 house types were identified:
- Old, un-insulated – 7
- New Build – 18
- Retrofitted - 35
- Holiday home - 6
Average areas and fabric data was obtained from Scottish EPC database, and with the help of online resources Google Earth Pro. CIBSE method allows the peak heating load (kWp) to be calculated, which represents how much heating energy is needed to maintain the design indoor conditions (18-22°C) at design outdoor conditions (-3.76°C). Peak load allows the heating unit to be adequately sized (in this case heat pumps for future scenarios) and also to construct an hourly demand profile for the whole year. The heating demand is a result of the temperature difference between the indoor and outdoor air. By using weather data [10] in given location, it is possible to calculate annual heat demand (kWh) and create an hourly energy demand profile for the whole year.
Calculation of peak heating load (kWp)
Identical inputs of each house type for manual calculation were as follows:
Calculation of peak heating load (kWp)
Identical inputs of each house type for manual calculation were as follows:
- Average design indoor temperature (DOT): 19°C
- Location adjusted design outdoor temperature: -3.76°C
- Altitude above sea level: 15m
- Exposed location
- Continuous heating (except holiday homes - intermittent)
- For DHW requirements: occupancy 3 people
- DHW storage temperature: 60°C
House type specific inputs
|
Old, un-insulated:
Calculated heat load – 17.86 kW Storeys – two Floor height – 2.8m Average (between different room types) air exchange rate: 1.95 air changes/h Roof pitch – 35° Total floor area – 151 m² See table below for fabric details: |
New Build:
Calculated heat load – 4.21 kW Storeys – single Floor height – 2.4m Average air exchange rate : 0.78 air changes/h Roof pitch – 0° Total floor area – 95 m² See table below for fabric details: |
Old, un-insulated:
New Build:
|
Retrofitted:
Calculated heat load – 7.26 kW Storeys – two Floor height – 2.5m Average air exchange rate – 1.3 air changes/h Roof pitch – 0° Total floor area – 118 m² See table below for fabric details: |
Holiday home:
Calculated heat load – 7.45 kW Storeys – two Floor height – 2.5m Average air exchange rate – 1.3 air changes/h Roof pitch – 0° Total floor area – 118 m² Heating mode – intermittent See table below for fabric details: |
Retrofitted:
Holiday Home:
Calculation of kWp to kWh and hourly heating demand profile
For annual heating requirements (kWh) for each house calculations, heating degree days from 'www.degreedays.net' was used [10]. The closest metrological measurement station to the island of Gigha with quality data, was “Islay Port Ellen (6.25W, 55.68N). Base temperature (allowing for internal and solar gains) of 15.5°C was used.
Following the formula:
kWp ÷ (ΔT) x HDD x 24
Where:
kWp – heat loss at design outdoor temperature
∆T – Difference between average indoor air temperature (19°C) and outdoor design temperature (-3.76°C)
HDD – Heating degree days in given location
24 – Hours of the day, as HDD are sum of hours below the base temperature.
Total energy demand for all domestic properties:
499.6 ÷ (19 - (-3.76) x 2108.5 x 24 = 1,110,798kWh/y
Hourly heating demand profile
HDD are recoded in half hourly time steps, but the unpaid access to the database, allows to download HDD on daily time steps.
The following algorithm was applied to create hourly time steps HDD:
More details on “HDD and Heat demand” sheet - download here:
For annual heating requirements (kWh) for each house calculations, heating degree days from 'www.degreedays.net' was used [10]. The closest metrological measurement station to the island of Gigha with quality data, was “Islay Port Ellen (6.25W, 55.68N). Base temperature (allowing for internal and solar gains) of 15.5°C was used.
Following the formula:
kWp ÷ (ΔT) x HDD x 24
Where:
kWp – heat loss at design outdoor temperature
∆T – Difference between average indoor air temperature (19°C) and outdoor design temperature (-3.76°C)
HDD – Heating degree days in given location
24 – Hours of the day, as HDD are sum of hours below the base temperature.
Total energy demand for all domestic properties:
499.6 ÷ (19 - (-3.76) x 2108.5 x 24 = 1,110,798kWh/y
Hourly heating demand profile
HDD are recoded in half hourly time steps, but the unpaid access to the database, allows to download HDD on daily time steps.
The following algorithm was applied to create hourly time steps HDD:
- Peak heating load (hourly average load + 30%) occurs for 8 of 24 hours (7am-9am and from 5pm-9pm).
- Off peak heating load (remaining load ÷ 16) occurs on remaining 16 hours of the day.
- For more natural load profile, each hourly load figure was adjusted by ±5% randomness factor.
- Daily Domestic hot water kWh was spread evenly during 6 hour period (4-6am and 3-5pm).
More details on “HDD and Heat demand” sheet - download here:
| gigha_heat_and_electric_demand_profiles.zip |
To add this hourly heating load to the electric heating load profile following assumptions and simplifications were made:
- For 18 new builds, 100% of the heating and DHW load is supplied by electricity
- For 7 old properties, 40% of the heating and DHW load is supplied by electricity
- For 35 retrofitted houses and 6 holiday homes, 30% of the load is supplied by electricity
