ORKNEY'S ENERGY FUTURE
DOMESTIC HEATING
METHODOLOGY
INTRODUCTION
The aim of these calculations was to find the electricity demand that will result from decarbonising domestic heating, which accounts for 16% of the total non-renewable energy demand in Orkney.
Homes using electric heating, i.e. electric heaters or heat pumps, were considered decarbonised as the electricity source can be renewable and their electricity consumption is already accounted for in the total Orkney electricity demand. Therefore, only homes currently using a non-electric form of heating were considered in the calculations: 4,900 homes. It was assumed that these are the oldest of the housing stock in Orkney, i.e. the houses from pre-1919 to 1982.
HEATING DEMAND
The total annual non-electric domestic consumption in Orkney in 2018 was 91.8 GWh [1]. It is assumed that this consumption is wholly attributed to the aforementioned 4,900 homes.
To find a daily energy consumption, the 2018 Scottish domestic gas consumption daily figures [2] were used to create an approximate annual heating profile on a daily basis which was then scaled down to the reflect the magnitude of Orkney’s consumption (0.14% of Scotland’s consumption).
To obtain the heating output in these homes from this energy consumption trend, it was assumed that 7% [3] of the energy was used for cooking and that all of these homes were heated using an oil boiler which was 90% efficient [4]. Therefore, the annual heating output was 76.8 GWh. A low estimate value was chosen for oil boiler efficiency as this would give a higher heating demand and therefore account for years where heating demand may be greater.
This daily demand was then broken down into an hourly rate for space and water heating using hourly trends from ‘when2heat’, an open-source data platform for heating profiles [5]. Four days’ worth of trends were used, so that there was a different profile for each season. The annual ratio of space to water heating was 78:22 (Figure 1), which was found to be consistent with values for the UK’s total heating breakdown [6].
Figure 1: Breakdown of total annual domestic non-electric heating consumption in Orkney.
ENERGY DEMAND REDUCTION
To be eligible for the Domestic Renewable Heat Incentive (RHI), a home must be of a certain energy efficiency standard before the renewable heat technology is installed [7]. Therefore, it was assumed that all homes being retrofitted with a heat pump will be renovated with energy saving measures before installation to become more energy efficient.
A model was created in the building performance simulation modelling tool, ESP-r. This was based on the worst-case scenario home in Orkney and was used to determine the percentage energy demand reduction associated with each energy saving measure (Figure 2). This home was assumed to have been built pre-1919 and have a floor area of 140m² (Scottish average for rural homes of this period [8]). The surfaces considered were external walls, ceiling and windows as analysis of heat transfer showed that these accounted for greater than 80% of the heat loss. An air infiltration rate of 1.40 air changes per hour (ACH) was assumed in the model which is presented by [9] to be appropriate for “leaky buildings”.
Figure 2: Example of an older style home in Orkney and its ESP-r model.
A study by Baker [10] conducted in-situ measurements of traditional Scottish solid wall buildings and found that indicative U-values were 1.1 ± 0.2 W/m².K. A 650 mm traditional sandstone wall with U-value 1.255 W/m².K was therefore chosen to model the external walls (Table 1). The ceiling was modelled as a traditional wood floor with a slate roof. The original construction material of the windows was single glazing which was reported by a retrofit project on the Orkney island of Westray to be common [11].
Table 1: Construction materials and corresponding U-values used in model of worst-case scenario home.
After applying the energy saving measures, the sensible heating load was approximately 9 kW and the total heat loss by convection at opaque and transparent external surfaces (walls and windows) and conduction through the ceiling were reduced (Figure 3).
Figure 3: Heat gain and heat loss before and after energy saving measures were applied. Sensible heating load and heat loss both reduced after modelling insulation to external walls and ceiling and double glazing. Heat loss considers convection at opaque and transparent external surfaces and conduction through the ceiling.
Results from the model indicated an energy saving of 34% from internal wall insulation, 25% from loft insulation and 10% by changing from single to double glazing. These values are consistent with reported heat loss through solid walls, an uninsulated roof and single glazing which can be up to 45%, 25% and 20%, respectively [12] [13] [14].
To calculate the overall reduction in energy used for heating, it was assumed that only the pre-1919 homes with 4+ bedrooms require all three types of energy saving measure and that all other homes being decarbonised are only fitted with internal wall insulation.
Based on the figures for the number of homes in each of these categories, a weighted overall energy reduction was calculated to be 42.8%.
Figure 4: Annual trend for heating output on a daily basis for all non-electric homes in Orkney before and after energy saving measures have been implemented.
HEAT PUMP SIZING
Using the model built in ESP-r of the worst-case scenario home, the maximum heating load over the course of a year was found to be approximately 9 kW, which would indicate that a 9 kW heat pump was required.
However, thermal bridges are not accounted for in ESP-r, and studies show that these can increase a home’s energy demand by up to 42% [15]. It is assumed that thermal bridges are avoided as far as possible during renovation, so the increase factor was reduced to 1.2. Therefore, the worst-case scenario home will require an 11 kW ASHP, which was found to be a realistic value for a home of this size and quality [16].
The heat pump sizes for the rest of the homes were found by first using the UK Government Domestic RHI calculator [17] to estimate the annual heating demand of different sizes and ages of homes in Orkney. Based on this, the peak heating load of these individual homes could be found using Equation 1:
(1)
where:
-
kW = peak heating load (kW)
-
ΔT = design indoor temperature (21°C) minus design outdoor temperature (-2.65°C)
-
kWh = annual heating demand
-
HDD = heating degree days
HDD was found to be 2806.2°C days using an online calculator [18], weather data from Kirkwall Airport weather station and a base temperature of 15.5°C.
This equation was derived based on methods for sizing solar thermal heating systems [19]. Once the peak heating loads for ten different types of home being decarbonised were calculated, the value for each was rounded up to nearest whole number to find the corresponding size of heat pump required (Table 2). Using this method, the heat pump size required for the worst-case scenario was also found to be 11 kW, validating the results of the model built in ESP-r.
a
p
Table 2: Rated power of ASHP required for each type of property being decarbonised.
ASHP ELECTRICITY CONSUMPTION
The electricity consumption of an ASHP will be less than its heating output due to the high efficiency (over 100%), or Coefficient of Performance (COP) of heat pumps.
The predicted hourly COP values for an ASHP were calculated using weather data from Orkney based on the Equation 2:
(2)
Where ΔT is the load temperature (80°C for space heating and 55°C for water heating) minus the mean outdoor temperature corresponding to that hour [20]. This accounts for seasonal fluctuations in heat pump performance (Figure 5). The hourly ASHP electricity consumption values were then obtained by dividing the corresponding heating output for that hour by the calculated COP.
Figure 5: Graph showing calculated hourly values for heat pump COP based on space and water heating for one year.
The total electricity consumption used for space and water heating by the retrofitted homes with decarbonised heating systems was found to be 19.0 GWh/year, calculated by summing the hourly electricity demand values. This is a 78% reduction from the energy consumption of the homes prior to being retrofitted with an ASHP and energy saving measures of 85.2 GWh (Figure 6).
Figure 6: Domestic heating energy consumption on a monthly basis before and after homes are retrofitted with an ASHP and energy saving measures are implemented.
The potential electricity demand of a decarbonised domestic heating sector has now been calculated and can therefore be used to utilise a portion of the surplus electricity.
IMPLEMENTATION
To achieve the goal of 100% decarbonisation of the domestic heating sector by 2030, it is assumed that 10% of the homes (490 homes) will be decarbonised each year between 2021 and 2030 (Figure 7). This is based on ASHP installation taking two days [21] and there being three installers in Orkney [22] [23] [24]. Therefore, a minimum of 9 years would be required for installation in all homes.
The additional electricity demand from these homes was added to the current electricity demand of the domestic buildings sector, which includes electricity from utilities and already electrically heated homes (Figure 7).
Figure 7: Annual total domestic electricity consumption in Orkney projected to the year 2030.
AVAILABLE DOWNLOADS
The spreadsheet with the demand calculations can be accessed here.
REFERENCES
[1] Department for Business, Energy and Industrial Strategy, "Sub-national total final energy consumption statistics: 2005 to 2018," UK Government, 2020.
[2] Scottish Government, "Daily Energy Demand," Scottish Energy Statistics Hub, [Online]. Available: https://scotland.shinyapps.io/Energy/?Section=SystemSecurity&Chart=DailyDemand. [Accessed 7 April 2021].
[3] Eurostat, "Energy Consumption in Households," June 2020. [Online]. Available: https://ec.europa.eu/eurostat/statistics-explained/index.php/Energy_consumption_in_households#:~:text=Main%20cooking%20devices%20require%206.1,final%20energy%20consumed%20by%20households.. [Accessed 7 April 2021].
[4] Boiler Guide, "Best Oil Boilers 2021: Pros, Cons and Prices," [Online]. Available: https://www.boilerguide.co.uk/articles/best-oil-boilers. [Accessed 7 April 2021].
[5] Open Power System Data, "When2Heat Heating Profiles," 6 August 2019. [Online]. Available: https://data.open-power-system-data.org/when2heat/2019-08-06. [Accessed 7 April 2021].
[6] UK Government, "Estimates of heat use in the United Kingdom in 2013," 2013. [Online]. Available: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/386858/Estimates_of_heat_use.pdf. [Accessed 12 April 2021].
[7] Ofgem, "Domestic Renewable Heat Incentive," 7 March 2021. [Online]. Available: https://www.ofgem.gov.uk/system/files/docs/2021/03/drhi_essentialguide_forapplicants_mar2021_v7.pdf. [Accessed 7 April 2021].
[8] Scottish Government , "Scottish House Condition Survey: 2019 Key Findings," National Records of Scotland, 2020.
[9] M. Menconi, N. Painting and P. Piroozfar, Building Energy Simulation of Traditional Listed Dwellings in the UK: Data Sourcing for a Base-Case Model, L. J., H. R., C. A. and J. L., Eds., Singapore: Springer, 2020.
[10] P. Baker, "Historic Scotland Technical Paper 10," Historic Scotland, Glasgow, 2011.
[11] Changeworks, "No One Size Fits All," in Solid Wall Insulation Conference, 2012.
[12] Energy Agency, "Solid wall insulation," 2021. [Online]. Available: https://www.energyagency.org.uk/en/solid-wall-insulation_46988/. [Accessed 12 March 2021].
[13] Energy Saving Trust, "Roof and loft insulation," 2021. [Online]. Available: https://energysavingtrust.org.uk/advice/roof-and-loft-insulation/. [Accessed 12 March 2021].
[14] Energy Saving Secrets, "Saving Energy Through Double Glazing," 20 September 2017. [Online]. Available: http://www.energysavingsecrets.co.uk/savingenergythroughdoubleglazing.html. [Accessed 12 March 2021].
[15] H. Ge and F. Baba, "Effect of dynamic modeling of thermal bridges on the energy performance of residential buildings with high thermal mass for cold climates," Sustainable Cities and Society, vol. 34, pp. 250-263, 2017.
[16] Boiler Guide, "What size heat pump do I need?," [Online]. Available: https://www.boilerguide.co.uk/articles/size-heat-pump-need. [Accessed 6 April 2021].
[17] UK Government, "Domestic Renewable Heat Incentive Payment Calculator," [Online]. Available: https://renewable-heat-calculator.service.gov.uk/Default.aspx. [Accessed 7 April 2021].
[18] BizEE Degree Days, "Degree Days Calculated Accurately for Locations Worldwide," [Online]. Available: https://www.degreedays.net/. [Accessed 25 March 2021].
[19] S. A. Kalogirou, "Solar Space Heating and Cooling," in Solar Energy Engineering, Academic Press, 2014, pp. 323-395.
[20] O. Ruhnau, L. Hirth and A. Praktiknjo, "Time series of heat demand and heat pump efficiency for energy system modelling," Scientific Data, vol. 6, no. 189, pp. 1-10, 2019.
[21] A. Callow, "Air Source Heat Pump Installation," The Eco Experts, 25 March 2020. [Online]. Available: https://www.theecoexperts.co.uk/air-source-heat-pumps#:~:text=%E2%9C%94%20Easy%20to%20install%3A%20the,no%20more%20than%202%20days.. [Accessed 14 April 2021].
[22] Jill, "Sanday, Orkney," Plumbing and Renewables, 31 October 2019. [Online]. Available: https://plumbingandrenewables.co.uk/sanday-orkney/. [Accessed 14 April 2021].
[23] "Heat Pump Installation," Stephen R. Paterson Ltd, [Online]. Available: http://www.stevenpaterson.co.uk/index.php/services/heat-pump. [Accessed 14 April 2021].
[24] S. Gray, "Installation," Heat Orkney, [Online]. Available: https://www.heatorkney.com/Installation.html. [Accessed 14 April 2021].
[25] National Insulation Association, "Loft Insulation," [Online]. Available: https://www.nia-uk.org/understanding-insulation/loft-insulation/#:~:text=The%20recommended%20depth%20for%20loft,or%20220%20millimetres%20for%20cellulose. [Accessed 14 April 2021].