A Closer Look at Energy Demands: Quantification and Characterisation

1. A Closer Look at Energy Demands: Quantification and Characterisation

2. Why is demand data important? having information on likely energy demands is a key requirement of a low energy building deisgn we can assess if design criteria have been met allows us to target demand reduction measures … or we can size low carbon energy supply equipment

3. Aggregate Energy Demand • the simplest way to describe the energy consumption of a building is to provide an aggregate value • is consumption over a time period (a year) • lumps all energy demands together (heat/electrical) • often expressed in kWh/m2 … or as a non-dimensional rating (e.g. SAP) • …useful as a performance metric – not that useful for design

4. Disaggregating by Load Type • dissaggregation gives us more detail • e.g. looking at what the energy was used for • useful for targeting demand reduction measures • … or to select energy supply technologies

5. Disaggregating by Load Type

6. Temporal Characteristics • load data can be disaggregated in different ways: • by type • spatially (by location) • temporally (by time)

7. Temporal Characteristics

8. Calculating Energy Demands we’ll look at how we can quantify and characterise each of the major energy loads in turn

9. Calculating Heating/Cooling Demands • we are spoiled for choice when it comes to determining space heating or cooling demands • simplified methods: • Standard Assessment Procedure SAP * • degree-day method** (daily or monthly heating/cooling demands) • basic UA calculation • more comprehensive methods: • building simulation (hourly  minutely heating or cooling energy demand) * produces a rating not a value, **does not adequately account for internal and solar gains

10. Climate Data • the starting point for a heat load calculation is climate data • this could be as simple as an average annual external air temperature • or as detailed as hourly readings of temperature, solar radiation, wind speed and wind direction

11. Calculating Heating/Cooling Demands • SAP is used more for compliance checking (with building regulations) than as a design tool • gives an energy rating score (1-100) using a ‘tick-list’ based on the building design

12. Calculating Heating/Cooling Demands • degree day/U-value methods are energy balance based • DD assumes that is ext. air temp > 15.5oC there will be no heating load • the assumption is that when the ext air temp reaches this point internal heat gains in the building will keep the temperature ~ 18.5oC • does not adequately account for equipment gains … or solar gains (increasingly important in well-insulated dwellings) • doesn’t account for thermal dynamics caused by building fabric

13. Calculating Heating/Cooling Demands • for each day (or longer period) calculate the accumulated degree days • calculate the associated energy demand • (kWh)

14. Calculating Heating/Cooling Demands • a basic UA calculation can be uses to produce annual daily or hourly demand data • the calculation could be performed once for T equal to an annual average to give an annual energy consumption • … or 8760 times with hourly external temperature readings and temperature set points to give hourly space heating demands

15. Calculating Heating/Cooling Demands • … again, does not adequately account for equipment gains or solar gains • doesn’t account for thermal dynamics caused by building fabric Qf - fabric Qg - gains Qs - solar Qh - heat Qi - infiltration

16. Calculating Heating/Cooling Demands • the most robust approach is to use a simulation tool to calculate heat load • building is typically decomposed into hundreds of volumes • an energy balance is set up for each ‘volume’ which includes fabric energy storage • internal heat gains, solar gains calculated using climate, geometric and schedule information • solution of all of the individual energy balance equations gives the heat flows and temperatures throughout a building typically at hourly or sub hourly time intervals • computer and software required and usually does more than calculate heat demand data

17. typical output is as follows: Calculating Heating/Cooling Demands

18. Calculating Heat Gains • to effectively calculate heating/cooling loads we need to calculate the other energy inputs (solar and internal heat gains) • solar gain is typically calculated within building simulation tools as part of the heat gain calculation or can be pre-calculated using climate data, geometric data and glazing data • internal heat gains (people and equipment) are typically prescribed and are a “boundary condition” for the heating/cooling load calculation

19. Calculating Heat Gains • the basis for these is typically a prescribed schedule detailing: when people are ‘active’ and when equipment is functioning • typically occupancy ‘profiles’ are developed • these are then used with heat gain data to calculate time series heat gains, that are used as boundary conditions for modelling time-series performance

20. Calculating Heat Gains • data is available for people and equipment (for example):

22. Calculating Heat Gains • using occupancy/equipment profiles and heat gain data enables a time-series heat gain profile to be developed

23. Calculating Electrical Demand • the electrical demand profile can be derived from the heat gain profile for electrical consuming equipment or vice versa • can assume that 100% of the electrical demand is eventually degraded to heat • a few exceptions e.g. lighting with in built extract • also it is possible to simulate the operation of daylight controlled lighting using a simulation tool (e.g. ESP-r) or some lighting design tools (e.g. DIALUX) • there are also free tools to generate electrical profiles

24. Calculating Electrical Demand • the characteristics of electrical demand can be significantly affected by time-averaging • generally the higher the time resolution the more realistic the electrical demand profile

25. Calculating Hot Water Demand • as with occupant and equipment gains, hot water demands are typically calculated using a pre-defined draw-schedule • this indicates the total draw being taken from a storage tank or needs to be supplied from a device

26. Calculating Hot Water Demand • again these are highly intermittent and significantly affected by averaging • the resulting time-series heat demand (W) can be calculated if the hot water supply temperature and the feed water inlet temperature are known or assumed

27. Calculating Resulting Emissions • calculation of emissions associated with energy use require the desegregation (by type) of energy demands • …and carbon emissions rates (cx) for the different fuel types • the carbon emissions are determined by multiplying the energy consumption over the period analysed by the appropriate rate: