1. Energy Sources for Buildings
Dr Nick Kelly
Mechanical Engineering
University of Strathclyde
Glasgow

2. Energy Sources for Buildings
a building can draw power from a variety of sources
typically this has been from centralized sources the electricity network or the gas grid
… less typically buildings could use solid fuel or bottled fuel
a building can also tap into local renewable energy sources (wind, solar)
both centralized and local energy conversion are in a period of rapid change

3. Drivers for Deployment
the UK is a signatory to the Kyoto protocol committing the country to 12.5% cuts in GHG emissions EU
reduction in EU greenhouse gas emissions of at least 20% below 1990 levels; 20% (average) of all energy consumption to come from renewable resources; 20% reduction in primary energy use compared with projected levels, to be achieved by improving energy efficiency.
UK Climate Change Act 2008
self-imposed target “to ensure that the net UK carbon account for the year 2050 is at least 80% lower than the 1990 baseline.”
5-year ‘carbon budgets’ and caps, carbon trading scheme, renewable transport fuel obligation
Energy Act 2008
enabling legislation for CCS investment, smart metering, offshore transmission, renewables obligation extended to 2037, renewable heat incentive, feed-in-tariff
Energy Act 2010
further CCS legislation
plus more legislation in the pipeline ..

4. C entralised Energy Sources
electrical power production in the UK and Scotland in particular is undergoing a period of radical change
8GW of capacity in 2009 (up 18% from 2008)
Scotland 31% of electricity from renewable sources 2010
… significant capacity of new offshore wind and nuclear power will come on stream between now and 2025

5. Centralised Energy Sources

6. Centralised Energy Sources
the legislative driver behind the significant increase in large scale renewables is the Renewables Obligation [Scottish Renewables Obligation]
requires utilities to source an increasing quantity of their energy [electricity] from renewable sources
… no real change in gas supplies
though biogas (methane) can now be injected into the gas network

7. Local Energy Sources
microgeneration lags far behind larger scale generation
120,000 solar thermal installations [600 GWh production]
25,000 PV installations [26.5 Mwe capacity]
28 MWe capacity of CHP (<100kWe)
14,000 SWECS installations 28.7 MWe capacity of small wind systems
8000 GSHP systems
an insignificant amount of built environment energy is derived from these sources

8. Promoting Microgeneration [1]: Technology Deployments
Carbon Trust ‘micro CHP accelerator programme’
deployment of 87 demonstration micro CHP units
disappointing carbon savings reported
final report never released
Energy Savings Trust Heat Pump Trials
29 ASHP and 54 GSHP systems installed and monitored
some disappointing COPs measured due to poor systems design
Warwick wind trials
some catastrophically poor performance reported due to poor location of turbines (-ve electrical power production)

9. Promoting Microgeneration [2]: Legislation - ELECTRICITY
15% of total energy provision from renewables by 2020
… 2% in 2009
in order to boost installation to meet UK and EU legislative targets UK government introduced FIT (2009) and RHI (2011)
Feed-in-Tariff (FIT) (replaced previous grants and tax allowances):
Technology
Scale
Tariff level (p/kWh)
Tariff lifetime (years)
Solar electricity (PV)
≤4 kW (retro fit)
41.3 25
≤4 kW (new build)
36.1
Wind
≤1.5 kW
34.5 20
> kW
26.7
Micro CHP
≤2kW
10.0 10
Hydroelectricity
≤15 kW
19.9

10. Enabling Microgeneration [3]: Legislation - HEAT
Renewable Heat Incentive (RHI) qualifying technologies:
air, water and ground-source heat pumps
solar thermal
biomass boilers
renewable combined heat and power
use of biogas and bioliquids
injection of biomethane into the natural gas grid
tariffs to be announced by the end of 2010
proposed levels
installations must be accompanied by energy efficiency improvements to dwelling
Solar thermal
18p/kWh
Biomass boiler
9p/kWh
ASHP
7.5p/kWh
GSHP
7p/kWh

11. Enabling Microgeneration [4]: Legislation – EPBD2
minimum energy performance requirements to be set for all new and refurbished buildings and compared against requirements calculated in accordance with cost-optimal requirements;
energy use of technical building systems to be optimised by setting requirements relating to installation, size etc. covers heating, hot water, air-conditioning and large ventilation systems;
all new buildings developed after 2020 to be nearly zero energy buildings, with an earlier target date of 2018 where the building will be owned and occupied by a public authority;
EPBD2 will be implemented by Member States by 2012–13.
installations must be accompanied by energy efficiency improvements to dwelling

12. Conclusions
radical change in UK energy mix at large scale due to very challenging GHG reduction targets [domestic and EU]
huge growth in on/offshore wind, biomass combustion
microgeneration lagging far behind, low numbers of installation in comparison to rest of Europe and North America
technology field trials yielding poor results (mainly due to poor installation)
FIT and RHI (and eventually EPBD2) are strong drivers for growth BUT
installer skills base is lacking
industry and supply chain infrastructure relatively immature in the UK

13. Estimating Energy Yield
in low energy building design calculating the likely energy yield or fuel consumption of low-carbon devices is as important as calculating the likely demand
this requires different approaches for solar devices/cogeneration heat pumps or wind
typically, however we need to do some form of resource modelling ….

14. Solar Devices
the starting point for a solar calculation is an estimation of the total solar radiation falling on a surface (W) at any point in time
additionally a performance model of the solar energy conversion device is required
calculating the total solar irradiation is beyond the scope of this class, but a spreadsheet and explanatory notes are provided to allow you to do this

15. Solar Devices
we would normally use historical climate data appropriate to the site for which we are modelling
this data can then be manipulated to estimate the total solar irradiance falling on a surface of arbitrary orientation and size
a common format of climate file is the Test Reference Year (TRY)
TRY files are available for a large number of sites around the world

16. Solar Water Heating
flat plate solar collectors are the most common and familiar solar energy conversion device.
they are generally used for water heating and form part of an active solar heating system.
flat plate solar collectors work in both direct and diffuse sunlight.

17. Solar Water Heating
a typical active solar heating system will comprise, collectors, heat exchangers, storage tank, pumps and pipe work.

18. Solar Water Heating
the operation of the collector is very simple: shortwave solar radiation is transmitted through the glass cover and absorbed on the back plate.
absorption of solar radiation causes the back plate to heat up; this heat is removed by the water running through the tubes.
as the back plate will itself emit increasing quantities of longwave radiation as it heats up, however the glass cover is opaque (does not transmit) this longwave radiation, so it is effectively trapped inside the collector increasing its efficiency.

19. Solar Water Heating incident solar radiation collector plan view
reflected solar radiation
convective losses
insulated back plate absorbs solar radiation and re-emits longwave
tubes
long wave losses
collector plan view
glass cover

20. Hottel Whillier Equation
A useful equation for the calculation of heat recoverable Qr (W) from a flat plate solar collector is the Hottel-Whillier equation:

21. Dr. N Kelly : Solar Energy
Photovoltaics
convert solar radiation to electricity
make use of the ‘photoelectric’ effect where a photon striking an atom can liberate an electron in photovoltaic devices the liberated electrons flow into an external circuit – giving rise to an electric current
relatively low efficiency process 4%-20%, with typical efficiencies of 12% (first solar cell had an efficiency of 6%)
efficiency dependent on many factors but primarily the material and construction of the photovoltaic device
Dr. N Kelly : Solar Energy

22. Dr. N Kelly : Solar Energy
PV Performance
to maintain the operation of the cell at the optimum point requires power electronics – maximum power point tracking
optimises the power yield from the PV as Itot and T vary with time
without power point tracking the performance of PV could be far from optimum!
Dr. N Kelly : Solar Energy

23. PV Model
a simple equation to model PV performance is:

24. SWTG Model Simple 1-D flow model :
The model of the DWT is a fairly simple 1-D developed by Grant and Danneker
The power output (work output from the rotor) is a function of:
the free stream air velocity U;
the pressure coefficient differential  across the rotor;
the DWT entry area A and ;
a duct velocity coefficient (Cv1.0) which has been empirically derived.
A more detailed derivation of the formula shown here is given in the paper.

25. SWTG Model
Power output is expressed as a function of the available power in the wind:
available power in the wind
power coefficient
Now the power output from a wind power device is often expressed in terms of the available power in the wind and a power coefficient.
In the equation shown the power available in the wind is the bracketed term and the power coefficient is made up of the terms to the left of the bracket.
Clearly the higher the power coefficient then the more power that can be extracted from the wind.
Also notice that the power extracted is proportional to U3 this has important implication for integration with BSIM, as we will discuss later.

26. SWTG Model note that the power output of the DWT is  U3
much higher power output from high wind speeds (e.g. gusts)
use of model with hourly averaged wind data could lead to underestimation power output
The use of typical “hourly” averaged simulation data is problematic with this model as the power output is proportional to U3 . Higher wind velocities will therefore give disproportionately higher power outputs.
Use of averaged data will therefore lead to an underestimate of the power output from the turbine.
To try and compensate for this a statistical treatment of the wind velocity supplied by ESP-r is added into the model.

27. SWTG Model
Variation of wind speeds about the mean is a function of U and the turbulent intensity I (Gaussian distribution)
Instantaneous wind speeds over the simulation time step are assumed to conform to a Gaussian distribution about the mean wind speed (or its components).
The equation above shows the probability density function for this distribution.
The form of the distribution is related to the turbulent intensity of the wind, note that the standard deviation in the PDF has been replaced with an expression with the wind turbulent intensity I and the averaged velocity U, which is a measure of the fluctuation of velocity about the mean.
Notice how increasing turbulent intensity causes the PDF to spread.
In the derivation of the PDF, the assumption is made that turbulent intensity is isotropic and the components of velocity behave independently of one another.
The PDF can then be used to calculate a time duration for a particular wind speeds and directions, these can then be used in the model to calculate power output over the time step.

28. SWTG Model
Results show the energy yield for different facades of a building and for different wind characteristics (turbulence levels)
If you’re interested the energy yield is roughly equivalent to that of PV (90-120kWh/m2) but of course the locations where the DWT could be placed on a building!!

29. Heat Pumps
with heat pumps we are interested in calculating the electrical power consumption of their compressor
this is a function of the energy delivered to the load and the performance characteristics of the heat pump
both the coefficient of performance and heat output of a heat pump vary depending upon the condenser and evaporator temperatures (temperature to which the heat is being delivered and temperature from which it is being taken)

30. Heat Pumps
assuming that the heat pump works, then we can assume that during its operation that the temperature of the space is relatively constant and so
the electrical consumption (W) of the heat pump is then given by:
Qo is the combined space heating and hot water load at some point in time

31. Heat Pumps
looking at performance over a time interval the energy supplied ( J ) by the heat pump should equal the energy demand ( J )
however if
F is the fraction of the time interval  t that the unit will be on (assuming on/off control) and overall electrical energy consumption ( J ) is

32. C ogeneration ( C H P)
for cogeneration we are interested in the fuel use, this is a function of the energy delivered to the load and the characteristics of the prime mover
The thermal and electrical output of a C HP unit are related by the heat to power ratio H:P
here the thermal energy supplied by the C HP systems over a period of time should equal the demand

33. C ogeneration ( C H P)
Again where the thermal output could exceed demand over a period of time  t, then the unit will only be active or a fraction F of that time period
this assumes that the device is heat load following and subject to on/off control