Energy Sources for Buildings
Dr Nick Kelly
Mechanical Engineering
University of Strathclyde
Glasgow
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
Drivers for Deployment
•
•
the UK is a signatory to the Kyoto protocol committing the
country to 12.5% cuts in GHG emissions 2008-20012
EU 20-20-20
–
•
UK Climate Change 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
–
•
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
–
•
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.
further CCS legislation
plus more legislation in the pipeline ..
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
Centralised Energy Sources
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
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
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)
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
Solar electricity (PV)
≤4 kW (new build)
36.1
25
Wind
≤1.5 kW
34.5
20
Wind
>1.5 - 15 kW
26.7
20
Micro CHP
≤2kW
10.0
10
Hydroelectricity
≤15 kW
19.9
20
Enabling Microgeneration [3]:
Legislation - HEAT
•
Renewable Heat Incentive (RHI) qualifying technologies:
–
–
–
–
–
–
•
tariffs to be announced by the end of 2010
–
•
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
proposed levels
Solar thermal
18p/kWh
Biomass boiler
9p/kWh
ASHP
7.5p/kWh
GSHP
7p/kWh
installations must be accompanied by energy
efficiency improvements to dwelling
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.
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
Estimating Energy Yield
• in low energy building design calculating the
likely energy yield or fuel consumption of lowcarbon 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 ….
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
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
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.
Solar Water Heating
• a typical
active solar
heating
system will
comprise,
collectors,
heat
exchangers,
storage
tank, pumps
and pipe
work.
collector
insulated storage tank
hot water loads
heat exchanger
pump
pump
cold water feed
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.
Solar Water Heating
incident solar radiation
collector plan view
reflected solar radiation
convective losses
long wave
losses
glass cover
tubes
insulated back plate absorbs
solar radiation and re-emits
longwave
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:
Q r  I tot A  UA(TP  Ta )
Photovoltaics
• convert
solar
electricity
radiation
to
• 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
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
PV Model
• a simple equation to model PV
performance is:
I tot
1   [T  25] p
Pmp  PSTC
1000
SWTG Model
• Simple 1-D flow model :

U
SWTG Model
• Power output is expressed as a function
of the available power in the wind:
power coefficient
1
available power in the
3
WT max  C p   AU   wind
2

C pMAX  0.59; usually 0.4 or less
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
SWTG Model
• Variation of wind speeds about the mean is a function of
U and the turbulent intensity I (Gaussian distribution)
ProbabilityDensityUmean=5
1.80
IU 
 1  u  u 2 
 
exp  
2
 2  IU   
1.60
1.40
I=5%
1.20
I=10%
I=25%
u  2  v 2  z  2
I
3U 
I=50%
1.00
Probability Density
f (u) 
1
0.80
0.60
0.40
0.20
0.00
-5
0
5
-0.20
Wind Velocity
10
15
SWTG Model
Power Output Frequency of Occurrence
Power Output by Orientation
10000
800
700
1000
averaged
500
5% turbulence
10% turbulence
400
20% turbulence
30% turbulence
300
Frequency
Power Output (kWh)
600
averaged
5% turbulence
10% turbulence
20% turbulence
30% turbulence
100
10
200
100
1
0
South
West
East
Orientation
North
0
500
1000
1500
Total Power Output (W)
2000
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)
COP  f1 (Te , Tc )
Q o  f 2 (Te , Tc )
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
COP  f1 (Te )
Q  f (T )
o
2
e
• the electrical consumption (W) of the heat pump is
then given by:
Q e  Q o / COP
• Qo is the combined space heating and hot water
load at some point in time
Heat Pumps
• looking at performance over a time interval the energy
supplied ( J ) by the heat pump should equal the
energy demand ( J )
Q o t  Q d t
• however if
Q o t  Q d t
FQ t  Q t
o
d
• 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
Ee  FQ e t
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
 f HHV  Q f  thQth eQe
m
• The thermal and electrical output of a C HP unit are
related by the heat to power ratio H:P
H : P  Q th / Q e
• here the thermal energy supplied by the C HP
systems over a period of time should equal the
demand


Qth t  Qd t
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
Q th t  Q d t
FQ t  Q t
th
d
• this assumes that the device is heat load following and subject
to on/off control