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

– 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 ..

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:

– 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

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

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 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 ….

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.

insulated storage tank

heat exchanger

pump pump

collector

hot water loads

cold water feed

insulated storage tank

heat exchanger

pump pump

collector

hot water loads

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

reflected solar radiation

convective losses

insulated back plate absorbs solar radiation and re-emits longwave

tubes

long wave losses

collector plan view

glass cover

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:

)( aPtotr TTUAAIQ

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

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!

PV Model

• a simple equation to model PV performance is:

pTI

PP totSTCmp ]25[1

1000

• Simple 1-D flow model :

U

SWTG Model

• Power output is expressed as a function of the available power in the wind:

lessor0.4usually;59.0

2

1 3max

pMAX

pT

C

UACW available power in the wind

power coefficient

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)

2

2

1exp

2

1)(

UIUIf

uuu

Probability Density Umean=5

-0.20

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

-5 0 5 10 15

Wind Velocity

Prob

abili

ty D

ensi

tyI=5%

I=10%

I=25%

I=50%

U

zvuI

3

222

SWTG Model

SWTG Model

Power Output by Orientation

0

100

200

300

400

500

600

700

800

South West East NorthOrientation

Pow

er O

utpu

t (kW

h) averaged

5% turbulence

10% turbulence

20% turbulence

30% turbulence

Power Output Frequency of Occurrence

1

10

100

1000

10000

0 500 1000 1500 2000

Total Power Output (W)

Freq

uenc

y

averaged5% turbulence10% turbulence20% turbulence30% turbulence

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)

),(

),(

2

1

ceo

ce

TTfQ

TTfCOP

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

)(

)(

2

1

eo

e

TfQ

TfCOP

COPQQ oe /

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

tQtQ do

tQtQF

tQtQ

do

do

tQFE ee

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

eeththff QQQHHVm

tQtQ dth

eth QQPH /:

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

tQtQF

tQtQ

dth

dth