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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

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Energy Sources for Buildings

Dr Nick Kelly

Mechanical Engineering

University of Strathclyde


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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

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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 ..

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C entralisedEnergy 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

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Centralised Energy Sources

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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

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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

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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)

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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):

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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

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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

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  • 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

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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 ….

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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

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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

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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.

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Solar Water Heating

  • a typical active solar heating system will comprise, collectors, heat exchangers, storage tank, pumps and pipe work.

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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.

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incident solar radiation

collector plan view

reflected solar radiation

long wave losses

convective losses

glass cover


insulated back plate absorbs solar radiation and re-emits longwave

Solar Water Heating

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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:

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  • 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

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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

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PV Model

  • a simple equation to model PV performance is:

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SWTG Model

  • Simple 1-D flow model :

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SWTG Model

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

available power in the wind

power coefficient

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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

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SWTG Model

  • Variation of wind speeds about the mean is a function of U and the turbulent intensity I (Gaussian distribution)

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SWTG Model

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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)

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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

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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

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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

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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

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