Heat and Power Sources for BuildingsHeat and Power Sources for Buildings

Overview

• energy requirements of buildings• traditional energy sources• carbon emissions calcs• LZC energy sources

– low-carbon energy sources– renewable (zero-carbon) energy sources

•space heating•hot water•electricity

– lighting– appliances– cooling–… also for space heating

and hot water

Energy Required

distribution: cables, ducts, fans, pumps, piping, etc.

delivery: radiators, underfloor heating, lights, diffusers, etc.

environmental system

control: thermostats, dampers, valves, timers, PID controllers, etc.

sources: boilers, chillers, electricity supply

Traditional Energy SourcesTraditional Energy Sources

• space heating – gas, oil or solid fuel boilers, direct electric, electric storage heating

• hot water - gas, oil or solid fuel boilers, direct electric heating

• electrical equipment and appliances – power from the grid

• … ultimate energy source typically fossil fuels

BoilersBoilers

• the main function of the boiler is to convert the potential energy of a fuel to heat

• In the UK this is typically in the form of hot water or steam (larger systems)

• boilers can be:

– condensing (recover latent heat from flue gases)

– combination (instant hot water)

• typical device efficiencies range from 70-90% depending upon age, features and fuel type

• fuels: natural gas, oil, solid fuel

GridGrid

• grid electricity ultimately comes from large central power stations:

– combined cycle gas turbine (η=50+%)

– coal/oil power station (η=35%)

– nuclear power station (η=35%)

• grid electricity carbon intensity: 0.53 kgCO2/kWh (DEFRA)

EmissionsEmissions

• how do we calculate emissions?

• example – natural gas:

• CH4 + 2O2 → CO2 + 2H2O

• (16) → (44) or 1 kg → 2.75 kgCO2/kgCH4 or x (12/44) = 0.75 kgC/kgCH4 (CO2 and Carbon coefficients resp.)

• energy content of nat. gas 93MJ/m3 or 51.12 MJ/kg or 14.2 kWh/kg

• so for an 80% efficient boiler, C emission for 1kWh of heat

• C = (energy/(efficiency x energy content)) x carbon coefficient

• C = (1/(0.8 x 14.2)) x 0.75 = 0.07 kg C/kWh = 0.24 kg CO2/kWh

EmissionsEmissions

• Similarly ….

• so for an 35% efficient coal power station C emission for 1kWh of electricity

• C = (energy/(efficiency x energy content)) x carbon coefficient

• C = (1/(0.35 x 10)) x 0.9 = 0.26 kg C/kWh = 0.94 kg CO2/kWh

distribution: cables, ducts, fans, pumps, piping, etc.

delivery: radiators, underfloor heating, lights, diffusers, etc.

environmental system

control: thermostats, dampers, valves, timers, PID controllers, etc.

LZC sources: CHP, PV, solar thermal, etc.

sources: boilers, chillers, electricity supply

Low Carbon Energy Systems

Combined Heat and Power (CHP)

• CHP (combined heat and power) is the simultaneous generation of heat and power from a single conversion device

• CHP technologies:

– ICE – internal combustion engine

– SE – stirling engine

– gas turbine

– fuel cell (SOFC)

CHPCHP

• CHP is classed as low carbon as it makes use of the waste heat produced by a thermodynamic cycle

• this is not done in conventional power generation – the heat is typically rejected to atmosphere

CHPCHP

25electricity

65heat

100 fuel

7 waste

72 fuel

83 waste

108 fuel

180 fuel total

90% eff. boiler

30% eff. power station

90% eff. CHP

10 waste

CHP

– the CHP prime mover depends upon the application

1kWe

>1MWe

Stirling

ICE (gas)

ICE (diesel)

Gas turbine

CHP

• typical device efficiencies : 85-95%

• heat/power ratios:

– 8:1 stirling engine;

– 2:1 ICE;

– 1:1 gas turbine

• fuel cell CHP is still a research area with lots of work to be done before these devices appear on the market

CHP

CHPBUFFER

TANK

DHW TANK

RAD NRAD A

T

T

T

T

CHPBUFFER

TANK

DHW TANK

RAD NRAD A

T

T

T

T

• CHP device coupled into heating system

Heat Pump

• heat pumps move heat energy from a low temperature heat reservoir to a high temperature reservoir (e.g. the building) using a refrigerant cycle

• heat pumps can use the ground, water or even the air as the low temperature reservoir

• the cycle is driven by a compressor, which consumes electricity

Heat Pump

• heat pump performance is measured using a quantity known as the coefficient of performance (COP)

• COP = useful heat output ÷ energy consumed by compressor

• so for a COP of 4 (typical) 1kWh of heat will require 0.25 kWh of electricity

• the cycle can also be reversed to surplus heat from the house can be returned to the ground (e.g. summer cooling)

• heat pumps (arguably) have the greatest carbon saving potential of any low carbon technology

• if powered using renewable electricity heat pumps become zero carbon devices

Heat Pump

Zero Carbon Sources

Photovoltaics• photovoltaic devices (PV) convert sunlight

directly to electricity

• PV is based on semiconductor technology

• the most common material used is silicon

• the basic unit of a PV system is the cell:

Photovoltaics• individual cells are wired together and

encapsulated in a panel

• groups of PV panels installed on a building are called an “array”

• silicon PV is typically 12% efficient

• so an incident solar intensity of 600W/m2 falling on a 1m2 panel will generate 72W

• typical energy yields are ~100kWh/m2/yr

• conversion efficiency is dependent upon:

– the PV material used

– temperature

– solar intensity

– the load

Photovoltaics• PV power is intermittent – the

amount being produced being determined by the solar intensity

• PV produced DC electricity – which can be used directly for battery charging

• connecting to AC loads requires the power from the panel is inverted

• PV is usually connected to the building’s electrical system via a power electronic interface

• this maximises the PV efficiency and converts ac → dc

Photovoltaics

Micro Wind• micro wind power devices generate

electricity from air flow around a building

• typical devices are horizontal axis machines – smaller versions of large scale machines

• typical device ratings are 1-5kW (@5-6 m/s)

• however the rated wind speed is rarely achieved in urban areas in practice (2-3 m/s)

• better suited to more isolated buildings or unobstructed air flow

Micro Wind• flow in urban areas is highly turbulent

and not ideal conditions for turbines

• wind speed and direction can vary wildly in short distances

• proper siting is critical to achieve the best yield

Micro Wind• the best site for a turbine can be

predicted …

Other Zero Carbon

• solar thermal

– flat plate

– evacuated tube

• biomass/biogas boilers

• hydrogen fuel cell