Evaluating energy efficiency for complex and integrated

Ho Hiang Kwee

Director, Urban Solutions, DNV Clean Technology Centre

Evaluating energy efficiency for complex and integrated energy systems

National Energy Efficiency Conference, May 24-25, Singapore

© Det Norske Veritas AS. All rights reserved.


May 2011

2

What is energy efficiency (EE)?

� From ISO 50001 (Draft International Standard) for Energy Management Systems

Ratio or other quantitative relationship between an output of performance,

service, goods or energy, and an input of energy

- Examples are conversion efficiency, energy required/energy used, output/input,

theoretical energy used to operate/energy used to operate.

- Both input and output have to be clearly specified …, and be measurable.

� Sounds simple enough, but ….

- Energy is not “homogeneous” - exists in many different forms and quality

- Some energy considered “free” (waste energy, renewable energy) – still part of input?

- EE definitions are different for different systems or technologies providing same energy

service, especially when different boundaries and “life cycle” assumptions are made

- EE is different from economic or cost efficiency, which is often the target of improvement

- EE varies with other “external” factors/parameters (e.g. ambient conditions, load/output

levels, quality of operation and maintenance)

� So, is EE a good indicator of energy performance and energy sustainability?



May 2011

3

Example: Producing useful heat (e.g. hot water or space

heating) from different technologies



May 2011

4

Device and system efficiency are different

For electric

heater, device

efficiency (> 90%)

and

System efficiency

(< 50%) are quite

different



May 2011

5

Heat pump energy efficiency – how to define?

Heat pump:

For low

temperature

heat, allows

very high

efficiency

(COP > 5)



May 2011

6

Examples of complex or integrated energy systems

� District energy systems - Cogeneration and tri-generation and other integrated

energy systems (e.g. incorporating power and water production)

� Unconventional air conditioning/HVAC system;

- Including use of thermally activated technologies utilizing waste heat and/or renewable

energy (absorption refrigeration, adsorption refrigeration, heat-driven dehumidifiers, heat

driven ejectors etc.)

� Integrated bio-energy plants and bio-refineries producing power, fuels,

chemicals

� Hybrid solar power plants producing power, heat, cooling



May 2011

7

Efficiency of cogeneration and tri-generation plant

� Possible (misleading) definition of energy efficiency of cogen plant

� Typically, will get EE of > 70%

� But what if EE of power generation sub-system is very low (e.g. < 20%), and most of output energy is in the form of low grade heat?

HV(fuel)energy Input

producedenergy Useful

×

+==

fuel

outoutcogen

m

PQ

&η

� Is this high EE cogen system better than a combined cycle power plant producing power at efficiency of 50%?

� Considering “efficiency” (COP) of electrical driven heat pump can be > 5, it is clear that the EE of cogen plant is not that impressive anymore

� Similarly, because the COP of absorption and adsorption technologies are still very low (relative to mechanical/electrical chillers), the high EE of tri-generation systems using sorption technologies for cooling may not mean that much without further detailed evaluation



May 2011

8

Can 2nd Law analysis based on exergy provide better insights?

� Possible to use “exergetic efficiency” as a more rational parameter for “energy efficiency” and energy sustainability?

� Exergy takes into account the quality of energy (in terms of its work potential), hence enables fairer comparison and assessment of energy efficiency

� In previous example, exergetic efficiency would be given by

where R is the energy grade function

� However, methodology using exergy is considered complex and not widely used yet

R HV).(

)1(

(fuel)exergy Input

producedExergy

×

+−

==fuel

outout

out

amb

cogenm

PQT

T

&ε



May 2011

9 9

Energy

Sankey

Diagram

Exergy

Sankey

Diagram

Source: exergy.se/goran/thesis/paper1/paper1.html



May 2011

10

Other approaches to indicate “energy sustainability”?

� Consider using carbon footprint or emissions as another way to provide

comparisons of energy sustainability?

Adapted from Carbon Trust:

A Carbon Footprint is the total set of greenhouse gas emissions caused directly

and indirectly by an individual, event, project, organisation, or product expressed

as CO2eq

� Carbon accounting approaches utilise a common indicator of

environmental/energy performance, and sustainability - the production of carbon

dioxide (or greenhouse gases).

� Overcomes some of the problems associated with First Law approaches that do

not differentiate between different forms and quality of energy.



May 2011

11

Features of carbon footprinting as an indicator of energy sustainability

� Need to have appropriate system boundaries and framework (e.g. through the use of international standards or protocols or methodologies), to be used as a rational and transparent indicator/metric of energy sustainability

� Under UNFCCC flexibility mechanisms, various methodologies for (complex) projects and systems have been developed to enable carbon emissions reductions relative to existing practices to be evaluated and subsequently traded

� Other Standards and Protocols for carbon footprinting of organisations, projects, and materials/ products/ goods/ services have been, and are being developed, some emphasizing a life-cycle approach. These include:

- GHG Protocol (WBCSD/ WRI) – see example of framework for “projects”

- ISO 14064, and 14067 (draft) and associated standards

- BSI PAS 2050

� Need to differentiate between accounting for GHG emissions for

- Individual, organisation, country, world,

- Activity, event, project

- Materials, product, goods

- Services



May 2011

1212

GHG emissions : Definition (for organisation footprint)

� The GreenHouse Gas (GHG) emissions are classified by categories (scope 1,2 or 3)

depending on emission source :

� Scope 1 : all direct GHG emissions from the sources the company owns or controls.

� Scope 2 : indirect GHG emissions from consumption of purchased electricity, heat or steam.

� Scope 3 : other indirect emissions like extraction and production of purchased materials,

outsourced activities, waste disposal, transport, …



May 2011

13

Source: GHG Protocol (for projects)

GHG reporting

framework using

GHG Protocol

(for projects)



May 2011

14

Conclusions

� Conventional First Law based energy efficiency indicators are inadequate,

especially for integrated and complex energy systems, where different forms of

energy are either utilised or produced

� 2nd Law approaches (e.g. exergetic efficiency) provides a more elegant and rational

approach to evaluating energy performance and sustainability, but not well received

yet

� Carbon footprinting is receiving a great deal of attention, and being incorporated in

various “green” and sustainable initiatives (e.g. Singapore Green Mark)

� Many companies are evaluating their carbon footprint as part of Corporate

Responsibility reporting

� However, not all evaluations and assessments are conducted in a way that would

allow fair comparisons and benchmarking

� Hence, more work needs to be done to improve use of carbon footprinting as a

useful indicator of energy sustainability



May 2011

15

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Evaluating energy efficiency for complex and integrated