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Cogeneration
Cogeneration or CHP (combined heat and power) is defined as the
simultaneous production of electricity and heat using a single fuel
such as natural gas, although a variety of fuels can be used.
Before explaining more about cogeneration, first it is necessary to
understand a little about traditional generation methods.
Most power generation is based on burning a fuel, predominately coal,
although oil or gas are used, producing steam. It is the steam pressure that
spins the turbines that drive the generators to create electricity,
unfortunately this is inherently an inefficient process.
No more than approximately one third of the energy potential within original
fuel can be converted into steam pressure.
Electricity, a must in modern day society, is a double edge sword. The
traditional generation process feeds our need for power and lighting, yet the
process creates vast quantities of heat energy. Without a local user, this
precious resource has little economic value and is dumped to the atmosphere,
using the ubiquitous cooling towers or other convenient techniques.
Additionally, generator plants situated in remote locations also require an
extensive transmission network to reach the end user. This demands numerous
voltage transformations, coupled with lengthy cabling, creating
significant I2R transmission losses.
In contrast, cogeneration uses the excess heat, usually in the form of
relatively low-temperature steam or hot water exhausted from the power
generation turbines. Such steam or hot water is suitable for a wide range of
heating applications such as building heat, process, air conditioning and
domestic hot water provision.
Therefore effectively displacing the combustion of fossil fuels that would
have otherwise been required for these applications, with all the obvious
environmental implications.
Cogeneration is a highly efficient means of generating heat and electric
power at the same time from the same energy source, making it significantly more
environmentally friendly than conventional power plants. By displacing fossil
fuel combustion with heat that would normally be wasted in the process of power
generation, cogeneration reaches efficiencies that triple, or even quadruple,
conventional power generation.
The positive environmental implications of cogeneration stem not just from
the inherent efficiency, but also from its decentralized character.
It is impractical with current technologies to transport heat energy over any
distance, therefore cogeneration equipment is physically located close to its
heat demand. A number of environmentally positive consequences flow from this
fact:
1. Power is generated close to the consumer, reducing transmission losses,
stray current, and reducing the need for distribution equipment.
2. Cogeneration plants tend to be smaller, and owned and operated by smaller
and local companies.
3. Generally built closer to populated areas, which requires them to be held
to higher environmental standards.
Onsite cogeneration is well established overseas, especially in North America,
Germany and Scandinavian countries with other European countries following close
behind. Clean burning natural gas, with propane or landfill gas as a backup can fire
cogeneration equipment.
Cogeneration solutions provide a very efficient, 85% or more, local, on-site,
power generation system that utilizes waste heat to drive heating, process and
even air conditioning requirements. Moreover, it is presently the best
technology solution for use of fuel, having the potential to reduce human
greenhouse gas emissions by more than any other technology except perhaps public
transit.
Cogeneration solutions simply reduce waste, with only 10%-15% losses, compare that with the 55% or more using traditional generation methods and it is clear that cogeneration uses fuel more efficiently.
Cogeneration reduces waste - it is that simple
Benefits
Cogeneration uses fuel more efficiently it is really that simple - more watts
per dollar. Buying electricity from the utility companies means we, the users,
must pay the full cost for the fuel burnt, yet most of the energy is lost
to the atmosphere and the users must also cover the cost for the extensive
distribution network and transmission loss.
Cogeneration is a smarter choice, compared to traditional methods,
saves up to twenty four percent of the total energy costs, when the
spare heat energy is utilised for heating, process, or air conditioning.
It also reduces pollution, provides lower transmission losses and
benefits the environment too. The greenhouse gas emission (GHG)
reduction is a significant benefit.
Energy security is a key driver for many businesses. During blackout
periods, grid maintenance or brownouts, cogeneration keeps business
operational.
In addition to the energy savings and the environmental benefits, cited
herein the Kyoto protocol offers another opportunity. Cogeneration
systems, installed within non-Annex I countries (that includes Hong
Kong and China) qualify, under the Clean Development Mechanism (CDM), to
trade the credit (saved emissions) providing revenue.
Waste
Engineers abhor waste, must be a genetic thing, and waste energy
is particularly troubling.
The waste heat energy available at distal power stations, generally
classified as high grade heat energy, from the steam turbines, has the
potential to drive heating, air conditioning systems (using
absorption chiller plant) or serve other process needs.
Europeans, especially in Germany, cognisant of the need for winter heating,
employ systems that utilise the heat by-product to drive district heating systems
serving homes, schools, industry, offices and shops without the need for
the further burning of a primary fuel.
However, unless planned from the outset, it is simply not economical, with
generator plants remotely located many miles away from the potential
users. However, cogeneration provides both the electricity and
heat energy, where it is needed most, near the demand.
Steam Driven Air Conditioning
Here in Asia the primary consideration for the build environment
is cooling. The sub-tropical, high humidity climate drives the
demand for air conditioning throughout most of the year. And
although this may seem a strange concept at first, it isn't - heat
energy provides chilled water for Air conditioning systems.
Driven primarily by heat, the absorption refrigeration cycle generates cooling
for air conditioning systems. It is a simple and mature technology with few
moving parts compared to convention mechanical refrigeration systems
and exhibits long life.
A detailed analysis of absorption refrigeration is beyond the scope
of this article, however, I am sure some meat on the bone would be
pertinent at this point.
Absorption refrigeration uses heat instead of mechanical energy to provide
chilled water which can then be used to air conditioning buildings.
In an absorption chiller, the mechanical vapor compressor is replaced with a
thermal compressor, and comprises an absorber, a generator, a pump, and a
throttling device. Single stage absorption chillers, have been supplemented with
double effect models, improving efficiency.
In operation, a refrigerant vapour from the evaporator is absorbed by a solution
mixture in the absorber. This solution is then pumped to the generator. Where
using a heat source, the refrigerant re-vaporises.
The refrigerant-depleted solution is returned to the absorber via a throttling
device. The two most common refrigerant/absorbent mixtures presently used in
absorption refrigeration are water/lithium bromide and ammonia/water.
In contrast to traditional refrigerants, absorption refrigerants have no
ozone-depletion potential and no global-warming potential. Therefore
increasing expensive, climate damaging CFC's and HCFC's are unnecessary.
If accidentally released, they are not harmful to the environment. However,
since Ammonia is toxic for humans Ammonia/water systems demanding special
consideration to monitor and manage the risk associated with handling and
leakage.
Absorption refrigeration was once the domain of large complexes, university
campus, hospitals, power stations, and the like, where the over-riding
requirement for steam production provided an opportunity to chose and fund
large capacity absorption chillers.
Above: Single Stage Absorption Chiller courtesy of Trane
However that has changed, today manufacturer's have developed lower capacity units, powered by steam or hot water. It is noteworthy that hybrid absorption chillers have been developed, these units are direct gas fired.
Design Strategy
It goes without saying that the design strategy for a particular project or
application needs to careful consideration and a detailed load analysis is an
essential element.
For example, a complex with seasonal requirements, say situated in Beijing,
could be designed to use waste heat to provide heating during the winter and
power absorption refrigeration to provide air conditioning for the summer.
Kelcroft Consulting Engineers your prime consultant for engineering intensive projects, providing one stop solutions.
Additionally, other loads including domestic hot water or process hot water
requirements can be served.
Further energy savings can be achieved if effective energy techniques are
employed, utilising heat rejected from air conditioning plant, heat pumps or
free cooling, all of which lower the operational energy footprint.
Whilst it might thought that 100% capacity, providing total energy security
might be ideal, often economics dictate alternative part load solutions. Base
load, demand limiting or load levelling strategies are equally sound solutions,
dependant on the country, the utility, available incentives, and client
requirements.
Under certain climatic conditions, excess heat energy may still require heat
rejection, this will be unavoidable. However, the designer must carefully
consider the part-load requirement, and system characteristics in the financial
model.
Distributed Energy (DE)
Distributed Energy (DE) or DG (Distributed Generation) utilises a larger number
of lower capacity units interconnected over the transmission network.
DE provides a better
overall efficiency, since they are located where the both the power
and thermal energy is actually needed.
These power systems, often located onsite or “inside the fence”, are
designed and engineered to suit the end users power and energy
requirements. Locating generation facilities near the point of use, has numerous
advantages, efficiency improvements reduce GHG
emissions and lowers transmission losses mentioned herein.
However, onsite generation requires careful consideration, since the
environmental impact formerly remote, is brought to the doorstep.
Releasing combustion contaminants, and noise needs careful diligent
assessment and most likely compliance with stricter environmental
standards.
In other countries, deregulation of the power generation market provided the
necessary legal framework for DE,
permitting renewable energy, cogeneration and micro-CHP systems the opportunity
to co-exist on the transmission grid, and sell power back to the utility company.
Cogeneration also provides new opportunities for co-operation leveraging
advantages to improve competitiveness. A simple example would involve two
adjacent industrial facilities, one a heavy power user, the other heavy
steam user. Using a joint cogeneration plant both needs are met, dramatically
reduce their respective utility costs.
In addition to commercial capacity cogeneration installations, micro-CHP
plants, and micro-turbines, about the size of domestic washing machines, are
increasingly offering the benefits of cogeneration for the residential sector,
particularly where heating is a prime consideration.
Micro-CHP systems, often marketed in Europe as a drop-in replacement for the
conventional central heating boilers, will likely have little impact in our
sub-tropical climate where air conditioning for comfort is the key
consideration.
Above: WhisperGen AC Micro-CHP courtesy of Whisper Tech
having said that it is technically possible to couple micro-CHP systems with
small capacity absorption refrigeration to providing electricity, heat and air
conditioning. Residential capacity, direct gas fired absorption units are rapidly
developing, which may be better suited to conditions found in this part of the
world.
Barriers, Barriers, Barriers
In the USA, the NREL conducted a wide ranging study, covering more than
sixty five case histories and identified numerous technical and organisational
barriers[1]. Most, if not all, stemming from utility company citing
lack of experience, poor equipment familiarity or various charge backs schemes.
In some cases, utilities demand technical standards or requirements not demanded
from equivalent installations, in others, anti-islanding provided via inverter
circuit boards was usurped demanding 19th century mechanical relays. This
mindset often blocks viable projects whilst significantly delaying others.
Cogeneration in Asia does not require reinventing the wheel to succeed, barriers
must be overcome from day one, and that requires a utility company mindset
change.
Future Outlook
With increasing market acceptance of cogeneration, it must be realised that
cogeneration, including micro-CHP systems, etc, interaction with the
electricity network can occur.
A critical limitation, in many cases, will be the voltage
variation due to electricity feed-in. Whilst large capacity conventional
generating stations dominate, the grid can and will provide a stable reference
voltage. However, increasing numbers of DE systems may increasingly cause
voltage variations across the network, effecting cogeneration and renewable
energy systems connected to the grid.
Cogeneration not only poses problems for Distribution Transmission (T&D)
infrastructure, but can also offers advantages to the grid
such as congestion relief, load levelling and load shedding.
It is important to note that many of these impacts are
interpreted against the background of our current network, dominated by a
unidirectional system layout and large capacity power stations.
With a future power network moving towards bi-directional
electricity and distributed generation, both the generation structure, backup, and
the T&D system will have to adapt to this lively development.
Uptime and reliability is a key issue, indeed the traditional
generators may be called upon to provide back-up power helping lower capital costs.
However billing arrangements need to be carefully examined.
In summary, compared to the traditional generation, the provision of
ancillary power control services by cogeneration or
micro-CHP plants is more complicated but possible.
For this purpose, one key challenge will be grid interconnection,
and virtual grids featuring many cogeneration and micro-CHP plants
forming virtual power plants. These plants may also be able to reduce
network operation costs, especially if their operation can be controlled
within a virtual power plant.
Still in its infancy across Asia, the wider acceptance of DE, the
forth coming grid-interconnection code in Hong Kong and availability
of natural gas provides both opportunities and challenges to pursue
cogeneration.
Certainly barriers still exist and these must to be resolved, including
entrenched utilities, revisiting the legislative framework, and planning
regulation amendments - all these need to be tackled before the sale of
value-added "green" electricity to the grid provides revenue.
References
1. Making Connections - Case Studies of Interconnection Barriers and their
Impact on Distributed Power Projects, NREL, June 2000
About the Author
Mr John A. Herbert is managing director of Kelcroft Consulting Engineers, a full
service E&M consultancy headquartered in Hong Kong, serving South East Asia.
He is founder and chairman of CogenAsia. He has more than twenty four years
engineering experience, the last ten in independent practice. He has been involved
in the E&M design and construction management of major commercial, institutional,
and industrial facilities internationally, an accomplished energy expert and
maintained a litigation free record.