Waging war on heat waste with CHP25/01/2017
Waste heat is the weakest link in the energy supply chain. In an article published in Tomorrow's Energy Management Magazine, Chris Marsland, Technical Director for ENER-G explains how CHP can capture and utilise waste heat to give energy supplies a productivity boost.
Forty per cent of the UK’s energy is used to generate heat energy; and heat wasted from buildings accounts for half of total energy emissions. Such waste has been highlighted in a report by a coalition of industrial manufacturers and environmental groups, which states that a staggering 54% of energy used to produce electricity is being wasted via conventional power production at an eye watering cost of £9.5 billion per year.
The UK's energy productivity problems can be significantly improved by moving from centralised to decentralised production, using technologies such as CHP.
CHP is a well-proven technology, recognised worldwide as a viable alternative to traditional centralised generation. With CHP, an engine (most commonly fuelled by natural gas, but also using renewable fuels such ad biogas) is linked to a generator to produce electricity. CHP maximises the energy in the fuel and converts it into electricity at around 33% efficiency and heat at around 52%. Heat is recovered from the engine by removal from the exhaust, engine water jacket and oil cooling circuits.
CHP can thus achieve more than double the efficiency of conventional power production. In addition, the technology can reduce CO2 emissions by as much as 30%, as well as achieving cost savings of up to 40%.
Whenever a site has a demand for both electrical and thermal energy, the installation of CHP on-site should be considered.
If there is a site demand for cooling, any excess CHP heat can be used in an absorption chiller to produce chilled water for air cooling systems. The process of generating on-site cooling, heating and electrical power is known as trigeneration or Combined Cooling Heat and Power (CCHP).
CHP should always be considered when:
- Designing a new building
- Installing or replacing a new boiler plant
- Replacing or refurbishing an existing plant
- Reviewing electricity supply
- Reviewing standby electricity generation or plant
- Considering energy efficiency in general
- Exploring options towards building regulation compliance
- Reducing CO2 emissions and environmental impact
CHP is particularly beneficial for applications where there is a large heating or cooling demand, like hotels, hospitals, universities, leisure centres and industry. CHP also scores in terms of versatility since it is suitable for new and refurbished buildings, as a replacement for an ageing boiler plant, or in augmenting existing or new boilers.
In order to optimise the CHP system, sizing the unit is critical to the success of the project. The most suitable sites for CHP generally have year round demand for heat or cooling, where the unit will be run as 'lead boiler'. The thermal integration of any CHP unit should be carefully considered and investigated in order to achieve the maximum possible savings. Thermal storage may be used to increase heat utilisation.
The utilisation of the recovered thermal energy drives the economics of a CHP project. The CHP Quality Assurance (CHPQA) scheme monitors and assesses the quality of CHP schemes on the basis of energy efficiency and environmental performance. It incentivises high CHP efficiency via high heat utilisation. The CHPQA uses the CHP Quality Index to prevent excessive heat being rejected and discourage CHP units being operated as gas generators only. A minimum CHP Quality Index score of 105 at design stage and 100 during operation will achieve the category of ‘Good Quality CHP’, which currently provides the benefits of Climate Change Levy (CCL) exemption and Enhanced Capital Allowances (ECA) eligibility.
Types of CHP heat recovery
LowTemperature Hot Water (LTHW) Systems
The most common and simplest form of heat recovery from a CHP unit is in the form of LTHW (typically 90oC/80oC, although lower temperatures can be easily accomodated). This enables heat recovery from the oil cooler, engine jacket and the exhaust gas heat exchanger, in a common primary water circuit.
CHP units can also be designed to operate at lower return temperature. In some circumstances, it can be possible to use the CHP heat for domestic hot water use. This can be to supplement times then there is no or little LTHW load, such as summer months.
If the user requires a heat source in the form of steam then the exhaust gases (350oC – 450oC) from the CHP can be diverted directly into a waste heat recovery boiler.
Steam generation from CHP is best suited for units greater than 500kWe as the quantity of recovered energy below this value is small.
Absorption Cooling Systems (Trigeneration Systems)
This technology allows cooling to be produced from waste heat and is suitable for sites that have a large continuous cooling demand, for example air-conditioning or process cooling. Typically these systems require a system temperature of 6oC–12oC, which is particularly suitable for absorption chillers.
Absorption chillers can also be successfully incorporated into schemes that have a large electrical demand but may only have a relativity small thermal demand. The size of the CHP unit could be maximised to meet the site's electrical load profile with the thermal energy being used to drive an absorption chiller. The additional heat load would allow the plant to operate more efficiently, removing any seasonal variation element and improving the operational hours of the scheme. Most standard absorption chillers operate on either LTHW, MTHW or steam. Absorption chillers also require some form of heat rejection.
For the right applications, CHP can dramatically improve heat utilisation - providing an affordable solution to energy decarbonisation and productivity.