150 kW back-pressure turbine generator installed at the district heating plant of the Franciscan Sisters of Perpetual Adoration in LaCrosse, Wisconsin, USA.
‘HEAT-FIRST CHP’ – A NEW WAY WITH OLD TECHNOLOGY
To understand how this is possible, it is worth first considering the marginal emissions associated with heat recovery in a conventional CHP plant. In these plants, a prime mover (gas turbine, engine, etc.) is used to generate electricity, and the waste heat thus produced is recovered and used for local thermal processes. In such instances, it is commonly understood that any heat thus recovered is ‘free’. There is no marginal fuel purchase or combustion associated with the production and recovery of this heat, and it is treated as such. This obviously does not apply directly to gas turbines in which ‘supplementary firing’ is done to increase the quality of the exhaust heat. However, even in these circumstances – and as the language itself suggests – the marginal combustion is only assumed to be that associated with the supplemental fuel used, and not with the base heat provided by the turbine exhaust.
In fact, this heat is often considered to be better than free, since its recovery and utilization usually displaces a boiler or heater elsewhere, thus leading to a net reduction in emissions and heat-related expenses.
It is critical to recognize the guiding principle of this system design however: while the heat is recovered to improve overall economics, the primary objective of the CHP plant is obviously to produce electrical power. Once this power is generated, any heat that can be opportunistically recovered is free – but such a plant would never be built solely to produce heat. Since power is central to this architecture, it appropriate to label it as ‘power-first CHP’. First make power, then recover the heat that is available.
However, the term ‘combined heat and power’ necessarily implies two forms of energy, without any implied preference for either. While power-first designs are quite common, there is also a host of installations that can be considered as an inversion of this logic, or ‘heat-first CHP’. In these installations, the primary product is heat, and power is recovered from this heat stream only when it can be opportunistically extracted.
The potential benefits of CHP in commercial building applications could be more fully
realized if manufacturers learn to make ‘plug-and-play’ systems that make on-site engineering
less costly and time-consuming. To understand what it takes for CHP systems to become
fully integrated into a building’s existing energy systems, US researchers are testing
‘integrated energy systems’ at a major university and are ready to pass on some early lessons
learned, report Patricia Garland, Doug Hinrichs and Matthew Cowie.
Many energy experts and building owners understand the potential benefits of CHP for buildings – including the tremendous gains in energy efficiency. ‘In the US, roughly 67% of the energy contained in the fuel for electricity generation is rejected as waste heat into the environment,’ says Reinhard Radermacher, Professor of Mechanical Engineering and Director of the Center for Environmental Energy Engineering at the University of Maryland. Further losses occur in electrical power transmission. Dr Radermacher continues, ‘When this waste heat is made available at higher temperatures, then it can be utilized for dehumidification, air conditioning, or heating with advanced CHP systems. By doing so, energy efficiency can increase from 33% to as high as 80% for a CHP system, although the efficiency of electricity generation is reduced.’
CHP can also increase the reliability of a building’s power supply – a substantial advantage in today’s changing electricity market. A highly reliable power supply is vital to some companies’ computing, manufacturing and research functions.
Emissions of carbon dioxide and air pollutants like nitrogen oxide, sulphur dioxide and volatile organic particles can be substantially reduced with CHP. CHP can meet some or all of a building’s cooling, heating, or dehumidification load, reducing the need for electricity from the grid by an equivalent amount, thus leading to lower emissions. CHP systems are also more efficient and require less fuel than traditional systems. According to the US Department of Energy, CHP systems could reduce annual greenhouse gas emissions by at least 25 million tonnes of carbon if the Government’s goal to double US installed capacity by 2010 were met.