CHP has long been shown to produce benefits in terms of lower operating costs, greater system efficiency and reduced NOx emissions. Greenhouse gas emissions reduction may be the next driver for the technology in the US, writes Scott Curran of the Oak Ridge National Laboratory.
|Princeton Hospital is an example of a US facility that has turned to CHP – in this case a 4.6 MW gas-fired installation Photo: NRG Energy|
Pending and recent regulations and mandates are stoking demand for greenhouse gas (GHG) reduction solutions, including combined heat and power (CHP). In fact, CHP’s role in helping meet these regulations may drive its uptake.
GREENHOUSE GAS REGULATION
Evidence of short-timescale climate change is moulding national and international policies to regulate GHGs from sectors such as power generation, transport, industrial processes, waste disposal and remediation.
Criteria air pollutants such as oxides of nitrogen (NOx), carbon monoxide (CO), unburned hydrocarbons (HC) and particulate matter (PM) all have after-treatment technologies that can reduce them into more benign compounds. Catalysts or combustion techniques can also reduce or eliminate GHGs, such as methane (CH4) and nitrous oxide (N2O).
But, unfortunately, no catalyst is currently available for the most common and abundant GHG: carbon dioxide (CO2). The industrial practice of carbon sequestration and storage – except through biomass – is neither mature nor widespread and also carries risks.
Many governments are engaged in international climate change activities, including those under the United Nations Framework Convention on Climate Change (UNFCCC) and the Intergovernmental Panel on Climate Change (IPCC).Developing GHG regulations already include legislation such as the US federal government’s Executive Order 13514, which imposes reductions in GHG emissions from on-site power at federal facilities in the US, as well from their off-site or purchased power.
As authorities increasingly look to implement GHG regulations, facilities and municipalities intensify their search for solutions – which include increasing the efficiency of power production with CHP and distributed energy.
Three widely accepted GHGs result from stationary power generation through combustion: CO2, CH4 and N2O. To measure their global warming potential, each GHG’s global warming potential (GWP) is compared with that of CO2 over a given timescale (often 100 years) – see Table 1.
A widely accepted assessment from the Intergovernmental Panel on Climate Change views methane as a stronger greenhouse gas than CO2, with a GWP 21 times greater. But nitrous oxide, which is produced in very small amounts from stationary combustion, has a GWP 310 times greater than CO2. GHG emissions values are presented in terms of CO2 equivalent, taking into account all generated GHGs and their global warming potentials.
To report GHG emissions on a CO2 equivalent basis, emissions for each of the greenhouse gases are multiplied by their individual global warming potential and added. Methane emissions, in particular, are important to consider in distributed power generation systems, since most run on natural gas, which is primarily composed of methane. Small amounts of methane are also released due to a small amount of incomplete combustion. But the biggest bulk contributor to GHG emissions is, of course, CO2, which results from the burning of any hydrocarbon fuel. Carbon dioxide emissions make up 87–99% of all GHG emissions from stationary power, assuming proper emissions controls are in place.
Without the use of carbon sequestration, the only way of reducing CO2 emissions from the exhaust is by burning less fuel – which is accomplished through a combination of energy conservation techniques and increasing efficiency. Increased efficiency is where CHP provides a significant benefit.
As a side note, GHGs are also being considered on a lifecycle basis for the fuel, taking into account upstream GHG emissions from producing the fuel. Low-carbon fuels such as biofuels have a GHG reduction benefit on a lifecycle basis because bioenergy crops sequester more carbon during their growth than is subsequently released during the combustion of the fuel.
For the sake of simplicity, we will focus on conventional natural gas during this discussion. But readers should be aware that renewable and waste sources of methane such as landfill gas and anaerobic digester gas can result in even further reductions in GHG on a lifecycle basis. The amount these fuels reduce lifecycle GHGs depend on the local GHG regulations and credits.
Figure 1. Example of GHG emissions for a facility using purchased electricity and heating needs from a natural gas boiler
ACCOUNTING FOR CHP
The next variable in the CHP GHG reduction potential equation is the prime mover used and the efficiency and recoverable heat from the system. Major consideration for this article is given to prime movers in the 1 MW range, including reciprocating engines, gas turbines, micro-turbines and fuel cells. Total system efficiency is insufficient for calculating GHG reduction potential. Electricity generation and generated heat have different qualities and for larger facilities they often replace different fuel sources.
GHG emissions from self-generated or purchased electricity depend on what was used to generate the electricity (similarly for purchased heat and cooling). The GHG emissions associated with purchased electricity vary greatly across the US, depending on the electrical generation mix for the region and how much on-site generation capacity is from energy-efficient generators and renewable or nuclear fuel.
The US GHG emissions associated with electricity generation can vary from as low as 727 lb (330 kg) CO2eq/MWh of generated electricity to almost 2000 lb (900 kg) of CO2eq/MWh. Internationally, this varies not only from country to country but also regionally within a country. CHP’s role in reducing a facility’s GHG emissions depends on its electrical and heating/cooling needs and the GHG emissions associated with electricity in the region where it is located.
This is best illustrated with an example. We can examine the case of a small hospital in the US, meaning that purchased electricity carries with it about 1500 lb (680 kg) CO2eq/MWh. The hot water used at the hospital is generated on-site using a natural gas boiler operating at a thermal generating efficiency of 80%, which equates to about 600 lb (272 kg) CO2eq/MWh. For this example we can look at a state mandate requiring a reduction in 2013 GHG emissions from all hospitals and universities. In response to the new mandate, the hospital’s board of trustees approved a CHP project that would allow the hospital to meet all of its electricity and heating needs through CHP.
We can easily calculate the GHG emissions for power and heat requirements. To calculate the GHG emissions from the CHP system, we first have to decide which GHG accounting method to use. Local or national regulations will dictate which is used for regulatory purposes but we can pick one as an example.
Most GHG accounting methods account for the differences in quality between generated heat and generated electricity. The way quality is accounted for is the biggest difference between all of the methods, such as the efficiency method, the energy content method, the work potential method, the savings method and others. We will focus on accounting for quality in terms of thermodynamic quality (i.e. it is easier to produce heat than to produce electricity). This same difference in quality also manifests itself in a price difference of the two energy forms on the market. Purchased electricity usually costs more that purchased heat.
To illustrate the concept simply, let us assume the hospital uses an equal amount of electricity and heat, as in Figure 1, which might be unrealistic but makes the illustration easier to follow. For this example, we will look a CHP system that uses a 1.4 MW molten carbonate fuel cell (Table 2). This system uses natural gas with internal methane-to-hydrogen reformation. If the traditional system is replaced or even supplemented by a distributed generation (DG) CHP system with the fuel cell we can see a reduction in GHG.
As illustrated in Figure 2, there was a GHG reduction from moving the electricity consumption from the grid, which used the regional mix, to a more efficient prime mover that used natural gas. The second reduction came from moving the heat demand for the hospital from a high-efficiency boiler that ran on natural gas to the CHP system attached to the prime mover that did not use a separate fuel but instead used recovered waste heat as the energy source. This example – in which a building uses the same amount of electricity and heat – is for illustration purposes only and not intended as a substitute for an engineering analysis of an actual system taking into account local GHG regulatory accounting rules.
An important note to make concerning the location of the system and the regional electricity generation mix is that, while the example showed considerable GHG savings over the example US mix GHG profile, some areas in the US have an electricity mix with even lower GHG emissions due to a combination of nuclear power and renewable/ hydroelectric sources.
CHP AND GHG REDUCTIONS
We have looked at a single CHP system and found that, assuming equal electrical and heating needs, there can be considerable GHG savings potential depending on the regional electricity generation mix. The next question is what are the GHG savings potentials for CHP systems with other prime movers?
Figure 3. Comparison of GHG emissions from the US mix against various CHP applications, assuming an equal amount of electricity and heating needs
CHP systems work well with a number of prime movers in the size class we are considering and some have been purposefully designed with CHP in mind, as shown in Figure 3. Screening commercially available prime movers to determine the GHG reduction potential with CHP requires some care and, in particular, knowing what the reported numbers for a prime mover mean for CHP.
The 23 MW CHP plant at the National Institutes of Health campus in Bethesda, Maryland, has won an Energy Star award
The key performance data will be in the manufacturer’s specification sheet and will usually include power rating, fuel consumption, emissions and information regarding exhaust flow and temperature, and sometimes coolant flow and temperature information. Where the estimates for GHG emissions from CHP systems are really affected is with the recoverable heat assumptions from the prime movers.
The best way of calculating the heat recoverable would of course be to contact an applications engineer from the prospective supplier and match the system to the prime movers and the facility needs. However, as a first cut screening, the performance data from the specifications sheets can be matched to CHP screening tools such as the BCHP (Building-CHP) Screening Tool developed by Oak Ridge National Laboratory for the US Department of Energy, or by using engineering principles.
This discussion shows the potential for significant GHG reductions with CHP, depending on the installation location and local GHG regulation policy. Although the example was simplified, it shows that facilities looking to reduce the GHG emissions from electricity and heating needs should investigate the GHG savings potential offered by CHP or discuss the savings potentials with a qualified applications engineer. GHG regulations could in fact be a strong driver for increased efficiency and that means technologies like CHP will be well-positioned to meet the challenge.
Scott Curran is with the Fuels, Engines and Emissions Research Center at the Oak Ridge National Laboratory, Tennessee, US. Email: firstname.lastname@example.org