A combined heat and power system (CHP) is the cogeneration or simultaneous generation of multiple forms of energy in an integrated system. CHP systems consume less fuel than separate heat and power generating systems. According to the Environmental Protection Agency in their Combined Heat and Power Partnership report, (EPA), CHP systems typically consume only three-quarters the amount of energy separate heat and power systems require. By combining both heat and power into the same energy systems, efficiency gains for the total system. Heuristically, high temperature and high pressure fuel ratios results in higher efficiency systems. In addition, the thermal energy produced from the CHP system could be used to drive motor applications or to produce heat, steam, and hot water.
As an initial step to reducing greenhouse gas (GHG) emissions, natural gas turbines could improve overhaul efficiency of 65-80%. In addition, the CHP offers lower greenhouse gas (GHG) emissions in comparison to conventional standalone systems. Gas turbines CHP systems operate under a homodynamic principle called the Brayton cycle. The design characteristics of a CHP gas turbine provide: 1) high electric and total system efficiency; 2) high temperature/quality thermal output for heating or for heat recovery steam power electric generation; 3) offer options for flexible fuels such as propane, natural gas, and landfill gas; 4) high reliability with 3-to-5 years before overhaul running 24/7; and 5) significantly lower GHG emissions.
Figure 1 Gas Turbine CHP System
Figure 1 demonstrates the mechanics and variables of a CHP system. In summary, the CHP technology enables the supply of efficient heat and power while minimizing GHG emissions. Total CHP efficiency is defined as the sum of net power produced plus the thermal output used for heating divided by total fuel input.
The use of methane (natural gas) as the main fuel for the CHP system offers advantages because methane offers the highest hydrogen-to-carbon ratio among fossil fuels, thereby, combusting with the lowest GHG emissions. According to EPA data, the emissions NOx particulates from gas turbines ranges between 0.17-to-0.25 lbs/MWH with no post-combustion emissions control versus 1.0-to-4.2 lbs/MWH for coal fed boilers. The carbon content of natural gas is 34 lbs carbon/MMBtu in comparison to coal at 66 lbs of carbon/MMBtu.
There are two valuable metrics used to measure efficiency for CHP systems. One is the total system efficiency which measures the overall efficiency of the CHP system including heat and electric and the other is the effective electric efficiency which is useful in comparing the CHP electric production versus grid supplied power. These two metrics, the total system and effective electric efficiencies are important for evaluating CHP system. The following provides a guideline foe measuring these two efficiency metrics and can be found at EPA – Efficiency Metrics for CHP Systems
Figure 2 CHP Efficiency
The economics of the CHP system depends on effective use of thermal energy n the exhaust gases. Exhaust gases are primarily applied for heating the facility and could also be applied to heat recovery steam generators (HRSG) to produce additional electric power. The total efficiency of the CHP system is directly proportional to the amount of energy recovered from the thermal exhaust. Another important concept related to CHP efficiency is the power-to-heat ratio. The power-to-heat ratio indicates the proportion of power (electrical or mechanical energy) to heat energy (steam or hot water) produced in the CHP system. The following provides an overview of the economics of a CHP system.
Figure 3 CHP Economics
Figure 3 illustrates the economics of a CHP system in comparison to competing energy sources. While the CHP does not have the low cost of coal in producing electric, the economic value of reducing GHG emissions is quite significant and beyond the scope of this article. However, natural gas prices remain below that of oil and better ways of capturing heat exhaust will further improve CHP efficiency. The bottom line is that natural gas produces less GHG emissions than coal or oil therefore; businesses should consider the benefits of CHP as a source of heat and power.