By Pieter de Bod, Pr.Eng, LEED AP BD+C, CDCP, Green Star SA AP
A tri-generation system with carbon sequestration is defined as a ‘quad-generation system’, which is the production of electricity, heat and cooling from a fossil fuel like natural gas with the addition of recovery of carbon dioxide from engine exhaust to minimise harmful carbon emissions to the atmosphere. Tri-generation and quad-generation plants are truly marvels of ingenuity, engineering and forward-thinking design.
- Learn about the functionality and implementation of a tri-generation or quad-generation system
- Understand how absorption chillers play a role in tri-generation and quad-generation systems
- Recognise the benefits of carbon capture technology
Building owners or industrial businesses are increasingly seeking ways to use energy more efficiently as a direct result of some or all of the following factors:
- Dramatically increasing electric or energy rates
- Decreased power reliability (blackouts, brownouts, rolling blackouts, and other power interruptions) from the current power providers
- Competitive and economic pressures to cut operational expenses
- Reducing carbon emissions of air pollutants and greenhouse gases
Various technologies exist for clients that are looking to use energy more effectively and to decarbonise or achieve a net zero energy building (NZEB). A tri-generation or quad-generation system is a great alternative solution to their existing or new facility. This article will provide more information about the different types of tri-generation plants and look at quad generation and absorption chillers.
Tri-generation or CCHP
A tri-generation plant is a plant that comprises mainly of mechanical and electrical equipment to simultaneously generate electricity, hot water or steam and chilled water in a very efficient manner and from a single source of energy. The main energy input to a tri-generation plant is typically natural gas or biogas. The high efficiency of a tri-generation plant is a result of efficiently recovering engine heat produced by the power generator and using that heat to generate hot water or steam for buildings or industrial processes, or to generate chilled water or ice making for ice storage. This technology is also called a Combined Cooling, Heating and Power Plant or CCHP.
Methane rich natural gas, a fuel with a high methane content of between 70%-90%, allows for a cleaner burning process (in comparison to diesel fuel) and this allows harvesting of the heat with minimal clogging of the heat recovery devices. Harvesting heat from diesel fuelled power generators is hampered by the increased accumulation of soot on the exhaust heat recovery devices. Even with the best technology available, most diesel generator sets typically only convert about 33% of the energy stored in the fuel into electricity.[1] In other words, for every three barrels of diesel combusted, about only one barrel is converted to electricity and the remaining two barrels are discarded as waste heat in different forms. The phenomenon occurs due to the incomplete combustion of the fuel in the engine and other losses as friction.
Co-generation
Co-generation, also known as combined heat and power (CHP), is the simultaneous production of only electricity and heat but lacks the cooling generation component. CHP systems are implemented where cooling is not required. Cogeneration systems can also be equipped with carbon capture technology. Cogeneration plants are less efficient than tri-generation or quad-generation plants and require lower capital expenditure.
Figure 1 – Tri-generation. All figures supplied by Pieter de Bod
Figure 2 – Co-generation.
Hydrogen tri-generation
A hydrogen tri-generation plant is capable of generating three components: electricity, heat and hydrogen where natural gas or renewable biogas is converted inside a fuel cell into hydrogen, which then electrochemically reacts with air to generate power and heat.[2] Heat generated during the fuel cell’s process can be recycled and used for various purposes. Hydrogen produced from the fuel cell plant can be used to fuel zero-emission passenger vehicles. The system runs on natural gas or directly on renewable biogas to produce clean power. Tri-generation may be eligible for the State of California Low Carbon Fuel Standard and may be considered carbon-negative by the California Air Resources Board (CARB). Renewable biogas can be used as the fuel source, generating power and heat for the wastewater treatment process while producing green hydrogen for transportation.
Hydrogen tri-generation has the potential to create significant value for manufacturers that use heat in their production process. The fuel cell system’s high operating temperature improves the efficiency of power generation and provides usable thermal waste heat. Heat can be used for metal processing, glass manufacturing, petrochemical, material handling, and more. Fuel Cell Energy is demonstrating tri-generation at our North American manufacturing facility. Ultra-clean electricity powers the manufacturing process, heat is used throughout the facility and the hydrogen is used in the process ovens. Hydrogen produced on-site replaces the need to purchase hydrogen and transport it to the facility.
Figure 3 – Hydrogen tri-generation.
Quad-generation
Quad-generation plants incorporate the same technology as a tri-generation plan but include a scrubber in the exhaust system to capture the carbon dioxide (CO2) which is a by-product of a combustion process. The captured carbon dioxide can be either stored deep underground in geological formations or used in industrial sectors where there is a demand for high grade carbon dioxide like the food and beverage industry or to promote fruit and vegetable growth in horticultural greenhouses.
Figure 4 – Quad-generation.
Carbon capture, storage and sequestration
Carbon Capture, Utilisation and Storage (CCUS) technologies are an important solution for the decarbonisation of the global energy system as it proceeds down the path to net zero emissions. The rationale for carbon capture and storage is to enable the use of fossil fuels while reducing the emissions of CO2 into the atmosphere and thereby slowing down global climate change.
Technology that captures carbon dioxide from our atmosphere has existed for decades and is now being considered as a key method for fighting climate change. Carbon capture and storage (CCS) technology is a form of carbon sequestration that is set to play a central role in helping us reach net zero by 2050.
It is important to note that carbon is captured at point of emission in a quad-generation plant and transported to carbon storage sites and stored safely within the natural environment. One example of a storage method is to inject the scrubbed CO2 deep into saline aquifers. Saline aquifers are natural underground geological formations consisting of vast expanses of porous sedimentary rock filled with salt water. The injected CO2 can be stored permanently, away from absorption into the atmosphere and therefore decarbonisation. Saline aquifers have the largest identified storage potential among all other forms of engineered CCS.
The ‘Endurance’ aquifer, located in the North Sea off the coast of the UK, is one such formation, which sits approximately 1.6km below the seabed. Roughly the size of Manhattan Island and the height of The Shard building in London or the Empire State Building in New York, its porous composition allows for carbon dioxide to be injected into it and stored safely for potentially thousands of years.
In the US, multiple large-scale saline aquifers are now being used for CCS purposes, such as the Citronelle Project in Alabama. During its three-year trial period, it was successful in storing more than 150 000 tonnes of CO2 per year, which was captured from a nearby pilot facility.[3]
Absorption chillers
Cooling to buildings or industrial processes is commonly provided by chillers that generate chilled water. Chilled water is circulated in insulated pipes to air-handling unit or fan coil unit equipment to keep the building cool in warm climate. Chillers are typically electricity-driven compression type. In tri-generation or quad-generation systems, cooling is provided by absorption chiller(s), which uses the recovered heat from the generator. The fact that the heat input to the absorption chiller is harvested makes a tri-generation or quad-generation system very efficient.
In principle absorption chillers operate on ‘free’ recovered thermal energy from natural gas-powered generators, thermal energy that would otherwise be wasted to the atmosphere. As long as the power generator is operational and there is a cooling demand, chilled water will be produced by the absorption chiller, independent of utility power.
Absorption chillers use Lithium Bromide (a type of salty solution) and distilled water as refrigerants, both with an Ozone Depletion Potential (ODP) of zero and a Global Warming Potential (GWP) of zero. By controlling the temperatures and concentrations of lithium bromide, it is possible to control the leaving chilled water temperature. Hot water from the generator will be pumped to the absorption chillers by means of a pump(s), situated in the generator plant room.
Absorption chillers have been commercially available for more than half a century and are now regarded as matured technology, and they gained acceptance due to their capability of not only integrating with tri-generation systems but also because they can operate with other industrial waste heat streams that can be fairly substantial.
Absorption chiller technology represents an optimal solution for a year-round efficient source of cooling and heat, especially when used in conjunction with a gas engine cogeneration plant.
Heat output
Heat from the generator engine is recovered using multiple types of heat exchangers in various arrangements. The hot exhaust gas recovered from the generator engine can be used as an energy source for hot water or steam generation for an industrial process, or building heating system, or can be utilised as an energy source for a highly efficient, double-effect steam chiller. If steam is not required, hot water will drive the absorption chiller. The amount of steam, hot water or chilled water can be carefully modulated by facility management or control systems depending on the season (cool or warm) and industrial process (cooling or heating demands).
The ultimate heat recovery design depends on the client’s specific power, heating and cooling needs. A cost benefit analysis will be done by the MEP consultant to determine the best technical and optimal solution for the client. Some clients have a high steam or heating demand, while other clients may have a higher cooling demand or even require ice storage.
Implementation
Tri-generation and quad-generation plants are a reasonably good solution for clients that require constant cooling, heating and power at their facility and have sufficient natural gas available but don’t have sufficient utility power. For clients that are looking to decarbonise or achieve a net zero energy building (NZEB), a tri-generation or quad-generation system is a great alternative to their existing or new facility.
A Net Zero Energy Building is a method of design and construction that aims to achieve an energy efficient, grid-connected building, enabled to generate energy from a zero-carbon emission or renewable energy sources to compensate for its own energy demand. As a result, these types of buildings boast a net zero energy consumption such that the total energy used by the building on an annualised basis is roughly equal to the amount of zero carbon emission or renewable energy created on the site or at a nearby location. Facility owners and developers have demonstrated awareness in developing net zero energy buildings to meet corporate goals and regulatory mandates.
Quad-generation plants are uniquely positioned in the industry in that they can provide reliable clean power around the clock – unlike solar or wind power plants which depend on the natural environment to generate power.
A key factor determining the optimal tri-generation or quad-generation solution is to determine the hourly base power, cooling and heating load demands/requirements in a building or industrial process.
Good examples of buildings with high power, cooling or heating base load demands include (but are not limited to) the following:
- Hospitals
- Universities
- Manufacturing plants
- Data centres
- Large hotel resorts
- Large international airports
As mentioned above, a cost benefit analysis will be done by the MEP consultant to determine the best technical and optimal solution for a client.
Table 1: The Carbon Dioxide Emission Coefficients in USA are as follows[4]:
| Pounds CO2 Per Million Btu | Kilograms CO2 Per Million Btu | |
| Natural Gas | 116.65 | 52.91 |
| Coal | ||
| – Anthracite | 228.60 | 103.69 |
| – Bituminous | 205.57 | 93.24 |
| – Subbituminous | 214.13 | 97.13 |
| – Lignite | 216.40 | 98.16 |
| – Coke | 250.59 | 113.67 |
| Diesel and Home Heating Fuel (Distillate Fuel Oil) | 163.45 | 74.14 |
Table 2: Typical thermal efficiencies for power generation options[5]:
| Type of Fossil Fuel/Atomic Energy Facility | Thermal Efficiency |
| Natural Gas—Peak Power Turbine | 25% |
| Nuclear Power[6], [7] | 30%-33% |
| Coal-Steam Cycle | 38%-45% |
| Nuclear—Pebble Bed Modular Reactor[8] | 45% |
| Coal—Gasification Combined Cycle | 48% |
| Natural Gas—Combined Cycle | 50%-56% |
| Future (2010)—Natural Gas and Coal Combined Cycles | 60% |
| Future (2015)—Fuel Cell Combined Cycles | 70% |
When electrical power is generated, the energy in the fuel ends up as electrical power or as waste heat. When the fuel is used to power a heat engine, there are boundaries that determine the maximum quantity of the fuel energy that can be converted to electricity. A large amount of waste heat is given off at the power generating plant by the power cycle that drives the generator. The thermal efficiency of the power generation is defined as the electrical energy produced divided by the total energy released by the fuel consumed. The excess heat of the power cycle is rejected to cooling water systems as well as the exhaust gases that exit from the power plant stacks. Typically cooling towers or dry coolers reject the heat to the atmosphere, but there are other means to reject the heat like distributing the hot water to residential, commercial or industrial use.
Table 2 lists thermal efficiencies for power generation. Using natural gas for peaking power is about half as efficient as the combined cycle option. Trigeneration systems can have overall efficiencies of around 80%. As per Energy Efficiency Services Limited (EESL), tri-generation can reduce the end user’s primary energy demand by 60-70%, increase overall energy efficiency by almost 75%, and cut greenhouse gas emissions by up to 30%. The trigeneration system can provide 300 tonnes of refrigeration for every MW of power it generates, saving up to 195kW of electricity, and eliminating the need for investments in centralised cooling equipment and hot water boilers. Further, by creating a parallel source of electricity through captive generation, trigeneration can protect consumers against surging tariffs.[9]
The integration of the tri-generation plant with the site infra-structure to has to be carefully considered when sizing the system to ensure that the most efficient solution is achieved. By correctly sizing the plant, it will not only increase the efficiency of the building, but it will also be the most cost effective to maintain and provide years of reliable service.
System utilisation is of utmost importance; therefore, any excess hot water (water not used by the absorption chillers) will be used for space heating (or other heating needs). Hot water will be supplied to Fan Coil Units by means of piping and plate heat exchangers.
We believe that tri-generation and quad-generation technology is here to stay, and we will see more and more similar plants erected in the near future.
[1] Rehman et al., 2007
[2] https://www.fuelcellenergy.com
[3] https://www.nationalgrid.com/stories/energy-explained/carbon-capture-technology-and-how-it-works
[4] https://www.eia.gov/environment/emissions/co2_vol_mass.php
[5] Sustainable Nuclear Power, 1st Edition – December 8, 2006, Galen Suppes, Truman Storvick, eBook ISBN: 9780080466453
[6] http://www.uic.com.au/nip57.htm reports 33% thermal efficiency for nuclear power as reasonable assumption and high energy output as compared to energy input.
[7] http://www.nucleartourist.com/world/koeberg.htm confirms 33% efficiency after improvements
[8] http://www.worldandi.com/public/2001/April/nuclear.html.
[9] https://www.iiec.org/sdc/SDC_Tri-generation_factsheet.pdf
