At the 26th Conference of the Parties (CoP) that took place in 2021, India committed to reducing the energy intensity of its economy by 33-35 per cent in 2030 compared the 2005 levels. Cogeneration and trigeneration are technologies that can help improve energy efficiency and reduce the energy intensity of the economy as they raise the capacity utilisation of the plant by 30-35 per cent. These two technologies harness the thermodynamic heat loss that occurs during the operation of a thermal power plant for applications such as heat, ventilation and air-conditioning (HVAC). These technologies retransmit the residual heat to households and commercial businesses requiring heat such as steel units, brick kilns and smelting plants. This process raises the overall efficiency of the power plant up to 80-85 per cent, while providing an alternative source of revenue for the generating company.
The simultaneous generation process on-site minimises energy losses as compared to the separate generation and transmission of electricity and heat (and/or cold). Cogen reaches aggregate efficiencies as high as 80-95 per cent compared to the efficiency of separate generation processes of about 50 per cent, providing efficiency gains of more than 30 per cent and thus primary energy savings of more than 50 per cent.
Combined heat and power technology
During the electricity generation process, thermal power plants emit large quantities of waste heat, which remains largely unused. This waste heat from central power plants can be used to meet nearby heating or cooling demands through the district heating or cooling network. The plants using the heat for other industrial activities such as captive requirements or external grid demand are called cogeneration plants operating on the topping cycle principle. The steel manufacturing plants, smelting plants, brick kilns, etc. have captive capacity of topping type, and require heat at various stages of production.
Apart from these, there are cogeneration plants that use the excessive heat for further power generation. These plants operate on the bottoming cycle principle, also known as waste heat recovery. They use alternative fuels such as natural gas, fuel oil or coal biofuels to generate more electricity than conventional power plants.
The gas turbine compresses air and mixes it with fuel to generate electricity. In addition, a heat recovery generator (applying the principle of the waste heat recovery cycle) captures exhaust heat from the gas turbine and delivers it to the steam turbine for generating additional electricity.
The cogeneration and trigeneration plants are also more environmental friendly as they release smaller quantity of emissions into the atmosphere. Moreover, these plants use less fuel to generate more electricity. The topping technologies also enable the plant to harness an additional revenue source as the excess heat/cold can be sold by the generating entities to commercial and individual consumers.
Cogeneration in India
In India, the sugar industry has the maximum cogeneration plants. These plants use bagasse from sugarcane to generate and consume electricity internally, and utilise the excessive heat for heating purposes. Thus, the excessive heat helps the plant to generate more electricity, which is then either utilised or sold to the grid. As of October 2021, there are around 800 biomass and bagasse/non-bagasse based cogeneration plants with an aggregate capacity of 10,557 MW. The leading states with substantial bagasse-based cogeneration capacity are Maharashtra, Tamil Nadu, Uttar Pradesh and Andhra Pradesh. The leading states for biomass power projects are Chhattisgarh, Madhya Pradesh, Gujarat, Rajasthan and Tamil Nadu.
In recent years, several power plants in Haryana, Uttar Pradesh and Punjab have been exploring the utilisation of biomass pellets and briquettes in coal-based thermal power plants as an auxiliary source of fuel to reduce coal usage. A percentage of biomass-based fuel can be accommodated in coal plants without much modification to the technology. Furthermore, adopting this method will help reduce crop burning by farmers, a practice that contributes to significant emissions.
As per the MNRE, India has the potential to generate around 18,000 MW of renewable energy using biomass (including both biomass-based co-generation and otherwise). In addition, there is further potential for developing 7,000-8,000 MW of capacity from bagasse cogeneration.
Trigeneration in India
Trigeneration, or combined cooling, heat and power, is the simultaneous production of electricity, heat and cooling – all from a single source. The heat and electricity are produced by the regular operation of a CHP unit, while an absorption chiller is used for producing chilled water from the heat output. The overall efficiency of a trigeneration system can reach up to 85 per cent, or even higher, depending on system conditions.
The trigeneration market has been estimated at about 20,000 MW in India, with the bulk of the demand expected to be driven primarily by commercial real estate, airports, industries and service establishments. The initial phase of trigeneration adoption might be limited to cities where there is a stable gas supply network. Smart cities could also find this attractive. As of now, trigeneration technologies are exclusively deployed in highly dense establishments with a district cooling system (DCS) designed specifically for this purpose. An example of successful DCS systems in India is DLF cyber city (Gurgaon) with a capacity of 78,000 tonnes of refrigeration. For remote and rural areas, where gas supply has not yet been established, work is underway to explore the feasibility of LNG transportation to set up trigeneration in those areas as well.
According to the India Cooling Action Plan, 57 per cent of the nationwide cooling requirement will originate from buildings for space cooling, followed by transport air conditioning for perishables items, among other things, with a consumption requirement of 23 per cent. Refrigeration is expected to account for 20 per cent of the air conditioning requirement. ICAP estimates the Indian cooling requirement to grow by eight times by 2037-38 compared over 2017-18. Thus, the implementation and deployment of trigeneration technology needs to be incentivised by the government in order to scale up and meet the rapidly rising demand.
It is estimated that this technology has been successful in helping improve operational efficiencies by up to 75 per cent while reducing energy costs by 30-40 per cent. Furthermore, it helps in reducing carbon dioxide emissions by 35-55 per cent and reduces water consumption by 10-20 per cent. Notably, industrial units, commercial units and households meeting their cooling requirements from generation units with 30 per cent cooling output are able to reduce their consumption of refrigerant, a pollutant, by 60-75 per cent.
Programmes for cogeneration and trigeneration
The two main schemes introduced for promoting the installation of off-grid and decentralised biogas plants are the New National Biogas and Organic Manure Programme and the Biogas-based Power Generation (Off-grid) and Thermal Energy Applications Programme.
The government also notified a scheme to promote biomass-based cogeneration in sugar mills and other industries. The scheme was notified in May 2018 and was valid till March 2021, with consideration for extension. Under the scheme, central financial assistance of Rs 2.5 million per MW of surplus exportable capacity for bagasse cogeneration projects and Rs 5 million per MW of installed capacity for non-bagasse cogeneration projects was provided to plants utilising biomass, such as bagasse, agro-based industrial residue, crop residue, wood produced through energy plantations, weeds and wood waste produced in industrial operations.
According to the revised biomass policy introduced by the Ministry of Power (MoP) in October 2021, all fluidised bed and pulverised coal units, that is, coal-based TPPs are required to use a 5-10 per cent blend of biomass pellets made of agricultural residue after assessing the technical feasibility and safety aspect. The MoP also mandates that this policy of co-firing biomass in TPPs will be effective for 25 years or until the useful life of the TPP, whichever is earlier, although the percentage of co-firing will be reviewed from time to time. The MoP requires coal-based TPPs to procure biomass for a contract duration of seven years or more in order to encourage the development of a robust biomass supply chain. Notably, this policy allows the discoms to meet their renewable power purchase obligation through purchase of electricity generated from biomass pellets.
In a similar vein, Energy Efficiency Services Limited (EESL) is offering trigeneration technology to industries and institutions in India through its unique pay-as-you-save business model for an integrated service offering. This includes equipment maintenance, and electricity, heat and power supply. While clients will not bear any upfront costs for technology installation, EESL will recover the capital and operating costs for the installations through demonstrated energy savings. In June 2021, EESL signed an MoU with Lemon Tree Hotels Limited for implementing energy efficiency and energy conservation measures at the properties of Lemon Tree Hotels. These energy efficiency measures will include deploying trigeneration technology at its properties as well.
The way forward
Net, net, the deployment of cogeneration and trigeneration technologies needs to be scaled up in the coming years through sustained investment by both private entities and the government. These technologies reduce emissions and fossil fuel usage, increase production and efficiency, and improve the quality of power generated. Hence, it is imperative for the government, private enterprises and financial institutions to collaborate on developing a coherent policy framework for cogeneration and trigeneration as they have multidimensional benefits for the power sector.