Way of Sustainability
One of the important features of the CCHP is to provide cooling by utilising the rejected heat instead electricity. This solution is realised by the thermally activated technology, which is dominated by the sorption cooling...
Dr. Neeraj Agrawal
The population growth and technological advancement exhibited in the last two decades along with the desire for higher life standards and comfort levels have led to an unprecedented increase in the energy consumption worldwide. Energy consumption has increased from 7140.7 Million tons of oil equivalent (Mtoe) in 1980 to about 12875.6 Mtoe in 2010. Asia and Oceania have the largest share in the energy consumed in 2010 with about 37.9% followed by North America with 23.1% and Europe with 16.4%. India’s electricity consumption stands at 587 billion kWh in 2006, out of which currently 8% is being used by the commercial sector and 25% in the residential segment. 30% of energy consumed in the commercial sector in India is on account of HVACR. Energy demand is increasing and more than 25% of total electricity consumption is in residential/commercial sector put together. Projected annual increase in energy demand is 5.4 billion kWh in residential and commercial sector in India.
The total world energy consumption exhibits that the oil still the dominant resource with 33.1% of the global energy consumed followed by coal (29.9%) and natural gas (24%). Renewable energy resources contribution to the over all world energy consumption pattern is still less than 9% with 6.6% of hydro-electric power and less than 2% for all other renewables combined. This heavy reliance on conventional fossil fuels has led to an increase in the global energy-related CO² emissions by 1.4% to reach 31.6 Gigatonnes in 2012 with a historic peak exceeding 400 ppm in the atmosphere in May 2013. In addition, energy uses in buildings, mainly electric power, heating and cooling/refrigeration, contribute to about 20 – 40% of the over all energy consumption with similar contribution to carbon dioxide emissions. The majority of these buildings depend on large central stations or plants to provide their electricity demands employing oil, natural gas or coal as fuel resources.
However, the operation of these central stations is usually character -ised by high rates of energy losses mainly in the form of waste heat. With additional losses in the electric power transmission through high voltage lines and in the transformers, only 35 – 45% of the over all energy produced by these stations is delivered to the final user. Thus, the high investment cost and high incremental risks of these stations along with their high energy production environmental foot print and complex design favour the switch to more efficient and compact decentralised energy production systems and facilities.
One of the well known and age-old technology, Combined Heat And Power generation systems (CHP) better known as cogeneration, allows the simultaneous generation of heat and power in a single energy process. It is estimated that the installation of 1 million micro-CHP units, with size range of 1–10 kWe, in the UK residential sector would allow an annual cost reduction of about £176 million on the energy production and the mitigation of 2.1 million tons of CO².
Although Combined Heat And Power (CHP), as a proven and reliable technology, mainly used in large scale centralised power plants and industrial applications, provides various technical, economic and environmental advantages compared to the separate production of heat and power in conventional Separation Production (SP) systems, such systems efficiency and capability decrease dramatically in hot climates especially in the summer months where the need for heating is minimal. Thus, a balanced and continuous heat and electricity demand profile all over the year is required to attain high cogeneration system over all efficiency. However, the case is very different in real climatic conditions where many regions exhibit a summer season with an increasing demand for cooling and air conditioning due to larger thermal loads, higher life - standards, new buildings design and architectural characteristics and the desire for high levels of thermal comfort. Combining heat and power system (CHP) with a thermally activated cooling technology by harnessing the discharged waste heat from power generation systems to fulfill heating and cooling needs with power generation known as the Combined Cooling, Heating And Power (CCHP) tri-generation system, an effective way to improve the overall efficiency and reduce the negative environmental impacts Green House Gas (GHG) emissions.
The concept of integrating various units to form a combined heating, cooling and power generation system was first introduced in the early 1980s for municipal cooling and heating. A typical tri-generation system comprises a prime mover, electricity generator, thermally activated technologies, heat recovery unit and a management and control unit. Over the last three decades, tri- generation systems have attracted considerable interest, especially small-scale systems (below 1 MWe), with the development of different options and alternatives for thermally driven cooling technologies and cogeneration units. Potential tri-generation users are small and medium-scale applications ranging from less than 1 kW to more than 10 MW including multi-residential dwellings and communities, office buildings, hotels, hospitals, commercial and shopping malls, universities, restaurants and food industry. Compared to the conventional separate way of energy production (heat by boilers and electric power by central stations) and conventional cogeneration units, tri-generation systems enhance the over all energy production efficiency with various technical, environmental and socioeconomic benefits on different levels.
Employing CCHP, overall fuel energy utilisation increases dramatically ranging from 70% to more than 90% compared with 30 to 45% of typical centralised power plants. In general, less primary energy is needed to obtain the same amount of electricity and thermal energy. In addition to the saving in primary energy, vast reduction in net fuel costs, transmission and distribution savings achieved. Based on a typical CCHP system, only 100 units of prime energy are needed for 33 units of electricity power, 40 units of cooling power and 15 units of heating power in summer day. The electricity generation efficiency of CCHP system is similar to centralised power plant, because electricity is consumed locally without loss on distribution lines, though small scale prime mover is less efficient than large prime mover in power plant. The keystone of full energy utilisation of CCHP system lies on the recovery of waste heat from prime mover. Further, CCHP systems increase the reliability of the energy supply network. Weather and terrorism are fatal threats to centralised power plants. A smaller more flexible and dispersed system, CCHP might prevent these threats from becoming reality, and controlled repercussions and fast recovery could be achieved if these situations occurred.
One of the important components of the CCHP is the prime movers technology. The technology can be divided into two categories, combustion based technologies such as Stirling engine, gas engine, Rankine cycle and reciprocating engine and electrochemical based technology fuel cells. Among these technologies, Stirling engine, Organic Rankine Cycle (ORC) and fuel cell driven technologies are relatively at development stage.
Reciprocating engines of the capacity in the range of 100 – 5000 kW are commonly employed in CCHP. Waste heat can be recovered at different levels, from exhaust gases at 200-400°C and from jacket water cooling and oil cooling at 90-125°C. Generally, reciprocating micro CHP systems have a total efficiency of about 80%. In one of the ICE integrated tri-generation with biomass gasification where gas produced by the gasification is used in engine to produce electricity and waste heat is recovered to provide heating needs and cooling power through an absorption cooling unit. Gas turbines can also be an attractive prime mover with different sizes and configurations for CCHP. Compared to ICE based systems, gas turbines are more compact and require less maintenance with hot gases released at a temperature of 250°C that could easily drive thermally activated cooling technologies. A tri-generation system using gas turbine with 100 MW power, 70 MW heat and 9 MW cooling capacity was developed. However, micro turbine applications in the residential and building sector are still very limited due to their low electrical efficiency and inflexibility to load profile changes.
One of the important features of the CCHP is to provide cooling by utilizing the rejected heat instead electricity. This solution is realized by the thermally activated technology, which is dominated by the sorption cooling. The difference between the sorption cooling and the conventional refrigeration is that the former one uses the absorption and adsorption processes to generate thermal compression rather than the mechanical compression using the rejected heat from the prime mover along with the electricity generating. This cascade utilisation of heat owes to the thermally activated technology. By introducing thermally activated technologies, the electric load for cooling is shifted to the thermal load, which can be fully or partially achieved by absorbing or adsorbing the discard heat from the prime mover. The main application of the sorption refrigeration is for CCHP systems in residential buildings, hospitals, supermarkets, office buildings and district cooling systems.
Mainly three types of the thermally activated technologies exist ie., absorption chiller, adsorption chiller and desiccant dehumidifier. Since the temperature of the discard heat from prime movers can lie in different ranges, thermally activated facilities should be chosen to couple with prime movers. For example, if the heat source temperature is around 540°C, then the suitable choice is a double-effect/ triple-effect absorption chiller based on the number of times the heat is utilised with in the absorption system.
An economical, efficient and of low emissions CCHP system should be designed with fully consideration of energy
demands in a specific area, prime mover and other facilities' types and capacities, power flow and operation strategy, and the level of GHG emissions. The selection of facility types belongs to the design of the system configuration, which emphasizes on the selection of prime movers according to current available technologies, and on the system scale. The existing CHP/CCHP sites in the market sorted by prime movers, 42% reciprocating engine, 23% steam turbine, 12% combustion turbine, 7% combined cycle and 16% others. With a selected CCHP system configuration, operation strategy is the key to achieve the most efficient way for the CCHP to operate. The operation strategy determines how much electricity or fuel should be input to the system according to the demands; which facility should be shut down to keep the whole system efficient; how the energy carries flow between facilities; and how much is the power one facility should operate at. With a designated configuration and an appropriate operation strategy suitable sizing and optimisation can make the system operate in an optimal way.
Finally it can be concluded that with the dramatic increase in the world primary energy consumption and the corresponding green house gas emissions, combined cooling, heating and power generation presents a promising technology providing multiple energy products accompanied with highly efficient energy production, green- house gas emissions reduction, higher energy supply reliability and lower operational and maintenance costs.
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