While utilizing heat generated through the combustion of mostly wood for cooking purposes they had innovated and developed ways to make maximum use of the heat being generated & in simplest manners by designing ‘Chullahas’ having minimum two and maximum four interconnected places to use heat to its maximum capacity. The demand of different energy forms being generated and used increased with time. Presently, heat energy is produced by using many means other than wood like petrol, diesel, coal and many other solid, liquid and gaseous fuels. Heat energy is also used to generate electricity, the life line of present day civilization.
Waste heat is by necessity produced both by machines that do work and in other processes that use energy, for example in a refrigerator warming the room air or a combustion engine releasing heat into the environment. The need for many systems to reject heat as a by-product of their operation is fundamental to the laws of thermodynamics. Coal-fired power station transforms chemical energy into 36%-48% electricity and remaining 52%-64% to waste heat. Industrial processes, such as oil refining, steel making or glass making are major sources of waste heat. Investigations indicate that about 50% of all fuel burned by industrial sources becomes waste heat, mostly low-grade. The thermal conditions of the industrial waste heat are industry dependent. Although small in terms of power, the disposal of waste heat from microchips and other electronic components represents a significant engineering challenge. Animals, including humans, create heat as a result of metabolism. In warm conditions, this heat exceeds a level required for homeostasis in warm-blooded animals, and is disposed of by various thermoregulation methods such as sweating and panting. Thus, every year, billions of rupees of energy are thrown away as waste heat.
In all forms of heat utilization, the wastage of heat is inevitable and also in very large quantities and its tapping was not given any thought before. It is estimated that 60% of that energy is wasted as heat. But with the growing demand of energy, depleting natural resources of fuels & with alarming environmental pollution, people have started to think about the tapping of waste heat from almost all sources for its optimum use. Technologies are being developed to make use of waste heat from machines, industrial units, and automobiles to human body. The concept of replacing existing energy devices with more energy efficient and economically competitive energy devices is still lacking. Efforts are being directed to develop energy conversion devices which can make use of waste heat and covert it (waste heat) into useful form of energy. In this article, new developments taking place to tap waste heat from various sources is discussed.
Thermoelectricity
Thermoelectric devices, which enable the conversion of heat into electricity, are still at an early stage in the energy innovation chain, but the principle behind how they work can help to highlight a crucial aspect of energy waste across the world that is often ignored in the policy realm. No doubt, emerging thermoelectric technology could give energy efficiency a whole new meaning by tackling the huge energy waste that happens before the watts even reach our homes.
The ability of thermoelectric materials to accomplish direct conversion between thermal and electrical energy in compact, durable, solid-state devices without moving parts and pollution free makes them an attractive technology for waste heat recovery applications. Presently, thermoelectricity is thought as a very attractive option to tap waste heat available at various sources in abundance and convert it into useful power without any further environmental effects. Thermoelectricity (based on Seeback effect) is the use of a system of suitable materials for converting heat into electricity. Basically, a thermoelectric device works just like an engine (based on Seeback’s effect): it converts, using a thermocouple (a combination of two dissimilar metals) whose junctions are maintained at a certain temperature difference, into an electric potential to generate power. For a given temperature output, a very efficient system will generate a high voltage, while a low-efficiency device will only create a modest voltage. In order to achieve real energy savings, the world would need thermoelectric devices with very high efficiency. Good thermoelectric materials should have high thermo power, high electric conductivity, and low thermal conductivity. Recently, there have been significant advances in direct thermal-to-electrical energy conversion materials and this has generated increased interest in the field.
Growing application
Scientists and engineers are working on efficient thermoelectric materials to design thermoelectric devices to harvest waste heat and turning it into electrical power. Taking advantage of nanotechnology and quantum effects, the technology holds great promise for heat recovery and thus making cars, power plants, factories and solar panels more energy efficient. In addition to scavenging waste heat, thermoelectric devices will improve efficiency of fuel consumption and limit the use and exhaust of greenhouse gases. At present, this efficiency remains discouragingly low: even state-of-the-art thermoelectric devices can only convert 10% of the energy from a waste heat source at 500 degrees. This is why thermoelectrics have only met success for limited ‘niche’ applications. Fortunately, this efficiency is only limited by the basic laws of thermodynamics, and there is considerable room for progress. New advances in nano-structured materials, resonant modes, insulating materials and other state-of-the-art physics could help boost thermoelectrics’ contribution to energy efficiency throughout the world. Thermoelectric is not a market-ready technology and is still at an early stage in the laboratory because the science behind the devices is so complex. Therefore, to reach market, thermoelectric will have to overcome a number of technological and policy related barriers. If the next generation of thermoelectric materials can be manufactured inexpensively, they could be used in more demanding applications. Some the applications are as:
- Cars and other light vehicles produce a great deal of waste heat in the engine’s exhaust and coolant. Automotive industries are hoping to increase fuel efficiency and eventually replace alternators and possibly even internal combustion engines with thermoelectric generators.
- Heavy equipment powered by diesel engines (such as tractors, earth movers, trucks) has medium exhaust temperatures and can make waste heat recovery via thermoelectric generation more efficient.
- Stationary power generation represents an enormous technical market for thermoelectric generators.
- Roughly a third of the energy consumed by industry is discharged as waste heat to the atmosphere or to cooling systems. These discharges are the result of process inefficiencies and the inability of manufacturing plants to utilize the excess energy. Industrial waste heat energy is considered to be an economic opportunity for waste heat recovery.
- The discovery of the unique properties of a new material for thermoelectrics may make it possible to recycle heat from computers and cars as electric power.
- With the cost effective thermoelectric devices, scientists think thin-film thermoelectric technology could eventually lead to slap-on thermoelectric patches (using body heat) that produce enough power to run battery-powered pacemakers, cochlear implants, brain stimulators, cellphone or iPod from our body temperature itself.
Latest developments
- Researchers get increasingly good at manipulating materials at the nanoscale and reported a material called tin selenide with the record ZT of 2.6.
- Recently discovered thermoelectric materials and associated manufacturing techniques (nanostructures, thin-film super lattice, quantum wells) have been characterized with higher thermal to electric energy conversion efficiencies.
- Mismatched alloys are a good match for the future development of high performance thermoelectric devices.
- Enhancement in thermoelectric performance can be achieved by reducing thermal conductivity through nanostructuring and structural modification of materials.
- Molecular thermoelectric device making use of quantum laws of physics holds great promise. Such devices could help to solve an issue currently plaguing photovoltaic cells harvesting energy from sunlight more efficiently.
- The thermoelectric devices in a flexible configuration can exploit small temperature differences occurring naturally in the environment of the application like ground-to-air, water-to-air, or skin-to-air interfaces.
- An international team of researchers from the US, India & Australia has demonstrated thermo-electrochemical cells (thermo cells) in practical configurations (from coin cells to cells that can be wrapped around exhaust pipes), that harvest low-grade thermal energy (temperature below 130OC), using relatively inexpensive carbon multi-walled nano-tube (MWNT) electrodes.
