Transforming waste heat into renewable heat

Tr ansfor ming waste heat into r enewable heat

Making use of heat lost from gas-fired tri-generation systems improves efficiency and reduces emissions. A new technique adds to the results achieved from existing methods of waste heat recovery. By Imrich Discantiny

Traditional co-generation systems produce heat for space heating and hot water and generate electricity. Moving to tri-generation allows growing demand for air conditioning for homes, offices and commercial spaces such as server rooms and switchboards to be met simultaneously or on a seasonal basis.

Tri-generation, or combined cooling, heat and power, is the process by which some of the heat produced by a co-generation plant is used to generate chilled water for air conditioning or refrigeration. Usually an absorption chiller is linked to the plant to provide this functionality.

At COM-therm’s power plant in Komárno, Slovakia, which serves 30,000 people in 10,000 dwellings, schools, hospitals and administrative buildings, waste heat recovery efforts initially focused on flue gases and aerothermal energy inside the plant using heat pumps with highly positive

COM-therm’s power plant in Komárno, Slovakia serves 30,000 people in 10,000 dwellings, schools, hospitals and administrative buildings.

results. The new technique focuses on the absorption process.

Absorption cooling has three circuits that exchange heat. Firstly, there is a circuit of hot water connected to the co-generation unit or another source of heat. Then there is a cooling water circuit that removes the heat released by the absorption cycle within the chiller. This low-potential heat (typically 30°C) has so far been considered unusable and has generally been released in cooling towers. The third circuit delivers cold water to the consumer.

Co-generation units usually work with a temperature gradient of hot circulating water at 90/70°C which if used directly would limit the cooling capacity of the absorption chiller. The hot circulating water must therefore be re-heated to at least 100°C which results in additional energy consumption.

Technical solution

Intelligent tri-generation technology uses the wasted low-potential heat to boost the temperature of the return hot circulating water.

The cooling water circuit that carries off the heat from the absorption chiller, and whose temperature is lower than the return temperature of the hot circulating water, is connected to the heat pump inlet to allow the lowpotential heat to be transferred. The heat pump outlet is connected to the return line of the hot circulating water.

This has the additional benefit of stabilising the activity of the absorp

Heat Distribution Circuit

Exchanger

Photovoltaic Collector

Thermal Collector Exchanger

Cogeneration Unit

Inverter

Resistance Spiral

Storage Tank

c The Komárno power plant has been trialling intelligent tri-generation technology with good results

x The components and connections of an intelligent tri-generation system

Heat Pump

Measurement & Regulation

Absorpton Chiller

Exchanger

Gas Boiler Cool Distribution Circuit

Heat Distribution Circuit

tion chiller by providing for a permanent temperature optimised gradient in the cooling water circuit of 25/30°C without the need for a bypass valve.

The previously wasted low-potential heat is what is termed “renewable heat” and its use increases the overall efficiency of the system.

Further benefits can be obtained by incorporating a storage tank in the hot circulating water circuit. This allows the stabilisation of the temperature conditions during various fluctuations in the system. The storage tank can also be used to store heat generated from other sources such as thermal solar panels or heat transformed from electric power generated by photovoltaic panels.

The precise design of the new system will vary according to the location and type of installation with the ratio of power generation, heat and cooling adapted to the needs of the consumer.

In the tropics, greater cooling output will be required and therefore a co-generation unit will be deployed with the ability to supply the required power; the heat supply boost required for the hot circulating water entering the absorption chiller can be covered by thermal solar panels in combi-nation with photovoltaic collectors generating electricity to heat the water in the storage tank. In mild climates, the system will typically include a gas boiler as a heat source.

The autonomy of intelligent trigeneration technology also allows it to be used in areas with underdeveloped electricity distribution networks as the photovoltaic cell system converts solar radiation to direct current electricity and can operate in an offgrid mode, galvanically isolated from the public grid.

Benefits and applications

Intelligent tri-generation technology:  Enables the provision of simultaneous heating/cooling from several heat sources (e.g. co-generation unit, gas boiler, solar thermal collectors, photovoltaic panels, water storage tank);  Ensures absorption cooling unit performance optimisation by achieving the optimal gradient of 25/30°C;  Stabilises temperature conditions during various fluctuations in the operation of the entire heating and cooling system;  Produces an effective increase in the efficiency of the heat pumps;  Optimises stabilisation of fluctuations of CHP power units and connected boiler room components;  Eliminates/reduces the need for cooling towers;  In conjunction with the first phase of waste heat recovery from flue gases, facilitates the installation of CO 2 separation technologies in central heating and power plants; and  Reduces gas consumption by at least 20%.

This latter benefit is reflected in the 2019 operating results for COMtherm’s Komárno plant which produced 51,100 MWhr of heat of which 10,700 MWhr (20.9%) was renewable heat. The average coefficient of performance (COP) of the heat pump section was 3.95. Additionally the plant produced 10,000 MWhr of power and 8,000 MWhr of cooling.

Apart from central heating and power plants, the technology has a wide range of applications such as waste water treatment plants, industrial facilities with constant heat/cold consumption, sports centres, schools, hospitals, hotels and office buildings.

Conclusion

Intelligent tri-generation technology efficiently uses all sources of lowpotential heat generated by power, heat and cold production, which have so far been considered unusable under industrial conditions and vented to the atmosphere by cooling towers. The technology for using low-potential heat can also be used in other industrial processes.

Transforming waste heat into renewable heat helps address global environmental protection and climate change issues.

Imrich Discantiny is a Senior Consultant with IGU Associate Member COM-therm (www.comtherm.sk) and a member of IGU’s R&D and Innovation Committee.