RecoORC is an EU research project on combined heat and power based on waste heat as a source.
In industrial processes there is a constant need for more efficiency. Energy is becoming more precious and costly, thus enhancements in efficiency are necessary competitive factors.
In the manufacturing sectors, such as steal, chemical and process industries high energy inputs are necessary. High temperature processes often cause huge amounts of low and medium temperature range waste heat. In many cases these exhausts are released to the ambient unused. The Organic-Rankine-Cycle is one outstanding concept to make use of those heat sources.
Within the last years the ORC technology has come a long way from experimental prototypes to reliable standard applications. Mainly in the sector of biomass conversion OR-Cycles took place in the past. In Germany, Austria, Switzerland and France cogeneration on ORC basis are widely spread.
In the field of industrial waste heat varying demand and supply make it rather complicated to apply the available standard solutions for biomass and geothermal ORCs. To find suitable technical solutions for a large number or different cases, this project will evaluate available and upcoming concepts. Bringing together interested users, industrial partners and the suitable technology in Belgium and Germany is the very aim of the project.
The ENOVA-Study estimates that 6% of primary energy can be saved by the potential of waste heat in Germany in a range of 60°C to 140°C. Main contributors are the steel, glass and ceramic sectors. In Norway the ratio of wasted heat in various industry sectors has been determined by a survey in 105 companies. The results show that cement industry and iron industry have a waste to secondary energy ratio of 40%, respectively 30%. Sweden covers 11% of its district heating demand by waste heat [BMWi, 2007]. For Belgium the potential of waste heat can be estimated in the same order of magnitude.
ORC
The Organic-Rankine-Cycle is a derivative of the Clausius-Rankine process. The process, as most generating processes, consists of two different pressure and temperature levels. Energy is transferred into the pressurized cycle to achieve overheated steam. The energy in this steam is expanded to a certain lower pressure and temperature level. The lower temperature level is created by cooling the so called sink. Various methods of sink cooling provide different temperature levels.
The higher the steam temperature and the lower the sink temperature the more energy can be generated by an expanding machine.
Low temperature (<250°C) cannot be used in an economic way to generate electricity utilizing steam. For such applications fluids that evaporate at lower temperatures than water have to be used. A large number of organic matters fit the requirement to put vapour processes into practice at any level between 50°C and 400°C.
Appropriate design regarding the choice of the fluid and the single components is mandatory.
ORC applications can be distinguished into temperature and dimension categories.
- Source 50°C-150°C
- Source 150°C to 200°C
- Source 200°C-400°C
- 0-25kVA Micro ORC
- 25-150kVA Mini-ORC
- 150-400kVA Medium
- 400-2500kVA Large
Possible heat sources:
- fossil
- biomass
- solar thermal
- geothermal
Possible waste heat sources:
- Residual heat from melting, casting, sintering(steel-, glass-, ceramic production)
- Friction heat from hydraulic systems (e.g. test rigs)
- Chemical processes
- Residual heat from other generating processes, as steam cycles CHP-engines
- Exhaust heat from combustion processes
- Heat from cooling applications. Condenser heat from thermal cooling or compression chillers.
Cycle fluids
The choice of the cycle fluid mainly governs the design and the operational behaviour of the system. The thermal source temperature and the desired sink temperature are the first and major design parameters. As organic cycle fluid a large number of compounds can be chosen according to the desired properties of the cycle.
- Thermodynamic properties: Molar mass, density, boiling pressure, evaporation enthalpy, heat capacity, viscosity
- Chemical aspects: Toxicity, climate effect, Ozone depletion effect, Irritant
- Legal aspects
The following groups of chemicals can be used for organic cycles:
- Alcohols, alkenes, alkanes
- Ammonia
- Refrigerants (fluorocarbons, chlorofluorocarbons)
- Siloxanes
Most organic fluids have an isentropic or positive slope of the saturated vapour line. This provides the possibility of complete expansion in the turbine within one stage or step without reheating or turbine bleeding. Thus ORC turbines can be constructed simpler and more economic. This so called “dry expansion” avoids impingement at the turbine blades and leads to long turbine life-cycle.
Expander types
To expand steam between two levels of pressure and generate mechanical work with, various concepts can be used. Rotary and reciprocating engines can be applied.
- Reaction turbine
- Piston engine, steam engine
- Scroll expander
- Screw-type expander (single, double)
- Roots expander
The expanders are attached to an alternator. Various configurations are possible: Synchronous, asynchronous, with or without gearbox. High-speed or low-speed.
Cycle configurations
There are different ways to design an OR-Cycle. Depending on the engine size, temperature levels and working fluid layout and complexity can be adjusted. All cycles have the same basic components: heat source with a certain heat carrier (water, thermal oil, exhaust gases) and a cooling cycle containing coolant (water, alcohol, brine etc.).
Simple Cycle
In its most simple form a cycle can be designed with two heat exchangers, a feeding pump and an expander. One heat exchanger is used as boiler one as condenser. A configuration like this would be preferable for a small and simple system at comparably low source temperatures. To avoid the usage of a recuperator a cycle fluid with a strong positive curvature can be used. In this case the outlet of the expander is very near to the saturation line and only a low amount of heat can be recovered from it until condensation.
Cycle with recuperation
The use of a recuperator can in many cases increase the efficiency of the cycle. An appropriate design of the recuperator assures that the condenser does not need to cool down large amounts of vapour. Size and design can therefore be optimized for condensation. By using a recuperator a minimum amount of heat is transferred to the sink and a maximum is kept in the cycle.
