ORC-based cogeneration systems
SARK Sp. z o.o. is currently engaged in efforts to implement innovative ORC cogeneration systems, intended for widespread use. The main purpose of these activities is to present to a large group of future customers the new technology, which makes it possible to exploit the huge energy potential of the biomass of agricultural and forest origin, and of other energy commodities available on the national and European markets. We offer the solutions that can satisfy the heat and energy needs of individual consumers (e.g. owners of single-family houses or agricultural farms), small and medium-sized enterprises or institutions (e.g. schools, offices, hospitals, hotels, etc.). High efficiency production of useful forms of energy and, in consequence, short payback period for investments can be achieved by adapting the size and power of the cogeneration system to the user’s needs.
The proposed cogeneration systems operate on the basis of the organic Rankine cycle in which low-boiling mediums (i.e. mediums which have a lower boiling point compared to pure water) are used as working fluid. This allows for the combined generation of heat and power at significantly lower temperatures than in case of large power plants. These devices enable the construction of micro and mini CHP plants – by means of cheap and easily available materials – which are able to operate without continuous surveillance of qualified technical personnel. In cogeneration systems for low and medium power levels, it is possible to use energy resources available locally (such as biomass of agricultural and forest origin, biogas, municipal waste or production residues), without their transporting over long distances. Low boiling point of the working medium allows us to effectively make use of the natural resources, such as sunlight and geothermal heat, as energy sources in cogeneration systems. A very popular solution is to use an ORC system for production of electricity exploiting the waste heat originating from industrial processes or being the side effect of the operation of other energy systems (e.g. heat from a combustion engine operating at a biogas plant). Due to scientific support from the Institute of Fluid-Flow Machinery PAS, located in Gdańsk, SARK Sp. z o.o. is prepared to meet nearly any customer need, depending on his energy needs and available fuels or heat sources.
Another important feature of modern ORC cogeneration system is the possibility of adapting it to suit specific application requirements. This is done by selecting appropriate (low-boiling) working medium, size and type of heat exchangers, expansion device power output, control options or even housing type. SARK Sp. z o.o. is principally involved in the production of systems with thermal powers ranging from 20 kW to 5 MW, which are capable of providing electric powers in the range between 1 kW and 1 MW. In such systems, from 5% to 20% of thermal energy can be converted into electricity, depending on temperature range and size of installation. The proposed systems can operate either inside (e.g. as central heating / hot water systems, also generating electricity) or outside buildings as pre-assembled container solutions, which are compact and easy to transport (e.g. systems exploiting waste heat from a biogas plant).
High-speed vapour micro-turbines are the most frequently used as expansion devices in the proposed systems. Such micro-turbines offer many advantages (compared to commonly used volumetric expanders), including high efficiency, durability, reliability, low vibration and noise levels, small overall dimensions and simple structure (only one rotating component – rotor). High-speed micro-turbines are coupled with highly efficient electric generators, not entailing the use of mechanical transmissions. SARK Sp. z o.o. is involved, in cooperation with the Institute of Fluid-Flow Machinery, in research on the so-called oil-free vapour micro-turbines which do not require oil lubrication due to use of bearings lubricated with a low-boiling medium. This greatly simplifies design, reduces the number of potential operational failures and allows to reduce manufacturing costs. Oil-free vapour micro-turbines are currently one of the most innovative expansion devices used in ORC cogeneration systems.
Low power ORC cogeneration systems
The smallest ORC cogeneration systems with a thermal power of up to 100 kW are the modern approach to the need of environmentally friendly sources of heat and electricity for private households, multi-family dwellings, lodging houses, small hotels and agricultural farms. A boiler adapted to burn biomass in the form of pellets (or other renewable or non-renewable fuels) is used as the primary heat source in the proposed solutions. The smallest systems of this type have thermal power of 20 kW, which allows to completely cover the heating needs of an average single-family house. Heat generation takes place concurrently with the production of electricity using the micro-turbine supplied with low-boiling medium vapour. The electric energy, several kilowatts of power, obtained in this way can either be used by the operator for internal purposes (e.g. lighting, audio/video devices and household appliances) or fed into the national power grid, improving energy balance of the house and lowering its life-time costs.
The described systems meet the definition of a micro-installation, and, in accordance with the Polish Law on Renewable Energy Sources (dated 20th February 2015), can have electric and thermal power of up to 40 kW and 120 kW, respectively. Fulfilling this requirement simplifies greatly the formalities related to the investment process and subsequent operation. In this case, the commencement of business shall not be required for energy generation, and there are solutions envisaged to facilitate sale of energy and its delivery to a distribution network.
In our cogeneration systems, electricity is produced by means of high-speed turbogenerators, the bearings of which do not require oil lubrication (the so-called “oil-free technology”). Low operating temperatures enable some turbogenerator components to be made of modern plastics, which significantly reduces their manufacturing time and cost. Low power ORC cogeneration systems equipped with such advanced turbogenerators can be considered as extremely innovative solutions since at the moment similar installations cannot be found elsewhere, either on the domestic market or on global markets. Commercially available cogeneration systems for electricity generation are based on other technologies that have been in use for many years such as reciprocating engines, gas micro-turbines and Stirling engines. Table 1 shows a comparison between the currently applied cogeneration systems and the proposed technology – ORC system with a micro-turbine. Only the Stirling engines and ORC systems can run on all types of fuel, including biomass (even of low quality). However, the Stirling engines emit more vibrations/noise and are less durable as compared to ORC systems. These advantages make the ORC system coupled with a micro-turbine the most promising solution.
Table 1. Technical performance of various technologies used in small cogeneration systems
| Technology | Reciprocating engine | Stirling engine | Fuel cells | Gas micro-turbine | ORC vapour micro-turbine |
| Total efficiency [%] | 70-90 | ≈90 | 55-90 | 60-85 | 75-90 |
| Electrical efficiency [%] | 25-45 | 5-25 | 35-55 | 15-35 | 5-20 |
| Fuel type | gaseous, liquid | all types | gaseous | gaseous, liquid | all types |
| Main area of application | houses, industry | houses, industry | houses, industry, transport | houses, industry | houses, industry |
| State of progress | finished product | finished product | pilot installations | finished product | laboratory research |
A system coupled with a Stirling engine is currently the most commonly used cogeneration technology for single-family houses. On today’s market one can easily find the device based on the above-mentioned technology with a thermal power of up to 14.5 kW and an electric power of up to 1 kW, which is fuelled with natural gas. The primary function of the device is to heat utility water used in the building as well as the building itself. The electric energy generated constitutes an additional product. The table below (Table 2) presents the comparison between the device that has been briefly described above and the proposed ORC micro power plant using the high-speed micro-turbogenerator.
Table 2. The selected features of a cogeneration system with a Stirling engine or vapour micro-turbine
| System with a Stirling engine | System with an ORC micro-turbine | |
| Thermal power [kW] | 14,5 | 20 |
| Electric power [kW] | 1 | 1,5 |
| Total efficiency [%] | ≈ 90%* | 75-90% |
| Inspection and maintenance intervals [months] | 12 | 24 |
| Wearing parts | Występują | Brak |
| Type of bearings | Klasyczne | Gazowe |
| Noise level [dB] | <46 | <43 |
| Purchase price (including assembly) | 14 000 € | 7 000 € (30 000 PLN) |
The comparison shows that the ORC cogeneration system coupled with a vapour micro-turbine has numerous advantages. It can reach a total efficiency up to 90% under the most optimal operating conditions. Its main advantages include: no wearing parts and the consequent longer inspection/maintenance intervals, low vibration and noise levels resulting inter alia from the use of noncontact gas bearings. Owing to a relatively simple micro-turbine construction the purchase price of a complete cogeneration device may be lower than in the case of competitive solutions, providing great technical parameters and better performance.* according to the manufacturer’s specifications
Medium power ORC cogeneration systems
Operating costs and pay-back period of the investment are the most important factors influencing the choice of a particular technology. This is particularly evident in the energy sector, where investment decisions are based on cost-benefit analyses. Medium power ORC cogeneration systems, which can deliver electric output powers of up to 1 MW in a single unit, require less investment than low power ORC cogeneration systems (costs per kWh produced). The purchase price of currently available ORC systems with ratings from a dozen to several tens kWe, which are specially designed for low and medium temperature heat sources, ranges between EUR 4000 and EUR 8000 per kWe. Such systems are presently offered only by several companies from Western Europe and North America. Despite high prices, there are more buyers than there are ORC systems available, which results in longer duration of delivery (at least several months). SARK Sp. z o.o. experience has shown that the construction of the ORC cogeneration system with a micro-turbine, by means of domestic technologies and subcontractors, may be considerably cheaper than the solutions commonly sold in the world. The estimated purchase cost of the ready-for-use installation should not exceed EUR 3000 per kWe.
The growing popularity of ORC power stations results in large part from a wide range of application options since the ORC module which generates electricity may be supplied from different types of heat sources. The use of cheap and readily available biomass, waste heat or a geothermal source as an environmentally friendly fuel seems to be very attractive. Together with the rise of environmental awareness of our society and the need to meet regulatory requirements relating to energy efficiency, more and more attention is paid to recovering industrial waste heat. Modern ORC installations offer economically viable production of electric energy from heat sources in which the temperature is at a very low level (e.g. water at 85 °C). Under such conditions, the use of other energy technologies is technically impossible or not cost-effective.
In the case of medium power cogeneration systems, the highest electrical efficiencies, with a view to ensuring high durability and reliability of an expansion device, can be achieved using multi-stage vapour micro-turbines. Depending on effective rated output of the system, high- or low-speed micro-turbines may be applied. Such micro-turbines can be coupled with electric generators that can be either stand-alone (synchronous/asynchronous) or be incorporated as part of a larger unit (i.e. a micro-turbine and generator are mounted in the same casing). Each of these solutions has its own advantages and disadvantages, which have to be taken into account in order to meet technical requirements as well as different expectations and possibilities of potential clients. The best specialists employed by the Institute of Fluid-Flow Machinery PAS, are involved in further research and development of new ORC turbogenerators.