Combined heat and power generation

Author: Dr. Klaus Hassmann, Cluster Energietechnik

Combined heat and power is used in large power plants as well as decentralized in the medium and small power range. Especially smaller CHP plants can be well adapted to the needs of the operator. This article addresses, among other things, circuit variants with different fuels, technical development potential, the effects of the soon to be expected regulation on the reduction of nitrogen oxides and cites selected quotes from politics and business on CHP. 

Many options for customized applications

• Customer groups (selection): housing industry, municipal facilities, EVU / municipal utilities, commerce and industry, hotels, retirement and nursing homes, hospitals, swimming pools.
• Fuels: natural gas, liquefied petroleum gas, heating oil, solar thermal energy, geothermal energy, biomass (e.g. waste wood), biogas, sewage gas, mine gas, also coal; the coal-fired power plants are to be either shut down or converted to another fuel in the short or medium term. There are still 139 coal-fired CHP plants in operation; their conversion is proceeding slowly despite a 0.6 cts/kWh subsidy for converted plants; 4 million t CO2 are to be saved by this measure. On the delays, just one example: Hamburg; the city wants to phase out coal by 2025. The district heating network is operated jointly with a large EVU with coal interests; a compromise has not yet been found.
• Type of application: Grid feed-in power/heat, power at all grid voltage levels, captive power/heat, ventilation, heating, air conditioning, refrigeration, process heat, compressed air, steam are some of the important options.

Variety of CHP applications *

• District heating network 5 km. Supplied u a 100 public buildings including 2 hotels and private customers. The heat requirement approx 8 million, electricity requirement 3 million kWh cover 3 BHKW with in sum 530 kW, a pellet boiler plant 300 kW, 2 natural gas peak boilers (3 and 1 MW), a gas heat pump (105 kW).
• Future-oriented treatment of sewage sludge: alternative to spreading the waste for fertilization on the fields. On the site are a biogas plant for dewatering and drying of sewage sludge; digester gas from the fermentation and biogas feed a CHP. Biogas is sufficient for operation and for drying the digestate and sewage sludge and also supplies 1800 households with electricity and the buildings on site.
• Deponiegas BHKW 600 kWel and 573 kWth. Admixture of natural gas up to 100% possible. Weak gases up to 15 vol% methane can be used for energy. The mixing section allows also liquefied gas. The plant runs in base load 8600 h/a. El. efficiency in landfill gas operation 42.5%, with natural gas 43.2%.
• BHKW 20 MWth and 92 MWel is to reduce CO2 by 20% per manufactured vehicle at the site of an automobile manufacturer - a contribution to the traffic turnaround.
• A gas-powered internal combustion engine can cover the annual compressed air demand for manufacturing processes of 1.3 million m3. Thanks to compressed air and heat 60% cheaper (45% less CO2) than el operated compressors. Low payback time
• Combining CHP with geothermal energy: For a 50 MW plant currently under construction, six boreholes were drilled at a depth of 2800 to 3100 meters. It is scheduled to go into operation in 2020. In the building of the CHP plant is a district cooling system with ice storage u a also for the supply of Grossmarkthallen be installed.
• Combination CHP with solar thermal, heat pumps and feed into a low-temperature heat network; heating center 120 kW, central WP 540 kWth. Peak load boiler 440 kW and PV 90 kW complete the system. To balance generation and demand, 2 stratified storage tanks (cold 15 m3 and warm storage 30 m3) are installed.
• BHKW for innovative ice heating - ice storage for heat and cold utilization. Thermal storage with water tank underground filled with 1 million liters of water. El. brine/water heat pump extracts energy from the water in winter to heat a laboratory - the water freezes. In summer, the ice cools the lab air via a pipe system in the floor. Storage tanks can release heat and cold in a time-shifted manner. Energy demand for a building was reduced by 59% compared to conventional heating and air conditioning technology.
• Developments with potential to bring CHP further forward
• Fuel cell: 2 technologies, Polymer Electrolyte Membrane (PEM) and Solid Oxide Fuel Cell (SOFC); technology is mature but still too expensive; 5 manufacturers offer heaters with electrical/thermal output of 0.2 - 1.5 kWel/0.61 - 1.0 kWth. The reactions of manufacturers are very different; some are waiting, others will continue to serve the niche market, only occasionally manufacturers will start mass production; they expect high volumes from 2022.
• Micro gas turbine: the technology is mature. A container generation is available in power sizes between 50 kW and 1 MWel to 1.4 MW thermal with 3, 4 or 5 turbines at almost the same efficiency even in the partial load range.

Further developments

• New limit values for medium-sized combustion plants will soon have to be complied with, for natural gas e.g. 100 mg/Nm3; this means a reduction by a factor of 3; new plants require SCR - catalysts; old plants must be retrofitted with them. Manufacturers are striving to build this component as effectively and compactly as possible with the smallest possible volume. The annual additional costs are estimated at 20 to 30 T€ for a 1 MW engine. An additional cost factor will be the verification of the emission values - the cheating with diesel sends its regards. Whether there will be a financial incentive for this important environmental protection measure?
• Preventive maintenance measures on engines are intended to increase the revision cycles or to detect defects early, which can lead to failure of the engine. Lubricating oil and cooling water analyses should contribute to this; samples are taken, for example, every 1000 operating hours, sent in and analyzed.
• Modern gas engines are converted from aluminum to steel pistons to increase the cylinder pressure. This means higher temperatures. The operation requires new, adapted oil mixtures; they should provide sufficient engine cooling.

Results of the 1st tender CHP 12/2017 and specifications of the 2nd tender CHP. Tender CHP 6/2018

There were 16 bids for new CHP plants, and four bids for upgrading old plants. The Federal Network Agency has published a list of the plant locations/companies of the seven successful bids on the Internet. Five bids with plants in the 1 to 10 MW capacity class were awarded, as well as 2 plants with a capacity of about 30 MW each. Three of the smaller awards were for modernizations. The average award value of just over 4 cts/kWh was well below the max bid value of 7 cts/kWh in the tender. In comparison: The average award value of the first PV tender was approx. 9, that for onshore wind approx. 6 cts/kWh. In the course of the further rounds, the surcharge value for PV has decreased to approx. 5 and for wind to 3.8 cts/kWh. It would be too early to draw conclusions from this 1st tender for CHP; systems designed to meet customer needs can be very complex and thus significantly more expensive than a simple CHP circuit. Additional environmental requirements also drive prices up.

Quotes to think about

• CHP with flexible operation make an important contribution in the future energy system sector coupling, security of supply and climate protection -  in a digital environment and after conversion to renewable fuels CHP is a transitional system with a very long life.
• Thinking in terms of opportunities and not risks is a prerequisite for the development of new business areas. The heat market offers opportunities galore.
• CHP is an indispensable bridge technology for the heat transition. As soon as the heat turnaround is given a high priority, CHP will increase significantly in market share.
• With proper CO2 taxation, CHP would have so many advantages that one can save the debate about subsidy rates and volumes.
• The time is ripe for a technological leap in German boiler rooms.

  The author's aim was to briefly present selected technical circuit options; manufacturers, operators and location are not mentioned; the trade journal "energie&management" served as the source of information.