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Combined heat and power

A CHP unit generates electricity and heat at the same time. The advantage does not come from the individual product but from the coupling: because the engine waste heat is used instead of being lost, the overall utilisation is considerably higher than with separate generation. Economically, however, this becomes an advantage only if both products are needed simultaneously – which is why sizing a CHP unit is essentially a question of operating hours.

Design and technology

A CHP module consists of a combustion engine as the drive, a generator for electricity production and heat exchangers extracting heat from cooling water, oil circuit and exhaust gas. Depending on the design, catalyst, gas control line, exhaust heat exchanger and acoustic enclosure are added. Available sizes range from a few kilowatts of electrical output for residential buildings up to the megawatt range for industry and district heating; fuels include natural gas, biogas, fuel oil, vegetable oil and pellets.

Characteristic of a module are the thermal and electrical rated output, whose ratio is called the power-to-heat ratio, the associated efficiencies and the lower modulation limit. Below this limit a module does not modulate further but switches off – a major reason for cycling when oversized. For heat extraction the return temperature is decisive: it should be well below the engine flow temperature so that the heat can be transferred completely.

Sizing

A CHP unit is deliberately sized smaller than the building heat load – often at ten to thirty per cent of it, thereby delivering roughly half to two thirds of the annual heat depending on the load profile. The target is 5,000 to 7,000 full-load hours per year; below about 4,000 hours operation is usually uneconomical. The remaining demand and the load peaks are covered by a peak-load boiler which at the same time provides redundancy. A buffer tank – as a rule of thumb 60 to 100 litres per kilowatt of thermal CHP output – extends runtimes, prevents cycling and allows electricity and heat demand to be decoupled in time.

Operating strategies

In heat-led operation the module follows the heat demand and surplus electricity is fed into the grid. This is the standard case and gives the highest runtimes provided heat demand is steady. In electricity-led operation the module follows the electricity demand and the resulting heat goes into the buffer, which can be economically advantageous with high on-site electricity demand and good storage capacity. Operation without grid feed-in limits generation so that no electricity is exported – sensible where feed-in is contractually or technically undesirable.

Modelling in EPOS-Plan

CHP modules are selected from a catalogue, filtered by fuel and thermal capacity class. Thermal and electrical output, overall efficiency and modulation limit are stored; in addition you define the buffer volume and the operating mode. The annual simulation balances hour by hour which share of the heat or electricity demand the module covers, how the store fills and when it switches off. Results are operating hours per module and on average, heat and power production, any heat surplus, fuel consumption by energy carrier and the emissions of CO₂, SO₂, NO_x, CO and dust.

The hydraulic integration also influences the result; the common variants with buffer, low loss header, series and parallel connection are described with principle diagrams under Fundamentals.