Block heat and power plants (BHPPs) are modular facilities or installations that generate both electrical energy and heat for local heat requirements. They may also feed heat for distribution into local heat networks. Their underlying principle is called cogeneration.
As prime movers for the generators, BHPPs may use internal combustion engines such as Diesel, plant oil and gas engines, but also gas turbines, stirling engines or even generate electricity directly with fuel cells.
Since BHPPs make use of the waste heat from the electricity generation, they achieve better capacity factors than the conventional combination of local heating and centralized power stations. Depending on size, BHPPs with combustion engines achieve an energy conversion efficiency of 25% to 44% of the fuel's total heat of combustion). BHPPs using fuel cells may achieve an electrical efficiency of 55% to 60%. If the waste heat is used locally and to the greatest possible extent, it is possible to reach 80 to 90% total efficiency relative to the primary energy content. While condensing boilers are almost 100% efficient, they do not generate any electricity. (Also note different efficiency definitions are used for condensing boilers.)
Common BHPP modules are available with electrical power outputs between one kilowatt (kW) and tens of megawatts (MW). Units below 50 kW are also called mini cogeneration plants, and installations up to 10 kW are known as micro cogeneration. Devices under 2.5 kW may be called nano BHPPs. Mini and micro BHPP are used to supply residential areas and business centers, hospitals, water parks and apartment buildings, but also small factories and housing developments. The nano class is even suitable for individual family homes. Cogeneration is also used by large central plants, typically with several hundred MW of electrical power.
Modes of operation
Heat controlled operation Heat controlled operation means that the power output of the BHPP is regulated to satisfy local heat requirements. In order to modulate the heat output, individual modules may be switched on and off, depending on demand. If there is only a single power module, either its power output is controlled with respect to the current demand or excess heat is used to intermittently charge a heat reservoir. The heat reservoir's size is often chosen such that the prime mover needs at least an hour to fully charge it. As the maintenance intervals are based on the power-on hours of the unit, intermittent operation is usually more efficient than modulated operation. As much as possible of the electricity generated by such plants is used locally while the excess is fed into the public energy grid against reimbursement.
Power controlled operation
In a power controlled BHPP, the power delivery is regulated according to the current demand for electricity or the unit's maximum capacity. Waste heat that is not required, is dissipated to the environment via emergency cooling systems, reducing efficiency. This mode of operation is frequently chosen for microgrids and islands. In Germany, certain BHPP running on renewable fuels are power controlled. Since minimum feed-in tariffs for renewables were legally mandated by the German Renewable Energy Sources Act or EEG (), the operators were able to maximize profit by maximizing electricity generation while disregarding excess heat generation. The latest changes of the EEG have attempted to curtail this kind of abuse.
Power oriented operation Under a power oriented regime, heat demand remains the main control parameter while satisfying the electricity demand is taken into account as much as possible. The BHPP is run when there is a demand for power, either using up the heat immediately or buffering it via thermal energy storage for later use. The BHPP does not have emergency cooling. When the power demand is low, heat requirements are mainly served from storage buffers, and the BHPP is added when heat storage runs low. This mode of operation maximizes the users' own use of generated electricity without efficiency losses and condensation operation.
Grid controlled operation Grid control means the power levels of several networked BHPPs are dictated by a central agency. This control center optimizes unit commitment across several decentralized cogeneration systems while taking into account economic factors such as common contracts for fuel supply and residual electricity feed-in. Grid control is the basic concept behind virtual power plants. As with power control, thermal energy storage is needed as a buffer to temporarily decouple heat generation and demand.
Layout The capacity of a heat controlled BHPP is chosen such that even under full load, it will only cover a part of the maximum heat energy requirements of the connected consumers while the rest is satisfied by a peak load boiler as needed. This ensures that the costly electricity generating plants are used more efficiently, running over longer time periods. They should ideally aim to be run for at least 7,900 hours per year but many only reach 3,000 to 5,000 annual hours of operation.
Using buffer storage, micro BHHP for residential buildings are operated monovalently, which means they are not augmented by a peak load boiler. Such a BHPP is not sized with respect to the basic heat energy requirements as described above but with respect to peak load like a conventional heating system. This kind of layout is put forth especially for mini BHPPs. Plants of this size are intermittently switched on and off (so-called "chopping"), shortening their life spans.
An existing heating system can usually be retained when converting to a mini BHPP with only minor changes. For one, there is the option of monovalent BHPP operation with increased heat storage capacity. If the heat is insufficient during the winter months, the existing boiler or a peak load boiler may be added (bivalent operation). If extra heat is rarely required, it can even be cost-efficient to add a simple electric heater basically working like an immersion heater. And even larger BHPPs can be optimized by using thermal energy storage, used to buffer heat demand peaks to avoid requesting extra heat generation outside of the cogeneration system (by a boiler). In addition, they allow temporary power controlled operation, meaning charging the heat storage during times of day when energy prices are peaking.
Ecology and environmental aspects The basic economy and the ecological concept of heat controlled operation is to use all of the generated heat while using as much of the electric power locally as possible. Surplus power is fed into the public electrical grid and reimbursed. Since this means that less conventional power station capacity is required for power generation, the increased use of BHPP replaces electricity generated in mid-load periods in fossil fuel "condensation" power plants (that only generate electrical power but no useful heat, mainly coal-fired power plants) and therefore helps to lower carbon dioxide emissions.
In total, BHPPs achieve much higher capacity factors (useful electrical energy plus useful thermal energy divided by energy input), compared to the regular mixed mode operation of local heating and centralized power grid. For example, a large modern steam–electric power station fired with bituminous coal is about 45% efficient. This means that more than half of its energy output is waste heat. Making use of this heat by way of district heating would incur significant transport losses (10–15%) while necessitating an elaborate and costly piping network since heat generation and heat consumers are usually separated over long distances for large central power plants. Transporting the electrical power through the grid incurs further losses of two to five per cent. BHPPs achieve an electrical efficiency between 25 and 38% (depending on type and size); the total efficiency of around 90% is only achievable when power and heat are used locally. The benefit of being able to use process heat locally relies on having suitable consumers. Residential buildings have seasonally different heat requirements which means the process heat can only be partially utilized during the summer months.
Maintenance expenditures for the engines (like oil changes and air cleaners, spark plugs for gas engines and such) and payments for service contractors worsen the overall economy since the specific outlay increases significantly for smaller and decentralised units, especially for micro BHPPs.
The use of palm oil as fuel is especially detrimental. Palm oil is cultivated on plantations in many developing countries and emerging markets at the cost of destroying rain forest, often even by way of illegal logging, despite the efforts of alliances such as the RSPO (Round Table of Sustainable Palm Oil). The remaining rain forests are being destroyed at increasing rates because of the rising demand for palm oil in fuels, food, cosmetics and such.
Emissions In contrast to conventional heating systems, BHPPs may not only generate exhaust emissions but also significant noise emissions and need to comply with applicable government regulations. Noxious gas emissions released by combustion include carbon monoxide, unburnt hydrocarbons and nitrogen oxide, for Diesel fuels also particulate matter ("soot") and sulfur dioxide. In the presence of sufficient oxygen, carbon monoxide and hydrocarbons can be converted to CO2 and are not critical below λ>1. However, enriching the mixture incurs the risk of unintentional methane release. The cylinder walls of combustion engines are much cooler than the mixture, which means that the combustive reaction stops in their vicinity, letting unburnt hydrocarbons escape. In lean-burn, the temperature of the mixture is lowered by the increased mass of the additional air, exacerbating this issue. If the exhaust is sufficiently hot (> 760 °C), the hydrocarbon fraction may be further reduced by thermal post-oxidation. In lean-burn operation, the exhaust is too cold to convert the unburnt hydrocarbons (or the methane in natural gas engines) via exhaust treatment, which means as a rule, any nitrogen oxide emission limits need to be met by the primary internal combustion itself, even though secondary exhaust treatment measures may help to comply with emission limits. The choice of exhaust treatment depends on the specific application and applicable regulations. Besides the budget and available space, the running costs of maintenance, repair and operation (upkeep) have to be taken into account. Finally, the total efficiency of the installation and the expected emissions are critical factors determining the suitability of any given BHPP design. Certain technical measures have proven effective since filtering out harmful fuel components from standard fuels is not economical.
Noise Noise emissions can be mitigated by structural engineering to minimize the transfer of vibrations, reducing them far enough to comply with applicable guidelines for residential areas.
Catalytic converters Catalytic converters transform NOx HC and CO in the exhaust gas at the same time, using precious metals, as long as the engine is run at a stoichiometric fuel rate (λ=1). Inside the converter, temperatures above 300 °C are needed for these reactions to occur and achieve conversion rates above 90%. For methane, especially high temperatures above 600 °C are required. A drawback is restricted engine operation which lowers the total efficiency of the installation.
Lean-burn Lean-burn engines are usually tuned to a lambda between 1.45 and 1.6. A potential increase of CO in the exhaust may be lowered again by adding an oxidation catalytic converter. Following