1 Introduction

1.1 The Origins of CHP

Conventional power generation entails large central plants connected to a grid supplying power to the end customer. While such systems offer central control, they suffer from significant thermal inefficiencies as well as line transmission losses. The end result is that the customer pays for these inefficiencies not only in the cost of power but also in power outages, power sags and surges, varying frequency, etc. The distribution system, which must be maintained and extended to meet expansion, is critical. In today’s context, it’s tough to expand the grid in real terms, not only from a practical perspective but also impossible in most metropolitan areas due to building density and the “Not In My Back Yard” principle [1].

Combined heat and power (CHP) systems (also known as cogeneration) generate electricity and thermal energy in a single, integrated system. In a traditional power plant that delivers electricity to consumers, about 30% of the heat content of the primary heat energy source reaches the consumer. However, the efficiency can be 20% for outdated plants and 45% for newer gas plants. In contrast, a CHP system converts 15–42% of the primary heat to electricity, and most of the remaining heat is captured for hot water or space heating. In total, as much as 90% of the heat from the primary energy source goes to useful purposes when heat production does not exceed the demand. Cogeneration works in parallel with the utility grid reducing the strain. Critical power quality needs can act as a filter to bring genuine, high-quality power that meets the client’s needs.

CHP systems have benefited the industrial sector since the energy crisis of the 1970s. For three decades, these larger CHP systems were more economically justifiable than micro-CHP due to the economy of scale. Since 2000, micro-CHP has become cost-effective in many global markets due to rising energy costs. In some cases (depending on local and state regulations), a cogeneration plant can be designed to generate excess power for sale onto the grid, creating an additional revenue source for the client. The development of micro-CHP systems has also been facilitated by recent technological advances in small heat engines. This includes improved performance and cost-effectiveness of {Stirling, steam, diesel, Otto} engines and gas turbines. The main difference between micro-CHP systems and their larger-scale kin is operating parameter-driven operation.

Industrial CHP systems generate electricity in many cases, and heat is a valuable by-product. In contrast, micro-CHP systems, which operate in homes or small commercial buildings, are driven by heat demand, delivering electricity as the by-product. Because of this operating model and the fluctuating electrical market of the structures in which they would tend to operate, homes and small commercial buildings, micro-CHP systems will often generate more electricity than is instantly being demanded.

1.2 Basic Principles

CHP is essentially a high-efficiency technology for generating power, usually in the form of electricity and heat on-site from a single fuel source [2]. What do we mean by that? Essentially applying a fuel source to a reciprocating gas engine, there is a rotational moment of that engine that drives into a generator to produce electricity that can be used on-site or exported to a local grid as a by-product of the electrical generation. The engine also generates heat from a number of sources. That heat can be utilised mainly as power in several ways, whether for space heating, process heating, cleaning, sterilisation activities, or a wide variety of applications.

Figure 1. Illustration of Gas Turbine Combined-Cycle generation. Source: https://commons.wikimedia.org/wiki/File:Gas_Turbine_Combined_Cycle_Generation_01.svg 

Primary heat sources from the unit: heat has picked up from the lubricating oil circuit, the engine jacket cooling circuit, and the intercooler, sometimes referred to as the turbocharger aftercooler circuit. Those are known as the low-grade heat available from the units; you also have a higher grade of heat from the engine exhaust, which could be directly linked to that hot water system or used to raise steam or directly fire exhausted [3].

So, looking at various applications and how one might integrate several engines into a system, we could look at integrating all the hot water into a single circuit that may be used to deliver heat to a heat consumer. We might also use our peaking boilers on site as part of the CHP installation such that the highest levels of peak demand might be satisfied from a natural gas-fired or an alternative fuel boiler plant. We may also want to include within the bounds of the scheme might be a heat vessel or heat buffer; that may be beneficial where the economic balance for operating units, i.e. it might be more economically viable to generate electricity during daytime hours. Yet, heat recovery may not be as pertinent during those times, so incorporating a thermal store into the circuit enables us to deploy that heat as and when required.

Taking that a step further, we might consider integrating a technology called absorbing chilling. An absorption chiller uses some form of cooling medium, often lithium bromide, and hot water off the engine (e.g. 90°C) to generate chilled water in the region of 6-12°C. This might be particularly beneficial in hotter countries where air conditioning of factory plants is required; also in applications such as data centres with a higher cooling load demand.

1.3 Renewables and CHP

CHP is essentially a high-efficiency technology for generating power, usually in the form of electricity and heat on-site from a single fuel source. Renewable micro-CHP systems, on the other hand, are a combination of micro-CHP technology and renewable energy technology, such as biomass gasification systems or solar concentrators. Integrating renewable energy sources with micro-CHP allows for the development of sustainable energy systems with the potential for high market penetration.

These technologies potentially have much to offer in helping us achieve our objectives of tackling climate change, ensuring cost-effective and reliable heat and electricity supply and tackling fuel poverty. As well as providing low-carbon energy to homes and small commercial buildings, micro-generation can offer the same service to community buildings, such as leisure centres and schools. On such premises, not only does the micro-generation installation help to reduce carbon emissions, but it can also help to inform and educate communities about energy and, hopefully, persuade people to minimise their own carbon footprint.

In recent years, with the rapid development of renewable energy sources, CHP plants are encouraged to enhance operational flexibility to reduce the renewables curtailment, including extending the load and reducing the start‐up time [4]. The rapid expansion of intermittent renewable energy makes the balance between electricity production and consumption in the grid hard to keep. Moreover, a better quick‐response ability is also required to continuously follow the changing electric power demand.

When accommodating intermittent renewable energy with a district heating scheme (DH), heat supply will fluctuate following the power output variation of renewable energy instead of daily periodic variation following heat demand. Thus, unsteady‐state energy analysis on the DH network is required to quantify the effect on heat users. Some action projects, such as Europe’s MICROCHEAP [5], intended to bring together industry specialists and research experts to focus entirely on renewable micro-CHP technology, coordinate and steer research in this field, and highlight the most promising technologies with the highest potential for market penetration in existing and future market conditions.

The following sections discuss the state-of-the-art technological options in the field of decentralised micro-CHP with biofuels regarding technology, cost, and environmental impacts and present a market survey concerning the possibility of future technology penetration in various regions.

Figure 2. Comparison of the efficiency of energy generation process using a conventional centralised method and decentralised fuel cell-CHP system. Source: https://doi.org/10.1016/j.rser.2014.10.080

2 State-of-the-Art Report

2.1 Micro-CHP and Biofuels

Renewable micro-CHP systems are a mixture of micro-CHP and renewable energy technology, such as solar concentrators or biomass gasification systems. Combining clean energy sources with micro-CHP allows for the expansion of sustainable energy systems, a high market penetration potential, reliable and cost-effective heat and electricity supply, and a highly beneficial environmental and economic impact on a regional scale.

The operating theory of the micro-turbine is similar to the gas turbine, except that most designs incorporate a recuperator to recover part of the exhaust heat for preheating the combustion air and increase the electric efficiency. Air is drawn through a compressor section, mixed with fuel, and ignited to power the turbine section and the generator. Hot exhaust gas from the turbine section is available for CHP applications (hot water heating or low-pressure steam applications). The high-frequency power generated is converted to a grid compatible 50 Hz through power conditioning electronics. Their compact and lightweight design makes micro-turbines attractive for many light commercial/industrial applications.

Most manufacturers are pursuing a single-shaft design in the 25–250 kW range, where the compressor, turbine, and permanent-magnet generator are mounted on a single shaft supported on lubrication-free air bearings. These turbines operate at speeds of up to 120,000 rpm and can run on various fuels, such as natural gas, gasoline, diesel, alcohol, biofuels, etc. With recuperation, efficiency is currently in the 25–30% LHV range, or even higher for new designs. Table 1 presents the general specifications of market micro-turbine cogeneration systems. Installed prices of $800–1200/kW for CHP applications are estimated when micro-turbines are mass-produced, while availability is similar to other competing distributed resource technologies, i.e., in the 90–95% range.

Table. General specifications of some market micro-turbine CHP systems. Source: https://doi.org/10.1016/j.rser.2004.07.005

Micro-turbines have substantially fewer moving parts than engines. The single shaft design with air bearings will not require lubricating oil or water, so maintenance costs should be below conventional gas turbines. Micro-turbines that use lubricating oil should not require frequent changes since the oil is isolated from combustion products. Only an annual scheduled maintenance interval is planned for micro-turbines.

Maintenance costs are estimated at 0.006–0.01$/kW [6]. Micro-turbines also promise lower noise levels and can be located adjacent to occupied areas. Micro-turbines have the potential to produce low emissions. They are designed to achieve low emissions at full load; however, emissions are higher when operating under reduced load.

Emission characteristics of microturbine systems based on manufacturers’ guaranteed levels. Today’s micro-turbines have greater efficiency and lower emissions of greenhouse gases than internal combustion engines. Low emission combustion systems are being demonstrated that provide emissions performance comparable to larger CHP turbines. The primary pollutants from using micro-turbine systems are NOx, CO, CO2, unburnt hydrocarbons, and a negligible amount of SO2. Without any post-combustion treatment, NOx emissions are targeted below 9 ppm using lean pre-mix technology.

2.2 Agro-residues

Biomass energy potential is promising among renewable energy sources due to its spread and availability worldwide. Apart from that, biomass has the unique advantage of being able to provide solid, liquid and gaseous fuels that can be stored, transported and utilised far away from the point of origin. Due to the negligible amounts of sulphur and nitrogen contents, biomass contains energy that, if adequately used, contributes less to environmental pollution.

Figure 3. Maise crop residues retained on the soil surface in northern Mexico. Source: https://www.flickr.com/photos/cimmyt/4688665449

Agricultural biomass utilisation is an issue of significant importance due to the considerably intensive regional agricultural activities performed worldwide and the critical amounts of produced residues. Crops by-products, fruit cores, rice husk, cotton gin waste, straws etc., provide a promising energy source in warm countries. Electricity production by Solid Oxide Fuel Cells (SOFCs) is one route to obtaining high power efficiency. Incorporating a SOFC in the conventional gasification turbine process can substantially upgrade the efficiency of biomass in energy conversion [7].

Biomass resources, especially agro residues, are distributed; thus, decentralised biomass conversion would avoid the extensive cost of biomass transportation. Traditional decentralised CHP plants suffer from low net electrical efficiencies compared to central power stations. Improving the electrical power yield of small-scale CHP plants based on biomass will improve the competitiveness of decentralised CHP production from biomass and reinforce rural development.

2.3 Processes involved in the integrated system with SOFC

2.3.1 Gasification

Gasification of biomass is a well-known technology, and according to the gasifying agent, the gasification process may be classified as air, steam, steam-oxygen, air–steam, O2-enriched air, etc. Gasification converts the solid material into a gaseous fuel as the main product and ash and non-reacted char as the remaining products. The quality of the feedstock, process parameters, and the gasifying agent may affect the gas composition. Air-blown gasifiers produce gas, with the major combustible components being CO, H2 and CH4. The gasification gas composition depends on the type of gasification process, gasification agent and the gasification temperature. The primary application of product gas will be its direct use in power and heat generation.

2.3.2 Tar Control

Tar control can be achieved either during the gasification process using appropriate primary catalysts or in the downdraft reforming stage. Catalytic reforming can be performed either with catalytic bed materials during or after gasification using a separate downstream catalytic reactor. Catalytic bed materials promote char gasification, water–gas-shift and steam reforming reactions and reduce the tar yield. With the addition of catalytic material in the reactor’s bed, there is no need for complex downstream cleaning. Natural minerals such as dolomite, limestone, olivine and iron ores, and synthetic minerals such as Ni-supported olivine, Fe-supported olivine, alkali metal-based material, and char can be added to the bed of the gasifier and catalyses tars. Carbon deposition decreases with the increase of steam/carbon ratio or current density.

2.3.3 Pyrolisis

Pyrolysis of biomass is a CO2-neutral process and can transform waste into energy and materials. Pyrolysis gas products can also be utilised for energy purposes. Different gas compositions can be produced depending on the type and conditions of the pyrolysis process. Biomass pyrolysis is generally a source of medium-level fuel, which can be directly used in engines, turbines and boilers for power production. Catalytic pyrolysis is considered the most attractive way of increasing bio-oil yields and reducing the problems of liquid pyrolysis products (polymerisation, high viscosity, and corrosivity) for better handling and treatment. Catalytic pyrolysis is a promising thermochemical conversion route for the lignocellulosic biomass that produces chemicals and fuels compatible with those issued from petrochemistry. The development of new catalysts helps the production of liquids with lower water content and higher heating value, rich in hydrocarbons, light phenols and other high-value chemicals.

Figure 4. Simplified flow diagram of the integrated process. Source: https://doi.org/10.1016/j.seta.2014.01.004

The simplified flow diagram of the integrated process, on which the present analyses was based, is depicted in Fig. 2. The integrated process includes (1) biomass inlet, (2) gasification air inlet, (3) produced biogas, (4) gasification residue, (5) steam inlet to reformer, (6) H2 rich mixture to cell, (7) air inlet to cell, (8) anode’s exhaust (unburned fuel), (9) cathode’s exhaust (depleted air), (10) gaseous burner feed from the cell, (11) burner’s exhaust to gasifying air heat exchanger, (12) burner exhaust to steam heat exchanger, (13) gasification’s air inlet to the process, (14) water’s inlet to process, (15) burner’s exhaust to SOFC’s air heat exchanger, (16) SOFC’s air inlet to the process, (17) heat supply to the turbine, (18) ambient air to the burner, and (19) low-quality heat generation.

2.4 Innovative Combustion of Solid Fuel in Heating Boilers

Providing economic and ecological sources for a safe supply of energy is a fundamental political goal and represents one of the highest priorities of the energy economy in Germany [8]. The expansion of thermal utilisation of biomass through biomass boilers represents a vital method or technology for the supply of households and many commercial enterprises with heat and hot water as well as electricity.

For the decentralised thermal utilisation of biomass, combustion technologies or biomass boilers with a thermal capacity of up to 200 kW are used. This section is about an innovative system, the so-called Low Emission Combustion-System. This combustion system is characterised by an integrated exhaust gas treatment (which is the cyclone as a combustion chamber and the downstream Packed-Technology as an integrated thermal oxidiser) and an innovative combustion air supply [9].

Figure 5. Cross section in a gasification boiler based on the Low-Emissions-Combustion-System (LEVS); (1) secondary air fan, (2) primary air fan, (3) door of the gasification room, (4) fuel hopper/filter, (5) gasification chamber, (6) adjustable grate, (7) fuel gas opening, (8) fuel gas channel, (9) primary air openings, (10) cyclone, (11) dust collection, (12) packed technology, (13) induced draft fan, (14) inspection flap, (15) loading door, (16) dip tube, (17) cyclone inlet channel, (18) water bag/heat exchanger. Source: https://doi.org/10.1002/ceat.201800118

It consists of a three-stage combustion unit with an innovative three-fan-air supply system. In the gasification chamber, primary air is supplied stoichiometrically by a pressure fan to adjust the gasification of solid fuels, the generated power, as well as the temperatures of the boiler and the exhaust gases. Due to its shape, the generated, burnable gas is led into the second combustion chamber, called the cyclone-combustion chamber. Here, the produced gasification gas gets burned, depending on the oxygen content inside the exhaust gas, by virtue of the supply of secondary air. Additionally, fine particles are agglomerated and separated by centrifugal forces in the cyclone.

The next step, the so-called Packed–Technology, makes up the main difference from other gasification boilers. It is a ceramic module as an integrated treatment stage for the oxidation of unburned exhaust gas components. The oxidation can be achieved due to the heat stored in the Packed–Technology during the combustion. This heat is released as soon as the temperature drops below the minimum oxidation temperature due to heat exchange. The particular architecture of the Packed–Technology increases the residence time of the flue gas by causing turbulent mixing and enhancing the mixing of combustion air and flue gas. As a result, the combustion of unburned components in the flue gas is improved. In the end, the low-emission exhaust gas submits its heat to the heating system and is finally discharged by a fan.

2.5 Residential Cogeneration

There is a growing potential for the use of micro-cogeneration systems in the residential sector because they have the ability to produce both valuable thermal energy and electricity from a single source of fuel, such as oil or natural gas, with high efficiency. In cogeneration systems, energy conversion efficiency increases to over 80% compared to an average of 30–35% in conventional fossil fuel-fired electricity generation systems.

The increase in energy efficiency with cogeneration can result in lower costs and a reduction in greenhouse gas emissions compared to the conventional methods of generating heat and electricity separately [10]. The concept of cogeneration can be related to power plants of various sizes ranging from the small scale for residential buildings to large-scale cogeneration systems for industrial purposes to fully grid-connected utility generating stations. Organisations that would benefit from cogeneration are those that could use the electricity and heat energy produced by the system. Consequently, cogeneration is suitable for building applications, provided that there is a demand for the heat energy produced.

Building applications suitable for cogeneration include hospitals, institutional buildings, hotels, office buildings and single- and multi-family residential buildings. In the case of single-family applications, systems design poses a significant technical challenge due to the non-coincidence of thermal and electrical loads, necessitating the need for electrical/thermal storage or connection in parallel to the electrical grid. However, cogeneration systems for multi-family, commercial or institutional applications benefit from the thermal/electrical load diversity in the multiple loads served, reducing the need for storage.

Cogeneration applications in buildings have to satisfy either the electrical and thermal demands, meet the thermal and part of the electrical demand, or satisfy the electrical demand and part of the thermal demand. Depending on the magnitude of the electrical and thermal loads, whether they match or not, and the operating strategy, the cogeneration system may have to be run at part-load conditions. Deficiencies may have to be made up by purchasing electricity (or heat) from other sources, such as the electrical grid (or a boiler plant). The surplus energy (electricity or heat) may have to be stored or sold. The surplus heat can be stored in a thermal storage device, such as a water tank or phase change materials. In contrast, surplus electricity can be stored in electrical storage devices, such as batteries or capacitors. In addition, a cogeneration system’s operation may depend on varying electricity prices, making cogeneration systems financially attractive in periods of high electricity prices.

3 At the Backbone of the Global Energy Mix

3.1 Challenges of energy systems on the climate neutrality path

The current challenges of energy systems show that intelligent sector coupling, including gas infrastructure as a seasonal and renewable storage option, allows for the climate targets to be accomplished. At the same time, this approach pragmatically reconciles the long-term prospects of climate neutrality with the opportunities available now.

Against this background, decentralised combined heat and power plants can provide a triple advantage. CHP systems are

  1. part of the variable renewable energy storage solution to re-electrify the wind and solar power stored in gas systems in a highly efficient manner,
  2. due to the reciprocal mode of operation, the natural partner technology for PV systems – in contrast to the combination with heat pumps, which has a counter-cyclical mode of functioning,
  3. system-relevant and can comply with the residual load highly efficiently as required.

Fossil fuels are still contributing to the global energy supply and are increasingly being phased out as the worldwide economy decarbonises. This loss of conventional power plant capacities harms grid resilience regarding voltage and frequency stability. The globally advancing digitalisation, the increase in electromobility, the conversion of the heating sector (e.g. through heat pumps) etc., will further raise the demand for electricity in the coming years – although the available capacity of many existing networks is already at the limit. The risk of blackouts is growing. Solar and wind energy are the central cornerstones of the world’s future energy supply, but they are only procurable in a naturally fluctuating way.

Energy storage is needed to reconcile the energy production mentioned above and its usage. Electric battery storage systems can be operated as ideal short-term storage systems, which can offset the volatility of electricity production in wind and PV power plants over hours or days at a maximum. Still, the question of the degree of recycling or the disposal of batteries often remains open.

3.2 Sector coupling

This concept is the basis for a real solution. When it comes to finding a systemic solution, the sectors of electricity, heat and mobility must always be evaluated as a whole. These hold a different share of the total energy requirements depending on the country, climatic conditions, infrastructure, etc. In Germany, for instance, only around 20% is accounted for by the electricity sector; in comparison, the mobility sector accounts for a third and the heating sector for a stunning 50% of the final energy demand. Soon, a mostly renewable energy scheme will be an efficient mix of energy sources and carriers – enabled by intelligent digitisation solutions.

Wind and solar energy expansion is a top priority worldwide to meet growing energy needs. Batteries act as short-term storage decentralised devices – e.g. in the mobility sector – and store, for example, the solar power harvested over the daytime to make it available for charging an electric vehicle in the evening. The scale of a typical general supply grid requires other storage capacities that reconcile the temporal offset of renewable electricity production and its usage months later.

As an energy carrier and storage medium, hydrogen has become the key to ensuring secure climate neutrality of the energy system. The German gas network has a storage capacity of 220 TWh [11]; the hydrogen can therefore be stored in large quantities in the existing gas network and removed on a seasonal basis as required. Due to its magnitude, the losses associated with H2 supply chain conversions should be minimised. Making good use of decentralised electrolysers’ waste heat (with efficiencies of up to 90%) combined with enhanced CHP systems (with efficiencies of up to 98%) at the location of the actual energy demand is an option.

By 2050, electricity and renewable gases of different origins are expected to directly meet all of Europe’s energy needs. Hydrogen is already added to the NG network in several places today, and component manufacturers in all sectors consider an increasing mixture ratio when picking up new products. Many natural gas pipelines are apt for conversion to 100% hydrogen so that a smooth and socially responsible transition to the green age can be achieved based on existing infrastructure.

3.3 Decentralised CHP as a flexible backbone technology

Thanks to the simultaneous and highly efficient electricity and heat production, every natural gas-operated CHP system is already helping reduce greenhouse gas emissions worldwide. Today, we can convert an installed natural gas CHP system to operate on hydrogen anytime so that “stranded investments” are avoided. Actually, only about 15% of the initial investment sum is to be calculated for this, thus creating an economical and future-oriented solution.

CHP arrangements operated with natural gas are not only a bridge technology paving the way to an entirely renewable energy system and then becoming obsolete. Instead, CHP systems with hydrogen can be the climate-neutral backbone power plant capacity, compensating for the variable electricity production from wind and solar power plants. Hydrocarbon-compatible CHP facilities enable a progressive, granular entry into the hydrogen economy. The existing infrastructure can be switched in parallel and synchronously with the growing hydrogen supply. It is not necessary to start or stop significant infrastructure projects abruptly.

Figure 6. Nyköping combined heat and power plant. Source: https://www.flickr.com/photos/vattenfall/3592352092

A CHP system is a PV system’s, ideal mate. A CHP system reliably provides energy and heat when the sun is not shining, unlike a heat pump, which depends on the availability of renewable electricity at periods when PV systems frequently do not produce.

In conclusion, it can be said that decentralised CHP systems help to address the issues with the energy systems mentioned above in the following ways:

3.3.1 Reliability of supply

  • No dependence on fluctuating energy sources
  • Demand-based supply of heat and electricity when the wind is not blowing and the sun is not shining
  • Digital integration into an overall renewable system via intelligent software and control solutions
  • Direct access to energy “at the touch of a button”
  • Decentralized CHP systems are rotating energy masses in the global balance and secure the grid frequency

3.3.2 Sustainability

  • A big portion of CHP systems installed worldwide are already operated with green gases
  • Highly efficient and resource-efficient use of natural gas to ensure the security of supply during the transition to complete climate neutrality
  • By using hydrogen, the CHP becomes a climate-neutral and, at the same time, demand-oriented energy supplier.

3.3.3 Efficiency

  • CHP devices installed today can be retrofitted for use with renewable gases
  • High total efficiency makes CHP a worthwhile investment regardless of gas type – both for the individual operator and for the economy
  • Business models exist and are emerging worldwide that make the demand-based provision of energy interesting for operators and thus offer investment incentives

4 References

[1] Doukelis, A., & Kakaras, E. (2011). The Integration of Micro-CHP and Biofuels for Decentralised CHP Applications. Green Energy and Technology, 177–195. doi:10.1007/978-1-84996-393-0_8

[2] https://www.epa.gov/chp/what-chp

[3] Combined Heat and Power Webinar, Clarke Energy. Available at: https://www.youtube.com/watch?v=w6ql4OaBO64

[4] https://www.iea.org/policies/627-access-to-the-grid-renewables-and-chp

[5] https://cordis.europa.eu/project/id/503138

[6] https://www.wbdg.org/resources/microturbines

[7] Zabaniotou, A. (2014). Agro-residues implication in decentralised CHP production through a thermochemical conversion system with SOFC. Sustainable Energy Technologies and Assessments, 6, 34–50. doi:10.1016/j.seta.2014.01.004

[8] Bundesministerium der Justiz und für Verbraucherschutz. 1. BImSchV Erste Verordnung zur Durchführung des Bundes-Immissionsschutzgesetzes (Verordnung über kleine und mittlere Feuerungsanlagen – 1. BImSchV)). Available on http://www.gesetze-iminternet.de/bimschv_1_2010//, 2010.

[9] Aleysa, M., Meriee, S., Akbary, N., & Ecker, M. (2018). Innovative combustion system for economic and ecological thermal utilisation of solid fuel in heating boilers. Chemical Engineering & Technology41(11), 2120-2131.

[10] Onovwiona, H. I., & Ugursal, V. I. (2006). Residential cogeneration systems: review of the current technology. Renewable and sustainable energy reviews10(5), 389-431.

[11] https://www.2-g.it/module/designvorlagen/downloads/2g_chp_narrative.pdf