Combined Heat and Power

This section discusses the use of fuel cell CHP in (domestic) built environment applications. Tri-generation can be considered a sub-set of CHP because the prime mover technology is the same and the operating aims alike. Thus, the discussions presented regarding fuel cell CHP in built environment applications provides a good basis to discuss tri-generation systems in Sect. 2.5.

Fig. 2.14 CHP in a domestic building (Hawkes et al. 2009)

CHP is defined as the generation of heat and power from a single fuel source, with the view to using both products. Figure 2.14 shows a typical domestic CHP system configuration. Fuel is supplied to the prime mover technology from the central network to produce electrical power and in the process generates heat. The electricity is used directly in the home, and if grid interactive, can be imported or exported as required. The heat produced in the electrical generation process is recovered and used in applications such as space heating or domestic hot water. By consuming this heat, system efficiency can be elevated from as low as 20 % to over 90 % depending on the prime mover technology and the extent of waste heat utilisation (Choudhury et al. 2013).

In built environment applications elevated system efficiency results in reduced primary energy demand, leading to decreased emissions and running cost for the consumer. However, as Beaussoleil-Morrison (2008) states, if the thermal output of the CHP system cannot be fully utilised, then the system cannot expect to deliver a net benefit relative to grid electricity and a highly efficient condensing boiler. Therefore accurate building energy load assessments and sizing of the CHP unit is essential (El-Gohary 2013). Currently, The Carbon Trust defines domestic scale CHP as any system up to 3 kWe. Because CHP and tri-generation systems produce electricity at point of use, they are often referred to as decentralised energy conversion devices. Decentralised energy conversion in the built environment has many advantages associated with it compared to traditional centralised conversion, including:

  • • Improved system efficiency, otherwise wasted heat is utilised (heating or cooling), therefore system efficiency can be elevated from as low as 30-50 % in central power stations to around 70-90 % (Choudhury et al. 2013).
  • • Decentralised energy conversion with CHP significantly reduces transmission losses, which account for 6-24 % in the European transmission network (Peht et al. 2006).
  • • Improved system efficiency and greater fuel utilisation leads to reduced primary energy demands, resulting in cuts to CO2 emissions and operating costs (Fubara et al. 2014).
  • • Electricity is regarded as having an economic value of roughly three times that of gas. Therefore converting lower cost gas (common fuel in CHP) to electricity allows households to recover cost and reduce energy bills. This is an important factor in the fight against fuel poverty (Staffell 2009).
  • • Centralised decarbonisation of electricity generation in many countries is problematic because of opposition to low carbon technologies such as renewables and nuclear. CHP in consumers’ homes offers an option to assist in both decarbonising electricity production and providing energy saving benefits directly to the home owner (Staffell 2009).

Figure 2.15 illustrates the benefits of generating heat and power at point of use with fuel cell technology. Note the efficiencies are illustrative and will vary depending on the systems used and countries involved. In a tri-generation system the CHP heat output is simply used to provide cooling in summer.

Currently, there are three key technologies used as prime movers in CHP and tri-generation systems in the built environment. All are at varying levels of commercial and technological maturity; internal combustion engine (ICE), Stirling engine (SE)—both combustion based technologies, and fuel cells. The performance and operational characteristics of these three technologies are summarised in Table 2.4. The data presented in Table 2.4 is indicative and used for comparative purposes. The data is quoted for CHP systems in the range up to 5 kWe. It should be noted that CHP performance, primarily electrical efficiency, is highly dependent upon fuel used, engine type, engine size and running mode. Larger capacity ICE and SE based systems will have higher electrical efficiencies (Bianchi et al.

Scheme of the advantages of decentralised energy conversion with fuel cell CHP compared to conventional centralised energy conversion

Fig. 2.15 Scheme of the advantages of decentralised energy conversion with fuel cell CHP compared to conventional centralised energy conversion

Table 2.4 Built environment CHP technologies (Wu and Wang 2006; Hawkes et al. 2009; Staffell 2009; Vourliotakis et al. 2010; Steinberger-Wilckens 2013)

Internal combustion engine

Stirling engine

Fuel cell

Capacity (kWe)




Electrical efficiency (%)



PEMFC—30 to 40 SOFC 40 to 60

Overall system efficiency (%)

Up to 90

Up to 95

Up to 85

Heat to power ratio



PEMFC—2 SOFC—0.5 to 1

Able to vary output




Fuel used

Gas, biogas, liquid fuels

Gas, biogas, butane












Vaillant ecoPower

EHE Wispergen

Baxi, CFCL

2012) . It is evident that fuel cell technology has some clear operational advantages, particularly when operating at the less than 5 kWe scale in the domestic built environment. Advantages include; higher electrical efficiencies, low heat to power ratio and near silent operation. However, the relative infancy of fuel cell technology has limited their extensive application and market involvement to date.

Because of the low electrical efficiency and correspondingly high thermal output of the internal combustion and Stirling engine, they should only operate when their thermal output can be fully utilised, otherwise the CHP system cannot expect to deliver a net benefit relative to grid electricity and a highly efficient condensing boiler (Beaussoleil-Morrison 2008). Fuel cells, however, with their higher electrical efficiency have much lower heat to power ratios; therefore their operation can be largely independent of thermal demand, making them a well suited technology for domestic CHP applications (Hawkes et al. 2009). Fuel cells can be operated in an electrically led manner, thus providing increased net benefit to the user. SOFCs have lower heat to power ratios compared to PEMFC technology, making them a well suited technology for domestic applications.

Figure 2.16 shows the annual CO2 savings per kWe capacity of various CHP systems compared to a grid electricity and boiler alternative (Maghanki et al.

2013) . The data shows the SOFC system has the best emission performance, with the PEMFC system second, excluding the 5 kWe gas engine. The high electrical efficiency seen in the SOFC CHP system means it can generate large emission savings due to a greater displacement of carbon intensive centralised electrical generation. Although the PEMFC system has a high electrical efficiencies in the 1-5 kWe range, as the electrical capacity of the gas engine increases to 5 kWe, its efficiency will also increase, therefore improving the emission performance, and in this case surpassing the PEMFC performance.

Annual micro CHP CO2 savings compared to grid electricity and boiler alternatives (Kuhn et al. 2008; Maghanki et al. 2013)

Fig. 2.16 Annual micro CHP CO2 savings compared to grid electricity and boiler alternatives (Kuhn et al. 2008; Maghanki et al. 2013)

Figure 2.17 shows that according to a report by Delta-ee energy consultants, fuel cell CHP systems for domestic applications outsold conventional combustion based systems for the first time in 2012, accounting for 64 % of global sales, illustrating a major shift in the domestic CHP market on account of fuel cells clear operational advantages (FCT 2013). Growth is being driven by Japan and to a lesser extent Germany, which together account for more than 90 % of yearly sales (Bradley 2013). However as Steinberger-Wilckens (2013) states, for fuel cells to produce a marked effect on the stationary market they need to match and surpass the performance of current CHP technologies such as SEs and ICEs.

Global micro-CHP sales by technology (Bradley 2013)

Fig. 2.17 Global micro-CHP sales by technology (Bradley 2013)

Fig. 2.18 A typical fuel cell CHP system for domestic building applications (Hawkes et al. 2009)

Steinberger-Wilckens proposes that meaningful performance indicators include; amount of CO2/fossil fuel avoided through the use of fuel cell technology or the total and electrical efficiency of the fuel cell system.

A schematic diagram of a typical fuel cell CHP system for domestic building applications is shown in Fig. 2.18. A fuel cell in principle is very simple, requiring few parts (even less moving), resulting in near silent operation and little maintenance required. However, in order to operate a fuel cell system i.e. a load supplied by a fuel cell, many auxiliary devices and interconnections are needed for both the correct operation of the fuel cell and the delivery of heat and power to the load. Some of these auxiliary devices have a power demand, thus they are a parasitic load on the system. The electrical efficiency of the entire system can be between a fifth and a third less than the quoted stack efficiency due to these parasitic loads (Hawkes et al. 2009). Auxiliary equipment also contributes to increased noise, vibrations and maintenance. A 1-5 kWe fuel cell CHP system can expect to produce 0-55 dB, whereas an ICE is around 95 dB (Hawkes et al. 2009).

The following list provides an explanation of the components found in the fuel cell CHP system shown in Fig. 2.18. In a tri-generation application, the thermal output is simply used in a heat driven cooling cycle.

  • • Fuel cell stack: where hydrogen and oxygen are combined to produce electricity, heat and water.
  • • Fuel processor: converts a hydrocarbon fuel such as natural gas into hydrogen and CO2 (not required in an SOFC system).
  • • Inverter, grid tie and power electronics: converts the DC electrical output of the fuel cell stack into AC electrical power to either serve the buildings energy demands or to be fed back to the grid.
  • • Waste heat recovery (WHR) system: used to recover heat from the fuel cell in order to improve; (a) the performance of the fuel cell stack and (b) the environmental performance i.e. make it a CHP system. In a domestic application flow temperatures of around 50-70 °C are common (Ellamla et al. 2015).
  • • Balance of plant: includes pumps, fans, valves, sensors, piping and control system, used to ensure the whole system functions in a safe, efficient manner for long term stable operation.
  • • Boiler: to provide peak thermal loads alongside the fuel cell.
  • • Thermal energy storage: i.e. hot water tank, to store the thermal output of the fuel cell.
  • • Smart meters: to measure and record energy conversion and consumption.
  • • Internet connection: to facilitate remote monitoring and data acquisition.

Extensive literature searches have highlighted that successful work has been carried out on the application of fuel cells in the domestic built environment; most significantly EneFarm's field trials of a 1 kWe PEMFC CHP system in Japanese households. The project has spear-headed the wider, global use of fuel cell CHP in the domestic built environment and the systems have confirmed annual CO2 emission reductions in the order of 750-1250 kg per annum are possible, illustrating the significant potential fuel cells have in assisting decarbonisation of the future domestic built environment (Elmer et al. 2015b). Ren and Gao (2010) have investigated a gas engine and fuel cell for domestic CHP applications in Japan. Results indicated that the fuel cell offered superior economic and environmental performance in such applications.

Section 2.4 has discussed the use of fuel cell CHP in (domestic) built environment applications. Next, Sect. 2.5 provides a review of the literature surrounding tri-generation systems, particularly those employing fuel cell and liquid desiccant technology.

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