**2. Background**

Cogeneration, also known as combined heat and power system (CHP), shows the ability to decrease primary energy consumption and reduce greenhouse gas emissions due to the increase in building energy efficiency [6,7]. Thermal energy demand can be supplied by a heater or a boiler, but the use of the CHP technology could provide the same thermal energy consumption and a fraction or the whole electrical demand depending on the CHP technology used. CHP technologies must be easily scalable for low power ratios to make them suitable for residential applications. CHP systems for buildings can be classified depending on the rated thermal power as micro- (1–5 kW) and small-scale (≤50 kW) units [8]. Because CHP systems produce the energy at the point of use, they can be referred as decentralized energy sources [9]. This advantage of decentralized generation includes an improved energy efficiency, which means an optimized fuel utilization that results in decreased CO2 emissions and primary energy consumption and a reduction in the national transmission losses, that account for 2%–11% of the losses in the European transmission network [10]. However, benefits of the distributed generation are only achievable if there is a proper energy management between generation and consumption [10]. Proper management would require of an energy storage system. Electrical energy storage (EES) improves the self-consumption ratio for small CHP units [11] and thermal energy storage (TES) eliminates system oversize and also optimizes the use of produced energy [12]. All the CHP units considered in this paper will integrate both EES and TES systems.

Figure 1 shows the different CHP technologies suitable to be installed in a residential building [13]. The zones have been delimited using the technical parameters obtained from the "Cogen Challenge Project" document [14], where micro-scale and small-scale cogeneration technologies are analyzed. Each technology is represented as a colored fuzzy area, delimited by four straight lines, two thick solid lines and two dotted ones. Thermal/electrical efficiency is indicated in the vertical axis and thermal/electrical rated power in the horizontal one. The represented surface covers the power range for the applicability in buildings, from single-family houses to blocks of apartments. The figure can be read as follows. Solid lines represent two possible CHP configurations in each technology. The right-side solid line of each area indicates the most common or typical "small-scale" unit of the technology and the left-side one corresponds to the smallest CHP unit possible as indicated in [14]. Solid lines can be understood as an operating point of the real CHP unit, where the upper extreme of the line is for the CHP unit thermal characteristic and the lower one corresponds to the electrical one.

**Figure 1.** Comparison of the technical parameters in different gas-fueled combined heat and power technologies.

For example, the solid line of the right side in the blue area means the "typical smallscale gas turbine CHP unit" as stated in [14], which means a rated electrical power of 250 kW and around 330 kW for its rated thermal power. The corresponding conversion efficiencies, around 30% and 40% for the electrical and thermal energies respectively, are read in the vertical axis. On the left side of the same fuzzy area, the electrical and thermal rated power of the smallest gas turbine CHP unit analyzed are 30 kW (26%) and 50 kW (47%), respectively. Both units are connected with the dotted lines that create the fuzzy area, which can be considered as an operational chart for the technology in the small-scale use. In other words, this area can be understood as the operating range for each technology. According to Figure 1 the following conclusions can be extracted:


kilowatts keeping a constant energy conversion efficiency when they are fueled from pure hydrogen [18–21]. When this pure hydrogen comes from a green production process, the energy obtained can also be considered as green or carbon-free.

FC-CHP are classified as a function of the fuel cell technology used in the power unit. The most common technologies in commercial units are based on polymer exchange membrane fuel cells (PEMFC), The most successful examples of these systems can be found in Japan and Europe [22,23]. PEMFC can be classified into low- (up to 80 ◦C) and hightemperature (from 120 ◦C to 180 ◦C) devices. They only differ in the working temperature required by the polymer used as solid electrolyte membrane. Low-temperature PEM fuel cell-based CHP systems are the most common. In this paper both PEM technologies are considered, paying special attention to a high-temperature PEM fuel cell-based micro-CHP system specifically conceived in the framework of the MICAPEM project that is been integrated into an existent nearly-zero energy house, developed and built for the international Solar Decathlon 2012 contest [24,25]. The use of high-temperature PEMFC is promising due to the improved chemical kinetics in the electrodes, better tolerance to CO impurities in the fuel, simplification of the water management because it is produced in vapor phase and simpler and compact heat recovery system because of the higher enthalpy of the thermal energy [26,27]. A majority of the significant studies in the literature involving a high-temperature PEM fuel cell-based CHP system are theoretical works [28–31] Only one report on tests in an experimental facility has been found [32].

The objective of this research paper is to expose, using numerical simulations, how fuel cell-based CHP systems can drive a potential reduction of primary energy consumption and CO2 emissions in the building sector. Numerical simulations are performed using preliminary results from the characterization of the high-temperature PEM fuel cell prototype built and tested to be installed in a demonstrative scale CHP facility. Once installed, the CHP technology will be evaluated and a novel oil-based refrigeration system for HT-PEMFC will also be tested, as explained in Section 3.3. In the same project framework, a hydrogen electrolyzer integrated with the solar system is also being installed to link with the green hydrogen source requirement objective.
