Energy retrofitting of residential buildings—How to couple Combined Heat and Power (CHP) and Heat Pump (HP) for thermal management and off-design operation
Introduction
Nowadays, the need for efficient conversion technologies promotes the deployment of combined heat and power (CHP) production and heat pump (HP), accentuating their role in the energy systems. In terms of the hydraulics, these units are installed just like any conventional boiler to provide heat to the energy system, while, in the CHP case, there is the opportunity to generate electrical energy too. Such technologies can foster the diffusion of distributed generation systems, taking into account the conformation of the power grids [1], [2], [3].
As regards the CHPs, they have problems to meet efficiently the dynamic thermal energy demand depending on their technology. So, they are usually provided by a buffer system, e.g. a water tank [4]. In that behaviour, the CHP works at rated load so that to have a higher efficiency. At the same time, the electricity surplus should be sold to the grid, whether a net metering option is allowed, otherwise it should be stored locally. Selling to the grid could be uneconomical when the electricity price on the spot market is lower than the CHP generation cost, while it could be economically feasible if associated to an electric storage [5]. Many technologies, so-called Power-To-Power solutions, are already tested in this combination. Among those ones, Compressed Air Energy Storage (CAES) shows promising performance even hybridized with Phase Change Material [43] or coupling renewable generation with fossil-fuel one by means of synthetic fuel technology [44].
Furthermore, the CHP designed to meet the thermal energy demand implies to account for the matching between the CHP and end-user power to heat ratio as much as possible in order to increase the plant profitability.
Referring to HPs, they are a promising option to reduce the energy-related greenhouse gases emissions in the building and industrial sectors thanks to the use of freely available heat such as ambient air, water, ground and thermal cascade as well. In the building sector, the electrically driven air–water compression heat pumps (AWHP) are the most common technology for retrofits owing to their relatively low investment cost, easy installation and little required space [6]. Its drawback consists of a reduced efficiency as well as lower thermal output caused by low source medium temperatures and larger required temperature difference during the coldest period of the year. In the industrial sector, the choice of refrigerant plays a key role. Indeed, thermodynamic properties, safety and transport issues promoted the use of refrigerants such as R744 (Carbon Dioxide), which could be used as coolant or in thermal cascade system [7], [8]. This latter provides the opportunity to integrate large scale Heat Pump in recovering waste heat from existing thermal cycles.
Having said, the HPs represent a more efficient solution owing to their COP but they work at lower input-output temperature. As matter of fact, the recent deployment of Heat Pumps in replacing conventional systems such as boilers has raised many issues linked to the new adjustments required to the local electricity distributors. Indeed, the HP is an adjunct electrical demand which entails the upgrade of the end-users’ electricity meter along with further transmission costs. For instance, in Italy the Authority for Electricity and Natural Gas (AEEG) imposes the purchase of a dedicated electricity meter for the HP installation [9]. So, considering the advantages of both CHP and HP technologies, a foreseeable solution could be their coupling to increase the overall energy efficiency for heating purposes as well as to reduce the associated CO2 emissions.
The substitution of energy system with new much more efficient one is a key issue in energy refurbishment of buildings and industrial processes. As a matter of fact, total substitution is often impossible. Furthermore, the on-off regulation widely used in the 20th century is so replaced with partial load conditions allowing reducing the downtime. However, load modulation for boilers as well as the reduced rotational speed of the engines usually corresponds to efficiency less than at rated load. For this reason, when old and new energy systems work at the same time, the overall efficiency decreases deeply compared to the declared one at rated conditions. So, civil and industrial applications require better system performance at partial load for each thermal machine to achieve a higher overall efficiency so as to reduce primary energy consumption. This latter, once measured in TOE (Tons of Oil Equivalent), is the main key performance indicator to develop energy management strategies. For instance, in energy refurbishment of public building stock, achieving goals of TOE reduction is driven by EU legislation [10]. To do so, Public Administrations are calling for thermal management services based on energy performance contracts where electric RES use is not allowed to get the primary energy saving. In this framework, the adoption of those combined systems would be able to accomplish the energy targets.
Specifically, a suitable matching of a CHP and a HP in the same energy system leads to higher efficiency, flexibility and safety along with deriving economic benefits. Few references are available in scientific literature about the combination of CHP and HP technologies. One of the first studies is related to a preliminary analysis showing the potential of a domestic scale hybrid CHP/HP plant in terms of energy utilisation, economic viability and emissions reduction [11]. Additionally, the incorporated heat pump gives a high degree of flexibility in meeting domestic energy requirements. Those considerations were confirmed by Smith et al. in a second study where demonstrated practically the advantages of heat-pump incorporation, giving an enhanced thermal delivery and higher total plant efficiency [12]. Then, from the exergy analysis carried out by the same authors [13], it emerged that by using an alternative refrigerant, operation at higher temperatures would reduce the exergy losses within the HP. In this framework, this paper analysed also the technical option of using the waste heat from CHP exhaust gases as a heat sink for the HP device. In the last decade, research projects focused on HP advances concluded that HP readily compliment with many renewable energy technologies to produce desired heat and power at reduced basic fuel input, in cogeneration design [14]. For instance, in small-scale distributed applications HP coupled CHP showed energy saving and emission reduction up to 50% compared to conventional layout [15]. From an energy point of view, the combined CHP/HP system is equivalent to the well-known GEHP (Gas Engine driven Heat Pump), where the CHP electrical output matches perfectly the HP electric demand within a unique block. Several studies investigated on different GEHP layouts and their performance along with the comparison with HPs [16]. Considering the device behaviour during transients, under a constant speed operation, GEHPs are more efficient than electric HPs, while HPs are more efficient under variable speed operations [17]. Hence, both HPs and GEHPs are remarkably influenced by the engine speed. Anyway, GEHPs are more energy efficient in the low speed mode [18]. The unique block architecture entails a unique energy output, i.e. thermal. On the contrary, the CHP and HP connection by the Grid offers the opportunity to provide electricity during the thermal load modulation from end-user side. The advantage of the combined system consists of HPs operational flexibility throughout a varying load profile, by maintaining high coefficient-of-performance (COP) values [19]. This latter feature, i.e. a “quantitative” flexibility, is so important that some research activities focused on modifying the GEHP basic layout by adopting the hybridization concept from electric motor-assisted engines. Li et al. studied a HPGHP (Hybrid-Power Gas engine-driven Heat Pump) system where the electric output is stored in a battery-pack to assist the ICE at low rotational speed [20], increasing its thermal efficiency at partial load.
Moreover, a combined system is able to provide high and low temperature heat, simultaneously. Therefore, another significant aspect regards the “qualitative” flexibility. The option to provide only high grade temperature is also available by the use of double-stage electric HP or Trans critical Carbon Dioxide-based HPs [21], where the Heat sink is not a RES. Cutting-edge applications focus on fuel supply-side, involving eco-fuels such as Hydrogen or its mixtures [22], [23]. Whether Power to Gas option was considered, well-proven technologies could be fuelled with environmentally-friendly fuels without heavy technical issues [24], [25]. Differently, a shift towards Hydrogen economy requires a further effort, i.e. the deployment of Fuel Cell technologies that are already studied in literature for Heat Pump coupling [26], [27]. Additionally, another interesting feature of a system composed of different appliances is to decrease the size of them so that to simultaneously match minor safety requirements. For instance, power plants up to 3 MW of heat produced in the firebox of the boiler need a continuous monitoring system for pollutant emissions [28]. As aforementioned, a coupled system (CHP + HP) could use the electricity production of the CHP to feed the HP so that to avoid the upgrade of the transformer kiosk and deriving costs. Those latter issues were the focus of recent research in formulating electrical-equivalent load following strategy superior than conventional strategies from both economic and energetic point of view, better load coincidence and peak reduction, respectively [45].
Thus, as purpose, this work is focused on the following points:
- i
coupling two well-established technologies: a CHP and a HP (presenting a graphical method). This configuration could represent a viable solution to support the energy retrofitting;
- ii
an analytical method was designed and an oversize/downsize factor was defined due to the mismatch between the power sizes of the two appliances available on the market as well as the deriving technical issues;
- iii
the system efficiency was discussed for these more complex plants.
Section snippets
Energy system model
In this section, the reference energy model was presented. It is composed of a CHP and a HP which are electrically connected by the local grid. As depicted in Fig. 1, both CHP and HP thermal power outputs were released to the end-user. As regards the CHP electrical output, it could feed the HP partially or totally, depending on the machines size and the operating conditions at rated and partial loads required by the end-user. Those coupling issues are the core of the present investigation. In
Perfect coupling and design conditions
CHP technologies can be identified immediately by the numerical values of three meaningful parameters: electrical efficiency, heat recovery efficiency and Power to Heat Ratio, which are reported below, respectively.
Similarly, the HPs are distinguished by the Coefficient Of Performance, which depends on load control technology and outdoor temperature, as follows:Here, the Crossed Heat to Power Ratio was
Energy effects related to the size mismatch between commercial CHP and HP
The CHP and HP perfect power coupling does not occur very often, due to the machines commercial size available on the market. In order to assess the combined system energy penalties and gains caused by the contingent electrical power mismatching, an oversize/downsize factor was defined in Eq. (7), below:
Since the consumed electrical power by the heat pump can be deduced from COP equation, the size factor can correlate directly the CHPR with the COP as follows:
Case studies
In this section, a numerical simulation is provided to evaluate the energy and environmental benefits coming from the use of the combined system compared with the conventional ones, (i.e. condensing boiler and traditional boiler) by means of the elaborated method. In detail, partial heating loads ranging from 30% up to 100% were evaluated using data from experimental campaign on CHPs carried out by the authors previously [23], [32], [38], [39].
In Table 3, the measured CHP electrical and heat
Concluding remarks
Civil and industrial sectors require better system performance for the heat generation at either rated and partial load for each thermal machine [40]. Referring to existing and older plants, effective retrofitting solutions are required to achieve higher overall efficiency so as to reduce primary energy consumption as ratified by recent European Directives. In this framework, a suitable matching of a CHP and a HP leads to higher performance, flexibility and safety along with deriving economic
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