Cogeneration systems with electric heat pumps: Energy-shifting properties and equivalent plant modelling
Introduction
Distributed generation (DG) thermal prime movers [1] allow exploitation of cogenerated heat within a host of small-scale (below 1 MWe) applications, such as office buildings, hospitals, residential blocks, and so forth. The diffusion of combined heat and power (CHP) plants also on a small-scale basis could contribute to bring important energy saving [2], [3] and greenhouse gas emission reduction [4], [5], [6] with respect to the conventional separate production (SP) of electricity (in centralized power plants) and heat (in boilers). On the other hand, utilization of the electric heat pump (EHP) technology often proves to be an efficient and economical alternative to traditional boilers, as discussed in various studies relevant to different countries [7], [8], [9], [10], [11], [12], [13], [14], [15]. Combination of CHP and EHP systems could even enhance the benefits from both technologies.
Horlock in [2] shows how the heat generation efficiency improves in a combined scheme where all the cogenerated electricity supplies a cascaded EHP. Primary energy reduction is also highlighted in [16], presenting a CHP–EHP combination for large aggregated users (communities and industrial areas). Benefits from CHP–EHP coupling in district energy systems are also presented in [17], [18], where an environomic optimization is performed. Likewise, an exergetic analysis is run in [19], showing how a CHP–EHP scheme is exergetically more efficient than the conventional separate production, which also results in lower fuel consumption. Overall increase in energy efficiency is analysed in [20] through a model based upon incremental indicators, while the author in [21] discusses the characteristics of CHP–EHP systems in terms of energy saving. A domestic application of a hybrid CHP–EHP system is reported in [22], highlighting that the EHP brings additional economic and environmental benefits to the CHP system, by producing heat when the domestic electric demand is low. The role of EHPs in improving plant flexibility, performance and profitability in combined systems is further illustrated in trigeneration applications [23], [24], [25], [26].
The aim of this paper is to formulate a simple model, based on the first law of thermodynamics, useful for general and indicative assessment of CHP–EHP composite systems within different generation frameworks. Specific focus is set on the operation flexibility characteristics as well as relevant potential energy saving and emission reduction of such systems with respect to classical generation means. For this purpose, the concept of equivalent cogeneration plant is introduced. The equivalent cogeneration plant is characterized by equivalent electrical and thermal efficiencies, obtained by transforming the original CHP characteristics through the energy-shifting property of the EHP. Energy-shifting refers to the capability of an EHP to turn an electrical input into a thermal output, which increases the flexibility of the cogeneration system (particularly relevant in the presence of highly variable loads). Within the approach proposed, this is highlighted by introducing equivalent cogeneration ratios and a thermal multiplication factor (TMF) indicator (as illustrated in Section 3). In order to assess how the first law energy performance of the equivalent cogeneration plant changes with the operational conditions, energy saving analyses with respect to classical SP means are run. This is carried out by using the equivalent efficiencies introduced as the input entries to the classical fuel energy savings ratio (FESR) cogeneration indicator [2]. In addition, following the lines drawn in [5], [6], [27] for multi-generation systems, the energy model is extended to the environmental assessment by defining an equivalent CO2 emission reduction (CO2ER) indicator. The final outcome is a comprehensive energy and environmental modelling structure for the equivalent cogeneration plant. The generality and effectiveness of the approach are illustrated in detail through various numerical examples. In particular, cogeneration-only systems, EHP-only systems, and CHP–EHP systems for heat-only production are also assessed as special sub-cases of the general model introduced. Specific applications are presented for spreading small-scale natural gas-fed DG technologies, namely, the internal combustion engine (ICE) and the microturbine (MT), and their potential when combined with EHPs is assessed in current and perspective energy scenarios.
Section snippets
Energy system components
The CHP–EHP composite energy system under study here is composed of
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A DG CHP prime mover, producing electricity and heat (usually in the form of hot water). For small-scale applications, MTs and ICEs already represent mature technologies, while fuel cells represent a promising solution for the next future [28], [29], [30]. The equipment is typically fed by natural gas (NG), especially in urban areas with well developed distribution networks. The produced heat is usually distributed as hot water
The equivalent energy efficiencies
Fig. 1 shows a schematic model for the CHP–EHP composite system, in which the cogenerated electricity splits into a quota feeding the EHP and a quota directly supplying the user. The two quotas can be modelled by introducing the energy-shifting factor αQ (0 ⩽ αQ ⩽ 1), indicating the relative electricity quota supplying the EHPClearly, the higher is the value of αQ, the higher is the energy-shifting from electricity to heat. In fact, fixed the fuel input, if αQ increases, the
Underlying assumptions
Relations (4), (5), (6) illustrate how feeding the EHP with cogenerated electricity brings about a net increase in heat production at the cost of net electricity production. This could be seen, for instance, in analogy to CHP extraction steam turbines [2], [32], where increase in steam production would generally occur at the cost of reduced electricity output. In addition, in Section 3.1 it has been pointed out that suitable evaluation techniques are needed when dealing with different types of
Parametric analyses for different prime movers
In Fig. 2 the mapping of the equivalent electrical and thermal efficiencies (4), (5) against the energy-shifting factor is illustrated. The prime mover electrical efficiency is used as the curve parameter, and the overall efficiency is set to 0.8 for all the curves, a value typically encountered for a wide range of CHP technologies and applications. The COP for the EHP is set to 3, a common value for several air conditioning applications with capacity below 1 MWt in central and southern Europe.
Conclusions
Deployment of distributed CHP–EHP composite systems could play a key role within a worldwide transforming energy scenario where the highest priority is given to seeking solutions aimed at minimizing fossil fuel resource depletion and CO2 emissions, particularly for the generation of heat. In this outlook, simple and quick tools are needed for general assessment of the energy and environmental potential of such systems in different energy contexts.
In this paper, a novel approach for
Acknowledgements
This work was in part developed with the contribution of Regione Piemonte, Torino, Italy, under the research Grant C65/2004 “Territorial sustainability of distributed energy generation and interactions with electro-energetic systems”.
The author thanks Prof. Gianfranco Chicco, Politecnico di Torino, Torino, Italy, for his useful comments to improve this paper.
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