Integration of a magnetocaloric heat pump in an energy flexible residential building
Introduction
In the recent years, global agreements have been reached concerning the necessity of decreasing our dependency on fossil fuels [1]. To meet these goals, a large deployment of renewable energy sources (RES) and a sharp improvement of our societies’ energy efficiency are needed [2]. In many countries, the building sector is the largest energy end-user. In Europe, for instance, it accounts for around 40% of the total energy demand and indoor space heating represents 75% of the building energy needs [3]. Consequently, the development of sustainable low-energy buildings with efficient heating systems is essential.
Heat pumps are mechanical devices which transfer thermal energy from a cold reservoir with low exergy (heat source) to a warmer environment with higher exergy (heat sink) by means of heating/cooling thermodynamic cycles. Conventional heat pumps are typically electrically driven and operate with a vapour-compression thermodynamic cycle. This mature technology for heating is cost effective, presents low CO2 emission, and achieves high coefficient of performance (COP) ranging from 3 to 5 [4,5].
Heat pump systems are therefore an excellent solution to supply heating energy for indoor space conditioning and domestic hot water in countries where the heating demand is dominating. Indeed, in the case of Denmark, a study showed that district heating (in urban areas) and individual heat pumps (outside urban areas) are the best heating supply solutions with regards to costs, energy consumption and CO2 emissions [6]. Similarly, it was found that heat pumps are an environmentally and economically optimum solution for heating supply in the future RES-dominated energy systems of Germany [7] and the U.K [8].
Consequently, heat pumps are key components of the energy development strategies in many countries. It therefore leads to a rapid increase of the market demand. For example, in the European Union, between 2005 and 2014, an average of 720 000 units were installed each year. In 2014, the total European heat pump stock was of 7.5 million units. With the continuous tightening of the building energy efficiency regulations and the enthusiasm for the renovation sector, the heat pump market presents a significant growth perspective. It is estimated that, if all the European countries had the same market penetration as Sweden, there would be 36.9 million heat pump units running in Europe in 2020 and 85.9 million in 2030 [9].
Responding to the large public interest for heat pump technologies, industries and research teams continuously endeavour to develop new cost-effective technical solutions to be production oriented. Numerous technology options could be viable alternatives to the conventional vapour-compression devices for heating, ventilation and air conditioning (HVAC) applications in the built environment. This thriving topic of research and development covers diverse complex physical phenomena leading to technologies with various degrees of potential, maturity and limitation for heating and cooling purpose [10]. Among them, the application of caloric effects in solid refrigerant materials coupled with a sustainable heat transfer fluid is gaining large attention. These caloric effects are caused by a phase transition in the material resulting in a large adiabatic temperature change. Depending on its nature, the caloric effect inside certain materials is induced through a change of a specific parameter of its surrounding environment. The main caloric effects are as follows:
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Electrocaloric effect: adiabatic temperature change by variation of electrical field [11,12].
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Barocaloric effect: adiabatic temperature change by variation of hydrostatic pressure [13].
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Elastocaloric effect: adiabatic temperature change by variation of uniaxial mechanical stress [14,15].
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Magnetocaloric effect: adiabatic temperature change by variation of magnetic field [16,17].
In addition, the concept of the active regenerative cycle can be applied to these caloric effects. It allows the device to transfer heat up to the temperature scale in a useful temperature span for HVAC applications, which is much larger than the sole adiabatic temperature change due to the caloric effect [16].
Among these aforementioned potential alternatives to conventional vapour-compression devices, the magnetocaloric-based technology is currently the most studied and developed of all [18]. The magnetocaloric effect (MCE) is a reversible temperature change occurring in a magnetocaloric material (MCM) when subjected to an adiabatic magnetization or demagnetization. When a magnetic field is applied to the MCM, its magnetic entropy decreases and its temperature increases. Reciprocally, when the MCM is demagnetized, its magnetic entropy increases and its temperature decreases. This phenomenon can be ingeniously employed to create a cooling/heating thermodynamic cycle for heat transfer purpose. The reversible nature of the MCE in the solid refrigerant material enables the possibility of developing an efficient heat pump technology with COPs higher than conventional vapour-compression devices. Several types of materials and compounds present a MCE. Their magnetocaloric response is maximum at the ferro-to-paramagnetic phase transition occurring at their respective so-called “Curie temperature”. Gadolinium is commonly designated as reference material for the MCE at room-temperature [16,17].
A century ago, the MCE was observed for the first time by Weiss and Piccard [19]. The former was later explained in 1926 by Weiss, Forrer [20] and Debye [21]. It was then suggested by Debye [21] and Giauque [22] that MCE could be used for extremely low temperature cooling purposes. By the mid-1930s, magnetocaloric cooling devices were commonly used in laboratories to achieve absolute temperatures below 1 K [23]. In 1976, Brown experimentally demonstrated that a Gadolinium-based magnetocaloric device could be used for near room-temperature heating/cooling applications [24]. A major breakthrough was achieved by Barclay and Steyert when they developed and patented the active magnetic regenerator (AMR) cycle in 1982 [25]. With this configuration, the magnetocaloric solid refrigerant is used as a thermal regenerator. This allows a simplification of the device and a significant increase of the temperature span above the adiabatic temperature change induced by the MCE alone [16]. The AMR design is considered to be the most thermodynamically efficient for magnetic heating/cooling devices [26]. Consequently, the AMR configuration has been the foundation principle of almost all the magnetic cooling/heating systems ever since [16].
In the following decades, the magnetic heating/cooling technology gained popularity. Multiple laboratories and research groups built and tested their own AMR prototypes. Zimm et al. reported in 1998 that their near room-temperature magnetic refrigerator using superconducting magnets could operate with COPs above 6 [27]. A large MCE can be generated with the use of superconducting magnets, but these are costly and can be impractical for HVAC applications in buildings. Therefore, most of the recent AMR prototypes are using permanent magnets. In 2012, the rotary AMR device of Engelbrecht et al. was operating with a no-load temperature span of 25.4 K and a maximum cooling capacity of 1010 W [28]. In 2013, Okamura and Hirano tested a magnetic refrigerator operating at a COP of 2.5 with a temperature span of 5 K [29]. In 2014, Jacobs et al. presented a prototype generating 2502 W of cooling power with a 12 K temperature span and a COP above 2. In addition, the machine reached up to 3042 W of cooling power at a zero-temperature span [30]. In 2015, an Italian research group investigated experimentally a magnetic refrigerator coupled directly to a vertical borehole ground source heat exchanger (GSHE). The device could produce cold water below 15 °C for a cooling capacity of 190 W at a COP of 2.2 [31]. In another publication, the same research group indicated that this AMR device could operate at a maximum no-load temperature span of 11.9 K, and a maximum COP of 2.5 for a thermal load of 200 W [32]. In 2016, a Brazilian team reported that their magnetocaloric device had a maximum zero-span cooling power of 150 W and a maximum no-load temperature span of 12 K. With a thermal load of 80.4 W, the machine generated a 7.1 K temperature span with a COP of 0.54 [33]. The same year, a research laboratory at the Technical University of Denmark presented an AMR device capable of reaching 81.5 W of cooling power with a COP of 3.6 and a 15.5 K temperature span [34].
A comprehensive explanation of the magnetocaloric effect, materials and systems can be found in the publications of Smith et al. [16] and Kitanovski et al. [17]. The AMR system principle is described in detail in the articles of Engelbrecht et al. [28] and Lei et al. [35].
In line with the global endeavour to improve heating and cooling systems, the main goal of the ENOVHEAT project [36] is to develop, build and test the prototype of an innovative heat pump for building applications based on AMR technology (see Fig. 1). In addition to great potential for high COPs, AMR-based devices have the benefit of a low noise level operation, an absence of greenhouse or toxic gases, and the possibility for recycling the magnets and the magnetocaloric materials [16].
A previous study from the ENOVHEAT project [37] numerically demonstrated for the first time that such a magnetocaloric heat pump (MCHP) can be implemented in a low-energy single family house and provide for its space heating needs under cold weather climate conditions (Denmark). In this project, the MCHP is used solely for indoor space heating because domestic hot water production requires a higher temperature span. Nevertheless, this is an encouraging starting point for the development of this promising technology for building applications.
As shown in the GeoThermag project [31], a MCHP can be directly coupled with a GSHE. Similarly, in the ENOVHEAT project, a MCHP is integrated in a single hydronic loop including a vertical borehole GSHE and a radiant under-floor heating (UFH) without an intermediate heat exchanger or hot water storage tank. As shown in Fig. 2, this specific MCHP system configuration can run with appreciable COPs. It should be noted that these results are taking into account all losses and inefficiencies from the entire heating system and GSHE. Therefore, the results of Fig. 2 can look different from the ones of publications reporting COPs of the sole AMR operation. The MCHP of the current study can produce 2600 W of useful heating power with an average seasonal COP of 3.93 when operating at fluid flow rate of 2200 L/h and with a rotation frequency of 1 Hz [37].
One can also observe in Fig. 2 that the system’s COP drops dramatically above an optimum fluid flow rate. This is a typical behaviour for AMR-based systems [[38], [39], [40]]. In the case of passive thermal regenerators, small utilization factors associated to low fluid flow rates are favourable to obtain high regenerator effectiveness. However, for AMR, utilization factor and fluid flow rate must be adjusted according to the device’s characteristics and operation conditions in order to achieve optimal performance. At low fluid flow rates, despite the temperature span being significant, the small amount of heat transfer fluid passing through the active regenerator cannot produce a significant heating power. Then the heating power output and COP of the MCHP increase with the fluid flow rate until reaching an optimum point. Above this optimum point, further increase of the fluid flow rate disturbs the temperature profile of the AMR. This causes a drop of the temperature span and, consequently, a collapse of the MCHP heating power output. In addition, a higher fluid flow rate induces higher pressure losses in the hydraulic system, which also lessens the system’s COP. In order to increase the maximum achievable heating power at a higher fluid flow rate, it is necessary to increase the operation frequency of the MCHP. At a higher operating frequency, more AMR cycles are performed over a given period of time. Therefore, the rate of magnetic work performed on the MCM is increased together with the maximum power output. However, operating efficiently a MCHP at higher frequencies leads to new technical challenges [40].
In addition, if the MCHP is implemented in a multi-zone building with a simple fluid flow rate controller and independent thermostats in each thermal zone, the heating system operates on partial-load most of the time. As illustrated in Fig. 3, this leads to modest performances (average seasonal COP of 1.84) compared to the optimum operation point of this MCHP [37]. The development of control strategies adjusting fluid flow and operation frequency of the MCHP for efficient partial-load operation is thus very important [41].
The major drawback of RES is the intermittence of power generation. The increasing share of RES in electricity grids can thus induce major mismatch between instantaneous energy use and production, leading to grid instability. A paradigm shift is occurring in the field of energy system management and the concept of Smart Grids with massive RES penetration is emerging. The future Smart Grid systems are energy grid networks which can intelligently integrate the actions of all users connected and sending information to them, in order to efficiently provide sustainable and reliable energy supplies. These grid users are the energy producers, energy suppliers, energy end-users (which can also be energy producers: “prosumers”), and energy storage systems [42]. Consequently, the continuous increase of intermittent RES in the global energy mix induces a fast growing demand for energy storage and energy end-user flexibility [43]. For that matter, the building sector should not be considered as a simple passive end-user but, on the contrary, as a major active player which can help regulating the electricity production and consumption. Buildings’ energy demand can be modulated by means of thermal storage, HVAC usage adjustment, electric vehicles charging scheduling, plug-loads shifting, etc. These demand side management measures are commonly denominated as “Building Energy Flexibility” strategies [44].
As mentioned before, the heating need for indoor space conditioning is a major target for building efficiency improvement. It is also the case for building energy flexibility strategies. The large potential for thermal energy storage (TES) in buildings can be cleverly employed to shift heating use in time and thus reshape the overall power profile. To that end, it was found that passive TES in the indoor environment and building structural thermal mass is more cost effective than enlarging storage water tanks [45]. In conventional vapour-compression heat pump (VCHP) systems, most of the flexibility is provided by an accumulation hot water tank. However, there is no water tank in the current MCHP system implementation [37]. Indoor temperature set point modulation for passive TES in the indoor space could therefore be a solution to enable building energy flexibility [46] and improve the MCHP operation by increasing its running time at highest COP.
The current numerical study extends the previous research conducted on the integration of this innovative heat pump in dwellings [37] by implementing a new control strategy taking advantage of the building energy flexibility potential. Firstly, the MCHP system and the building study cases are described. The heat storage control strategy for improved heating system performances is then presented. Finally, the benefits of this control strategy concerning the heat pump operation are discussed and followed by a conclusion and suggestions for further work on that research topic.
Section snippets
Characteristics of the magnetocaloric heat pump
In this study, the MCHP is a rotary AMR-based device (see Fig. 4). The refrigerant is a solid MCM (Gadolinium) arranged as packed bed sphere (450 μm diameter) in trapezoidal shaped-cassettes regenerators (see Fig. 5). The tapered regenerators have a length of 59 mm, a height of 17 mm, an average width of 61.43 mm, and a gradient angle of −10 degrees [39]. Gadolinium is a well-known rare-earth element experiencing a large adiabatic temperature change when magnetized in the vicinity of its Curie
Building study case
The building study case of the ENOVHEAT project for testing the integration of the MCHP is a single family house in Denmark. This single-story dwelling has a heated floor surface area of 126 m2 and a geometry which is typical for modern Danish houses (see Fig. 7). It is a low-energy building with a yearly heating need of 16 kWh/m2 [48].
In the case of TES in the indoor environment, the building envelope performance and the structural thermal inertia are the main parameters determining the heat
Heat storage control strategy
As mentioned before, the original MCHP controller is simple and causes the heating system to operate with modest performances. This simple control strategy is based on a basic fluid flow regulation. Each of the UFH sub-circuit is equipped with a valve regulated by an ON/OFF controller. The valve is fully open (fluid volume flow rate of around 240 L/h) when the operative temperature of the room is below the indoor temperature set point. The valve is closed when the operative temperature of the
Results and discussion
The simulation results presented hereafter correspond to a four-month heating period from the 1st of January to the 30th of April under Danish weather conditions [50]. The “heat storage temperature span” is defined as the difference between the minimum and the maximum temperature limits.
Conclusion
This numerical study has tested the use of a heat storage strategy in order to improve the operation performance of a magnetocaloric heat pump integrated in a low-energy Danish residential building. This innovative heating system can be implemented in a single hydronic loop including a vertical borehole ground source heat exchanger and a radiant under-floor heating without intermediate heat exchanger or hot water storage tank. Because the magnetocaloric heating system presents a maximum
Acknowledgements
This work was financed by the ENOVHEAT project which is funded by Innovation Fund Denmark (contract no 12-132673) and was carried out partly within the framework of IEA EBC Annex 67 Energy Flexible Buildings.
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