자기냉동 Magnetocaloric refrigeration
Magnetocaloric technology operates on the magnetocaloric effect, a phenomenon in which a paramagnetic material exhibits reversible temperature change when exposed to a changing magnetic field.
자성재료에 자기장을 걸어주면 가열되고 가지장을 제거하면 냉각되는 현상인 자기열량효과(magnetocaloric effect, MCE)를 이용하여 저온을 생성하는 방법을 자기 냉동이라고 한다.
즉, 자성을 띄는 물체에 외부 자기장의 변화를 주면 온도도 변화한다. 자기열량효과는 모든 자성을 띄는 재료에서 나타난다.
Magnetic cooling operates on the magnetocaloric effect, a phenomenon in which a paramagnetic material exhibits reversible temperature change when exposed to a changing magnetic field. A magnetic cooling system applies a magnetic field to a paramagnetic material. This aligns randomly oriented electron spins in the paramagnetic material (A→B in Figure C-1), an exothermic process that raises the material’s temperature and causes it to reject heat to its surroundings (B→C). Upon removal of the magnetic field, the magnetic spins return to their randomized state, an endothermic process that cools the material (C→D). The material then absorbs heat from the space to be cooled (D→A). During this step, the paramagnetic material returns to its original state and the cycle starts again.
가지냉동은 자기장 변화가 생기면 온도의 변화가 생기는 자기열량효과를 이용한다. 자기냉동 시스템은 자기성 물질이 자기장에 존재할 때 적용한다.
A→B : 처음에 자기성 물질은 무질서한 방향으로 전자 스핀하고 있는데, 이를 가지런하게 만든다. 외부에서 흡열을 하여 물체 온도 상승 (흡열)
B→C : 뜨거워진 물체가 외부공기로 발열 (발열)
C→D : 자기장을 제거하면, 자기 스핀은 다시 무분별한 상태로 돌아가고 물체는 냉각된다. (발열)
D→A : 외부공기보다 더 차가워진 물체는 외기로부터 열을 흡수한다. (흡열)
Figure C-1: Magnetic cooling cycle
Source: Goetzler et al. (2009)
The temperature gradient and subsequent capacity of magnetic cooling systems varies with the strength of the applied magnetic field. Dieckmann et al. (2007) report that existing permanent magnets suitable for air-conditioning applications can only produce a magnetic-field strength of up to 2 tesla (T), yielding a maximum temperature change of only 9°F. Although stronger magnetic fields could induce greater temperature changes (e.g., 10 T could produce a temperature change of 45°F), obtaining such strong magnetic fields would require superconducting electromagnets that draw significant power. This parasitic energy consumption could negate some or all of the efficiency gains associated with magnetic cooling.
Therefore, some type of regenerative cycle is necessary for magnetic cooling cycle to be viable for space cooling. One approach to accomplish this is the active magnetic regenerator cycle prototyped by Astronautics Corporation of America. This cycle uses a bed of magnetocaloric materials layered with materials having progressively higher Curie temperatures.11 By successively applying a magnetic field to the bed (and thus shifting the temperature gradient across the bed) and coordinating the flow of coolant, the temperature difference between the high and low sides is spanned regeneratively, and heat can be absorbed from the cold source (the cooling load) and rejected to the higher temperature sink (Boeder et al. 2006). Figure C-2 presents the concept of the active magnetic regenerator cycle.
Figure C-2: The active magnetic regenerator cycle
Source: Boeder et al. (2006)
캐나다와 불가리아 연구진이 Applied Physics Letters에 발표한 논문에서 다중강성 합성물 HoMn2O5를 이용하여 자기열량효과를 이용할 수 있다고 밝혔다.
다중강성(multiferroic) 합성물인 HoMn2O5은 자기장이 변할때, 절연 행동을 보이는 물질이다.
그러나 매우 놀랍게도, 연구진은 거대한 자기열량 효과가 자기장 영역의 내부와 외부로의 이동 없이도, 그저 단순히 HoMn2O5 결정을 일정한 자기장 내에서 회전시키는 것만으로 얻어질 수 있다는 것을 발견했다(이는 표준 자기열량 효과를 드러내는 물질에 해당한다.). 이 발견은 자기 냉각 기술을 개발하는 데 있어 매우 중요한 단계이며, 가정적으로나 산업적 응용을 위한 효율적인 “친환경적” 냉각 시스템 구축을 위한 발판이 될 것이다. “회전 자기 열량 효과를 이용한다는 것은 냉각 장비로 흡수된 에너지를 크게 줄일 수 있다는 의미이기도 하다. 이는 단순하고, 효율적인 소형 자기 냉각 시스템의 미래 구축에서 새로운 문을 열 것”이라고 Balli가 말했다. 다음 단계로, 연구진은 HoMn2O5 결정과 이에 관련한 물질에서의 회전 자기 열량 효과를 향상시키는 가능성을 탐구하려고 한다.
Technical Maturity and Current Developmental Status
Although many institutions have been working on the magnetocaloric effect for the past 40 years, equipment using the magnetic cooling cycle is not yet commercially available. According to research publications, including Dieckmann et al. (2007), Liu et al. (2009), and Gschneidner et al. (2008), current research efforts have focused on either: a) improving the cooling capacity of prototype systems using current magnetocaloric materials and permanent magnets; or b) identifying or developing new permanent magnets and magnetocaloric materials. Most of these efforts focus on near-room-temperature refrigeration applications.
A number of leading scientists and engineers from around the world have formed a working group on magnetic refrigeration in the IIR (International Institute for Refrigeration) to promote magnetic cooling as a viable, energy-efficient and environmentally friendly cooling technology. Leading RD&D entities include the Center for Neutron Research (Liu et al. 2009) at the National Institute of Standards and Technology (NIST), University of Maryland and Iowa State University.
DOE is funding a magnetic cooling project co-researched by Astronautics and Ames National Laboratory through the ARPA-e program (ARPA-e 2013). Past prototypes constructed by Astronautics have had COPs of approximately 2.0; however, the goal of this project is to develop a 1–ton magnetic air conditioner with a COP of 4.0 that can also fit within the envelope of a window A/C unit. According to lead researcher Dr. Steven Russek, although Astronautics has not yet integrated all the components into a fully-functioning prototype system, their work is converging nicely toward this goal. Astronautics is eager to work with an industrial partner to help propel its technology to commercialization (Russek 2013).
Two companies claim to be in the process of commercializing magnetic systems for refrigerators. One, the French firm Cooltech, has announced that it plans to launch a magnetic refrigeration system for retail cold counters by the end of 2013, while the other, Camfridge, is working with Whirlpool to launch a domestic refrigerator within two years (Gaved 2013). However, neither of these companies has published any documentation of their prototypes’ performance or shared their progress with other players in the magnetic cooling field (Russek 2013).
Barriers to Market Adoption
A potentially significant barrier to the market adoption of magnetic-cooling technology is the volatile nature of the global market for rare-earth magnets, as described further in the “Cost, Size, and Complexity” section below.
Energy Savings Potential
Potential Market and Replacement Applications
Magnetocaloric technology is technically applicable to all heating and cooling applications for residential and commercial buildings. This technology is also technically applicable to all climate regions and building types. Because the electron spin alignment in paramagnetic materials is reversible, magnetocaloric technology can be used for heat pumps as well as air conditioning.
According to Gschneidner et al. (2008), the magnetic refrigeration system has the potential to reduce energy consumption by 20% over a conventional vapor-compression system.
If the prototype being developed by Astronautics succeeds in achieving an overall COP of 4.0, magnetocaloric cooling could save approximately 20% of energy compared to baseline vapor-compression technology. Dr. Russek asserts that this will represent the highest COP and largest cooling power ever demonstrated by magnetic cooling and that its COP could only improve with the use of better components (e.g., pumps).
Cost, Size, and Complexity
A credible projection of costs for magnetocaloric technology in HVAC applications is not currently available given the early stage of development. However, Dieckmann et al. (2007) and other publications note that the permanent magnets used to induce the magnetocaloric effect account for a significant portion of the cost of the prototype systems developed so far. Political factors (e.g., trade restrictions by China, which accounts for 95% of the world’s supply of rare-earth metals (Bell 2012)) are undeniably affecting market stability, but perhaps a more important factor is the increased demand for neodymium from other sectors. Witkin (2012) notes that increased demand of neodymium magnets for hybrid vehicles, electronics, and wind turbines is causing the shortage of neodymium worldwide, causing stakeholders to push for alternative materials. DOE’s ARPA-e program is funding several such projects under its Rare Earth Alternatives in Critical Technologies (REACT) grant program. However, RD&D efforts to improve the magnetocaloric materials and heat exchangers of magnetocaloric systems should allow the required size of the magnet to shrink, thus reducing the need for rare-earth materials (Russek 2013). According to Russek, magnetocaloric materials themselves are extremely inexpensive.
Although the prototype being developed by Russek’s team is supposed to fit within the envelope of a 1–ton unit, it is currently closer to the size of a 3–ton unit due to the size of the heat exchangers that were required to utilize a relatively small magnet. However, further development work could optimize these components and thus drive down the overall system size.
The magnetocaloric system requires several moving parts, including an electric motor to drive the active regenerator cycle and pumps to circulate the heat transfer fluid (Russek 2013).
Peak-Demand Reduction and Other Non-Energy Benefits
Because magnetocaloric systems are projected to operate at higher efficiencies than conventional systems, they could provide a commensurate reduction in peak demand.
Russek (2013) asserts that the pumps and motor utilized in his team’s prototype are significantly quieter than the parts found in a vapor-compression system.
Next Steps for Technology Development
Researchers have investigated magnetic cooling for many years but are seeing recent improvements through technological breakthroughs in materials science and related fields. These advances can continue the development of the magnetic cooling cycle by reducing the cost of magnetic materials. Assuming researchers can address this central challenge, magnetocaloric technology will become a more attractive option for HVAC applications.
Moreover, although magnetocaloric technology can technically be used in space-heating applications, we were unable to locate any documentation of any space-heating prototypes or performance data. Testing will reveal the capabilities of magnetocaloric technology in space-heating applications.
Table C-3 presents the potential next steps to advance magnetocaloric technology.
자기열량효과는 온도와 자기장 변화의 함수로 단열온도변화 또는 등온 자기엔트로피변화로 측정, 계산된다.
자기장 변화에 따른 온도변화를 측정하는 방법
자기장 변화, 온도 변화에 따른 자화(magnetization)나 열용량(heat capacity) 측정 방법
- 단열된 재료를 자기장에 노출시켜 온도변화를 직접 측정하는 방법
- 자화를 측정하여 Maxwell 관계식을 이용하여 자기엔트로피 변화를 계산하고, 자기장이 없을 때의 열용량을 가정한 후 단열온도변화를 계산하는 방법
- 자기장 없을 때와 있을 때의 열용량 변화량과 자기엔트로피 변화량을 계산하는 방법
2014. 05. 26 작성