Cooling/heating technologies applied in the HVAC industry ranges from simple natural cooling to advanced Direct Expansion (DX)-solutions, and all active cooling solutions are still based on the use of refrigerants in compressor-based systems, where the best available technology are systems with high efficiency compressors and less harmful refrigerants. The conventional active HVAC systems are characterized by high energy consumption, high investment and operation costs, low flexibility and large greenhouse gas emissions (Best and Rivera 2015). The introduction of Eco design renders many of the existing product lines obsolete, forcing the industry to develop new, more efficient products. Thus there is an increasing need to improve the energy efficiency of the built environment without compromising the indoor air quality and thermal comfort levels. This has resulted in the development of various techniques of better usage and conservation of energy for HVAC applications in buildings.
In addition, with the tight regulations and standards regarding HVAC systems and the limitations on the use of conventional systems with harmful refrigerants and working fluids, there is a large potential for environmentally friendly HVAC solutions making use of efficient and innovative technologies.
The potential of PCM as an eco-friendly solution in balancing energy oscillations in a building has been addressed in a number of scientific articles and experimental cases (Kasaeian et al. 2017). The majority of the work concerning the use of PCM for air conditioning in buildings have been concentrated on passive solutions (Ning et al. 2017), mainly integrating the phase change material within the building envelope components. Integrating PCMs in building components like plaster, gypsum board, concrete, and other building envelope materials has been investigated extensively in the literature and tested in projects (Microtek Lab. 2018), (Karaipekli and Sarı 2016).
While passive HVAC applications in buildings employing PCM have been widely investigated and implemented, active solutions and systems are still less mature. PCM-based products are generally on a research and laboratory testing level. So far it has been tested in laboratory settings with promising results and in a few one-off projects, as for example, PCM-Kompaktspeicher project, where a demonstration unit was built in the premises of Rubitherm GmbH, to be supplied with fresh air, which is cooled during the warm summer days by a PCM compact storage device (Stein und Partner 2018).
A number of research and development projects supported by Danish public funding programmes such as ForskEl, EUDP and InnoBooster has explored the potential in using flexibility potential in energy systems (e.g. heat pumps) in balancing the power grid, and also the integration of intelligent buildings is addressed in a series of projects (Energiforskning.dk 2018). There are ongoing projects introducing heat pumps and tele sites on the not yet fully developed demand response market, but this project will be the first to enable companies to introduce and mass market the technical possibility to a wider range of HVAC customers.
Inherent storage capabilities in building energy systems and robustness towards short term fluctuations makes building energy systems an ideal part of the solution to the problem of balancing a system based on renewable energy sources (Danish Ministry of Energy, Utilities and Climate 2016). The use of heat pumps in balancing the electricity grid has been well explored (Sørensen et al. 2013), and numerous projects have been conducted to prove the potential. While entire bulding energy systems are studied in large scale projects (COORDICY 2018), the great potential in ventilation and comfort cooling has yet to be explored on a larger scale in commercial projects. With a combined total energy consumption of 7,6 TJ for ventilation (5,4 TJ) and comfort cooling (2,2 TJ) as compared a total consumption of only 0,6 TJ for heat pumps, the potential in power reductions and flexible consumption is huge (Danish Energy Agency 2015).
Thanks to their substantial latent heat storage capacity, PCMs have a large potential to be employed in active HVAC applications. A large amount of energy can be stored in the isothermal process of melting the PCM, which is what happens when hot air is lead through the PCM module. In Fig. 1, where the principle of using only a sensible energy storage (blue dotted line) is compared to a latent energy storage (solid green line), it is shown that the energy potential is greater when using latent energy storage.
The use of PCM in connection with ventilation systems has been studied in a few earlier investigations. In one study, the PCM is used in a heating application for load shifting during peak hours (Stathopoulos et al. 2016). For that, a prototype of a PCM heat exchanger was built, and a numerical model was developed to study the behavior of the PCM heat exchanger. The study showed that the relatively simple numerical model could accurately reproduce the experimental behavior of the PCM heat exchanger. In another study, PCM was applied in a heat exchanger for a heating application in a ventilation system, to provide load shifting during peak hours (El Mankibi et al. 2015). A numerical model was developed, and a prototype was tested in a rectangular generic room. This study showed that when charging the PCM module at night, the performance of the heat exchanger greatly depended on the insulation of the heat exchanger due to the long periods of time where the PCM should remain charged. And thus, increased insulation prevented involuntary discharging of the PCM. During a large number of test configurations, the study showed that it was possible to perform load shifting without compromising thermal comfort or indoor air quality.