energies Review A Review of Heat Recovery in Ventilation Ewa Zender–Swiercz´ Faculty of Environmental, Geomatic and Energy Engineering, Department of Building Physics and Renewable Energy, Kielce University of Technology, 25-314 Kielce, Poland; [email protected] Abstract: The purpose of the article was to present information on heat recovery in ventilation systems and to highlight what has not been sufficiently researched in this regard. A lot of information can be found on methods and exchangers for heat recovery in centralized systems. Decentralized, façade systems for cyclical supply and exhaust air have not been sufficiently researched. It is known that these devices are sensitive to the influence of wind and temperature, hence heat recovery may be ineffective in their case. The literature describes the aspect of heat recovery depending on the location in climatic zones, depending on the number of degree days (HDD). Attention was also paid to the risk of freezing of heat recovery exchangers. The literature review also showed the lack of a universal method for assessing heat recovery exchangers and the method of their selection depending on the climate. Keywords: heat recovery; ventilation systems; recovery efficiency; energy-consumption 1. Introduction Buildings are a key sector in terms of energy users. Their exploitation and maintenance currently consumes up to 40% of the total energy demand in the world [1–6]. Heating, ventilation and air conditioning (HVAC) systems account for 40–60% of a building’s energy Citation: Zender–Swiercz,´ E. A needs [7–10], and the ventilation systems themselves account for 20–30% [11]. The heat Review of Heat Recovery in demand can be reduced by reducing the building’s heat loss by insulating and sealing the Ventilation. Energies 2021, 14, 1759. building envelope. However, such actions have consequences in the form of reduced air https://doi.org/10.3390/en14061759 exchange, and deterioration of indoor air quality (IAQ). This in turn has a negative impact on the performance and health of people staying in the room [12–15]. In order to improve Academic Editor: Angelo Zarrella the air quality in rooms, mechanical ventilation is used, where a large amount of heat is lost with the exhaust air. In order to reduce energy consumption, the heat energy contained Received: 29 January 2021 in the exhaust air stream is recovered. Its efficiency varies depending on the airflow (more Accepted: 19 March 2021 airflow means less efficiency) and the temperature difference between outside and inside Published: 22 March 2021 (higher gradient means better efficiency). Likewise, the way the air is distributed in the room is important. In the process of designing an installation, it is assumed that fresh air Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in and indoor air are ideally mixed if the flows specified in the regulations are adopted. In fact, published maps and institutional affil- with different locations of air inlets and outlets, the air may not be renewed, and this results iations. in an increase in the amount of air to achieve a minimum IAQ. This obviously increases the amount of energy consumed. A further increase in energy consumption results from the use of highly efficient filters that remove dust particles, which are currently the main public problem [16,17]. At the same time, it should be noted that the energy consumption of the building sector Copyright: © 2021 by the author. will continue to grow. New buildings start using energy, while the energy consumption Licensee MDPI, Basel, Switzerland. of existing buildings never falls but remains constant or rises. In order to globally reduce This article is an open access article distributed under the terms and energy consumption and CO2 emissions, old buildings should be modernised [18–21]. conditions of the Creative Commons Installation of centralised ventilation systems may not be possible in this case [22–25], Attribution (CC BY) license (https:// but there are options to reduce energy consumption. For this purpose, hybrid ventilation creativecommons.org/licenses/by/ systems or decentralised facade ventilation can be used [26–29]. The latter make it possible 4.0/). to adjust the capacity to the current needs of the amount of fresh air. Because of the lack of Energies 2021, 14, 1759. https://doi.org/10.3390/en14061759 https://www.mdpi.com/journal/energies Energies 2021, 14, 1759 2 of 23 ventilation ducts, they are equipped with small fans, which translates into low pressure losses [30]. Heat occurs in sensible and latent form. The sensible form of heat is that where temperature change is visible during the flow of heat. In turn, latent heat is the heat recovered from the moisture contained in the air stream. Commonly-used devices enable the recovery of sensible or total heat, i.e., the sum of sensible and latent heat [31–36]. The ventilation systems with heat recovery as a way to reduce the consumption of heat and cooling energy have been known since the 1970s [37]. The use of heat recovery in airtight buildings can reduce the annual energy consumption for heating and cooling by up to one-third [38–40]. The Energy for Buildings Directive (EPBD) even imposes an obligation to use heat recovery in mechanical ventilation systems [41]. The “Ecodesign” Directive [42] imposes requirements in the European Union on the efficiency of heat recovery. In the case of intermediate systems it is 68% and for the remainder is it 73%. Each investment in the heat recovery depends on profitability, building regulations and the scope of the renovation. Carlsson et al. [43] found on the basis of the conducted research that the combination of modernisation consisting in sealing the building envelope and the use of the ventilation with the heat recovery allows for a 78% reduction in total energy for space heating and 83% reduction in greenhouse gas emissions. The ventilation heat recovery devices include rotating thermal wheels, plate exchang- ers, heat pipes, run-around systems and heat pumps. Each type of heat recovery device has its own advantages, disadvantages and respective applications. The building attributes define the type of heat recovery equipment to be installed [25]. Air handling units with heat recovery can bring significant final and primary energy savings. However, the prof- itability of the analysed units with heat recovery differs significantly depending on the energy efficiency level of the building and the heat supply, the frost level, the energy price development and the real discount rate used for the analysis [44]. In the literature there are a review of types of the heat recovery exchangers and the review of the physical and performance parameters of heat exchangers. There is no review that indicates the types of ventilation systems where heat recovery has not been sufficiently investigated. The review described in the literature do not include the influence of climate on the heat recovery. The article reviews the available heat recovery technologies, the parameters that affect the amount of heat recovery, and the methods of assessing its effectiveness. The aim of the article was to show the gaps in research on heat recovery, to show what else should be investigated and what is not described in the literature. 2. Methods This systematic review identified all studies which investigated heat recovery in ventilation systems. So, for the current review a heat recovery was defined as exchangers using in the ventilation systems. Studies that investigated various types of exchangers, various methods of efficiency assessment and influence of various factors on efficiency were included. The main focus was on the heat recovery in the façade decentralized ventilation and passive ventilation systems. This action was due to the least available test results for the heat recovery in these systems. All included studies were published in the English language and we placed no country, date or publication restriction to the search. Although some studies provide additional details (e.g., methods of efficiency assess- ment or impact of HDD or external temperature on heat recovery), these are not universally provided and there is great variety in the way are categorized making them hard to use in a systematic review. The search strategy used was developed to find peer reviewed full journal articles and abstracts (subject to enough information), grey literature including conference proceedings. Electronic searches included Web of Science (1900–2021), Scopus (1900–2021) and ResearchGate (1900–2021) Database. These databases were chosen as they are key heat recovery databases, and it was unlikely that new studies could have been found elsewhere. Energies 2021, 14, 1759 3 of 23 A search strategy was developed to include all relevant keywords relating to heat recovery, ventilation, and visual search in each resource. In order for a record to be included in the initial search, the study must have included at least one word or phrase from among those entered. 3. Heat Recovery Exchangers (HR) in Heating-Ventilation-Air Conditioning (HVAC) Systems 3.1. Storage Heat Exchangers Storage heat exchangers work alternately, in the first phase heating up from the warm air stream, in the second phase giving up heat to the cold stream. They found their application in decentralised facade ventilation systems [45]. In this case, pay attention to the risk of condensation and to allow the condensation to run off by leading the duct for the installation of wall elements with a slope to the outside of the building. At the same time, the authors state a negligible risk of fungus and mould growth due to constant contact with alternating air flow, large temperature fluctuations, constant air movement and no possibility of stagnation. The spherical packed beds (SPD) as a method of heat accumulation are widely used in chemical reactors, grain dryers, nuclear reactors, and in heat storage in buildings and solar power plants.