Abstract
In this work, thermal, humidity and enthalpy recover efficiency of innovative energy recovery exchanger is presented. The system under analysis allows adjustment of the humidity recovery especially useful in the winter period and forefend energy use for an anti-froze system of energy exchanger. It is shown that the presented method can achieve the real value for humidity and thermal efficiency above 80% and 90%, respectively. Such high efficiency was possible to obtain because the proposed system does not require energy consuming anti-freeze systems. The presented system is able to work even in extremely adverse outdoor air conditions (-20°C and humidity 100%).
Introduction
In 2010 the EU introduced the Energy Performance of Buildings Directive (EPBD) on the energy efficiency of buildings, which states that after 2019 all public buildings will have to fulfil the requirements for nearly zero-energy buildings (nZEB) [1]. The ventilation, heating, hot water and cooling systems of buildings are responsible for half of the final energy consumption in the construction sector. Follow the national requirements defining the nZEB, these systems need to be designed to meet the criteria for the maximum use of non-renewable primary energy for ventilation, heating, domestic hot water and cooling. With the growing share of energy consumption, heat recovery seems to be essential solutions able to save signif-icant amounts of energy [2-4]. However, this required a new solution for ventilation able to maintain the desired indoor air quality [5, 6].
Ventilation, heating and air conditioning units are increasingly applicable in modern industry and for domestic purposes. Public buildings and private houses are often equipped with ventilation and air conditioning systems [7, 8]. The key part of such a system is a heat exchanger which is responsible for efficient energy recovery. In most solutions, parallel-plate heat exchangers are used. This type of heat exchanger was analysed by Vera and Linan [9] to provide formulations for construction design. Authors developed a 2-D model to evaluate ex-pressions able to describe parallel-plate heat exchangers. Detailed experimental and numerical analysis of the fluid-flow within the heat exchanger with elliptical tubes was performed in [10, 11]. For the parallel-plate channel flows DNS numerical simulation [12] provide very accurate results. Kragh et al. [13] analysed a new solution for counterflow heat exchange used in ventilation systems in low temperature areas. The performance of the heat exchanger was calculated based on the model and validated against experimental data. In another research, the influence of channel geometry on the performance of a counter flow heat exchanger was studied [14].
The most commonly used heat exchangers have a maximum efficiency of 90% [15]. However, this relatively high value is received under carefully selected laboratory conditions and may differ under real conditions, especially under low outside air temperatures [16]. With the growing share of heating loads and ventilation, the heat recovery seems to be the main solutions to reduce heat losses in well-isolated buildings [17]. In residential buildings, ventila-tion plays a substantial role in the total heat loss. Heat losses resulting from the ventilation can be in the range of 25-55%. For this reason, it is impossible to fulfil the regulations about an energy-efficient without well designed mechanical ventilation with the heat recovery. The catalogue sheets of heat exchangers usually delivery the temperature efficiency under opti-mum or standard conditions. However, the velocity of the air-flowing through the exchanger channels is an additional parameter, affecting the efficiency but this values it is not standard-ised. Because of that the companies usually based on their own appropriately adapt the air velocity to obtain as high as possible efficiency. During the winter months, the air tempera-ture may fall below –5 C and under such conditions, the danger of heat exchanger icing may occur and at a later stage of total freezing, appears. This can generate a significant decrease in system performance and is caused by water dropping out from the exhaust air and occurs at the contact of the heat exchanger surface with cold air drawn from the outside (air tempera-ture decline below the dew point) [18]. Under the influence of negative temperatures, the condensate accumulated in the exchanger starts gradually freezing, causing a significant re-duction of the heat recuperation as well as results in decreasing the effective air-flow surface thereby leading to a substantial increase in the flow resistance. In adverse situations, such an effect can lead to irreversible mechanical changes and system damage.