RESEARCH OF SINGLE ROOM DECENTRALIZED HEAT RECOVERY UNIT

New devices emerging on the market such as small ventilation equipment contribute to the need for research on their compliance with manufacturers’ data. They are new, poorly known and still not very popular. However, they have great potential to ventilate single rooms. Moreover, they are compact and inexpensive. One of them is single room decentralized heat recovery unit, commonly called compact recuperator, which can be installed in almost every building in the external partition, away from sources of pollution. It is characterized by a very simple construction. Its task, in addition to providing fresh air, is heat recovery using a small accumulated heat exchanger. Together with the accumulated heat exchanger, according to the manufacturers, it is able to obtain up to 90% of heat recovery from exhaust air. It is an energy-efficient way of ventilation of single and residential rooms in the temperature range of -20 to 50°C.

The compact recuperator is a part of a decentralized, single pipe ventilation system. Typical decentralized ventilation (DV) is a type of traditional mechanical ventilation that is implemented in a similar method to fan-assisted natural ventilation. However, fresh outdoor air is supplied and distributed into the room by passing through a compact decentralized air-handling unit rather than being utilized directly [1, 2]. Compared to centralized ventilation, DV has many advantages. DV minimizes duct space, can simplify individual zoning control in spaces and is less influenced by outdoor environmental condition [2]. Furthermore, the use of decentralized installation results in reduced operating costs. However, there are also some disadvantages: possibility of location in buildings and compliance with standards [3] in terms of air inlets and outlets location. This is the reason that sometimes they technically cannot be used [4].

Nowadays, the world attaches great significance to the saving of energy. Energy-efficient construction focuses mainly on the continuous reduction of thermal losses [5]. The airtight construction required by the current state of the art of the building shell increases the need for fresh ventilation air. According to standards [3], heat recovery is obligatory in installations with an air volume above 500 m³/h. Most often this is accomplished by recirculating air or just by the use of equipment for the recovery of heat and moisture. In small detached houses and in residential buildings, heat recovery devices such as recuperators are still seen as expensive and luxurious. However, in the future, in the light of new requirements, they will become a standard. Therefore, a compact recuperator is a noteworthy alternative, in particular for modernized and emerging residential and commercial buildings. The need for using such devices was also discussed by Amanowicz & Szczechowiak [6]. Considering the need to introduce solutions using heat recovery, also with a small supply air stream, in order to obtain a low primary energy ratio, the use of such solutions should soon be common in Poland.

The one-pipe ventilation system is relatively unknown and poorly supported by research. Most of the information comes from catalogs and brochures from manufacturers. In the literature, experimental and theoretical study of the heat exchanger with a periodic change in the flow direction for room ventilation [7], and stochastic analysis of compact plate-fin recuperators for microturbines [8] can be found. Research relating directly single room ventilation units with recuperative or regenerative heat recovery was conducted by Manz et al. [9]. More research results can be found directly on DV in the field of its energy analysis, for example [2, 10].

The aim of this study was experimental investigation of the work of the compact recuperator in real conditions. The device’s efficiency was determined based on measurements results and compared with manufacture’s data.

The compact recuperator offered by Alnor was tested (Fig. 1). A unit named HRU-WALL-100-25 with the amount of air flow of 25 m³/h was purchased in December 2018 for the price of PLN 1160. It is the smallest of the two types of units offered by the manufacturer. The exact dimensions of this device were given in Fig. 1.

Figure 1.

As stated by the manufacturer, HRU-WALL is a single room decentralized heat recovery unit for concealed installation. The unit comes with a ceramic heat exchanger which boasts a maximum heat recovery ratio of 90% (the nominal heat recovery ratio is 74.3% as per standards [12]). The energy-efficient EC fan changes the running direction every 70 seconds to alternate between air supply and air exhaust. Low energy consumption and extremely low operating noise make this heat recovery unit a recommended solution for non-stop operation. The heat recovery unit has three speed levels to choose from, depending on the size of the area and indoor demands. This device operates in the outdoor temperature rangeing from -20 to 50°C. It weighs 2.4 kg and belongs to the “A” energy class [9].

Measurements of air temperature and airflow rate were carried out for the tested recuperator. The first tests took place in winter, at a low outdoor temperature in order to measure the air temperature in the individual parts of the device during its operation in heat recovery mode. Fig. 2 shows the test stand. It consisted of the recuperator mounted in the plate with thermal insulation placed in the window frame together with the switch; five thermocouples connected to the thermometer, which recorded the measurement data; and the micromanometer for measuring the pressure difference between the room and the external environment.

Figure 2.

Prepared test stand enabled change of the pressure in the room by installing two small fans in the door with adjustable performance.

Mentioned thermocouples were arranged as shown in Fig. 3: in the outdoor environment; in the air intake; between the heat exchanger and the fan; behind the fan (air inlet to the room); and in the room.

Figure 3.

Temperature measurements in the indicated 5 measurement points were taken for three fan speeds in two different conditions: (1) in real conditions with natural air flow; (2) for balanced pressure indoors and outdoors. The measured temperatures were used to calculate the actual efficiency of the tested recuperator.

Measurement of air velocity distribution flowing through the device was also carried out in order to determine the air volume flow rate. The test was made using a thermoanemometer. The use of other measurement methods was in this case impossible. Prepared test stand (Fig. 4) consisted of tested recuperator together with a switch, a cardboard box with an air intake, a thermoanemometer installed in the channel, a laptop with a software for recording measurements and a micromanometer. Small fans were also attached to the box. Their speed was regulated using the power supply with adjustable voltage.

Figure 4.

Open-box reproduced conditions without differential pressure on both sides of the device; closed and sealed – overpressure conditions on the outside. Mounted adjustable fans enabled the change of pressure inside, measured by the micromanometer.

The air velocity was measured in 6 points at two axes of the device channel, in accordance with the standard [13]. All fan speeds were considered, for supply and exhaust.

For correct and accurate placement of the thermoanemometer in the tube, the tube halves printed in a 3D printer were used (Fig. 5).

Figure 5.

The measured air velocity values were averaged and the air flow volume rate for each fan speed for supply and exhaust was calculated. It was based on a product of averaged velocity and area of channel.

The graph shown in Fig. 6 shows the distribution of measured air temperatures for the second fan speed when pressures in the room and outside were balanced. The indoor and outdoor temperatures were almost constant during all measurements. The remaining temperatures show 70 seconds of air supply and exhaust operation in heat recovery mode. It clearly shows five work cycles of the HRU-WALL-100–25 compact recuperator. There was also a noticeable difference between the indoor temperature and the temperature measured between the exchanger and the fan. Theoretically, the difference should be very small, because there were no elements that could decrease or increase the air temperature. This may be due to the fact that in addition to the heat exchanger, heat accumulation was also carried out on other elements of the device such as fan elements.

Figure 6.

In order to obtain the efficiency of the device, the averaged air temperature in measured 5 points for 3 fan speeds was calculated, for the time step of 5 s. Table 1 and Table 2 show the calculated efficiency of the unit separately for supply and exhaust air. The exhaust efficiency means the amount of energy kept by the device in relation to the total energy-related to the temperature difference between the indoor and outdoor air, whereas supply efficiency, the amount of energy supplied to the air by the device in relation to the total energy necessary to warm the air to room temperature.

The efficiency was calculated from the ratio of heat supplied/received from the heat exchanger (Σ _q1) to the sum of that energy and energy needed for the air to reach room temperature (Σ _q2): (1) η = Σ q 1 Σ q 1 + Σ q 2 , %

The values of air flow volume rate used for calculations (converted into mass flow rate) were measured values (Table 3–5). It has been assumed that the average air flow volume rate is identical to the calculated average value from the measurements, and the ratio of the amount of supply air to exhaust air was adopted so that the amounts of energy delivered to the device and consumed were the same. The calculations were made in Microsoft Excel with the use of the Solver tool.

Air flow volume rate, according to manufacturer data, should be as follows: 25 m³/h for the third fan speed, 18 m³/h for the second fan speed and 10 m³/h for the first fan speed. Thus, the test confirmed the accuracy of the data provided by the manufacturer. Results differed by a maximum of 5 m³/h for atmospheric pressure. Moreover, the device was characterized by unevenness of the air flow in relation to the exhaust and higher efficiency of exhaust than supply mode. Therefore, the air velocity profiles for supply and exhaust mode were different, as shown in Fig. 7.

Figure 7.

On the air velocity profile for exhaust mode, the air was blown towards the channel walls. At the walls, the air velocity value was the highest. A small amount of air flowed through the channel. The centrifugal force produced by the fan threw air at the sides of the channel. Air velocity distribution was more even for the air supply, however, slightly more air flowed over the upper part of the channel, probably due to the fact that the intake has an inlet at the bottom. Air velocity values were lower than in the case of exhaust mode.