Improvement of ventilation with latent heat storage

VincencButala

REHVA Fellow

Professor

University of Ljubljana,

Faculty of Mechanical Engineering,

Slovenia

vincenc.butala@fs.uni-lj.si

UrošStritih

Slovenian IEA-ECES delegate

Assistant professor

University of Ljubljana,

Faculty of Mechanical Engineering,

Slovenia

uros.stritih@fs.uni-lj.si

RokKoželj

student

University of Ljubljana,

Faculty of Mechanical Engineering,

Slovenia

In the present paper improvement of the air solar collector ventilation system with latent heat storage (LHS) for the heating period is presented. LHS in phase change materials (PCM) improves effectiveness of the ventilation system and resolves mismatch between obtained heat from the air solar collector and its use for ventilation of the office. Application of LHS in the ventilation system increases average temperature of air for ventilation.

Key words: ventilation, latent heat storage, phase change materials (PCM), solar collector, thermal storage.

System operation principle

Operation principle of the ventilation system with the air solar collector and the LHS unit presented in Figure 1 is divided into the heating and cooling mode, meaning that the system can be used during the whole year. Operation principle in the cooling mode consists of two consecutive cycles. The first cycle (charging period) is carried out at night when the cold outside air is supplied with the fan to the LHS unit (number 1 on Figure 1), where air flows around compact storage modules (CSM) filled with the PCM. The heat in the PCM is transferred to the air flow and causes cooling of PCM, which solidifies and in this way accumulates cold. Air is then transported outdoors (2). The second cycle (discharging period) is carried out during daytime when cooling demand occurs. In the second cycle, warm outside air enters in the LHS unit (1) and transfers the heat to the solid PCM, which then melts. In this instance, the air flow cools down and then enters the room (3). In the heating mode, the cold outside air is transported with the fan through the air solar collector (4), where it is heated under influence of solar radiation. Heated air is then transported through the LHS unit (5) where it transfers the heat to the PCM, which liquefies and accumulates the heat in form of latent heat. In the morning or evening, or when the solar radiation intensity is insufficient, the cold outside air is transported through the LHS unit (1) where the PCM solidifies and with that releases the heat. Released heat from the PCM heats the passing air flow, which is than supplied to the room (3) at the temperature level that reduces the risk of thermal discomfort [1].

Figure 1. Schematic principle of ventilation system operation [1].

Air solar collector

The air-heating flat-plate solar collector, a commercial product of SolAir company [2], was installed in the investigated system. The air solar collector has an area of 1.638 m² and it was mounted vertically on the parapet below the office window (90° tilt angle of the collector) on South side of the building of the Faculty of Mechanical Engineering in Ljubljana with 17° azimuth. The solar absorber plate is made of aluminium with fin thickness of 0.2 mm and with fin width of 30 mm. Solar absorptance of the absorber is 93% with tolerance of ±2%. Its hemispherical emittance is 35% with tolerance of ±3%. The absorber is painted with black thickness insensitive spectrally selective (TISS) paint. The air flow channels are connected with no spacing in between and the air solar collector is connected to LHS unit inside of the office. The glazing of the air solar collector is made of extruded solid polycarbonate sheets with thickness of 4 mm and with solar transmittance of 90% ±1%. The thermal insulation is made of polyethylene and it is located on the backside of the air solar collector while there is no sidewise insulation.

Performance measurements of the air solar collector were done at Fraunhofer Institute for Solar Energy Systems ISE in Freiburg, Germany [3]. Power output of the air solar heating collector has been obtained through measurements under the steady-state conditions with the calorimetry method. The efficiency of the air solar collector reduced to the aperture area with radiation of normal incidence is 0.703. The value is obtained at the solar irradiation on the air solar collector plane of 961 W/m², thermal power output of 1290 W and air mass flow rate of 250 kg/h.

Latent heat storage

The casing for CSM plates was made of 8 mm thick PMMA with external dimensions of 725 mm x 460 mm x 420 mm. The LHS unit was thermally insulated with 50 mm thick EPS. The LHS unit contained 29 CSM plates filled with paraffin Rubitherm RT22HC [4] which has the melting range between 20°C and 23°C with the melting peak temperature of 22°C. The heat capacity of RT22HC between 14°C and 29°C is 200 kJ/kg ± 7.5%. The melting temperature of the PCM was chosen in a way to ensure maximum melting and solidification during the operation of LHS with consideration for both heat and cold storage. During discharge period of heat, the PCM should release heat within its own melting range which is within the comfort level. The melting point for used PCM is appropriate for Slovenian climate which is between 22°C and 23°C [1].

Figure 2. The LHS unit with CSM plates (left) and elements of experimental system (right).

On the left side of Figure 2 the LHS unit with visible CSM plates inside the LHS unit is presented. External dimensions of CSM plates were 300 mm x 450 mm x 150 mm and they were horizontally positioned in the LHS where the longer side was perpendicular to the air flow direction. The distance between panels (air gap) was 10 mm. Average mass of the filled CSM panel was 1361 ± 5 g, weight of the PCM (RT22HC) in the panel was 1003 ± 5 g, the volume of each panel was 1.42 L and the volume of the PCM was 1.3 L. Approximately 9% of the panel volume is empty in order to compensate for the volume expansion of the PCM and to avoid deformation of the panel due to higher pressure. Compactness of the LHS is 133 m²/m³ (ratio between surface of plates and volume of plates), density of stored heat is 16 kWh/m³ (ratio between stored heat and volume of LHS) [1].

Other elements of experimental system

Elements of the experimental system are shown on the right side the Figure 2. On the inlet of the LHS unit, the streamer is attached because of the air flow separation possibility. On the outlet side of the LHS unit, a grid is attached for mixing the air flow. Inlet and outlet air ducts that are connected to the LHS unit are made of PVC ducts with 100 mm diameter and are thermally insulated with 20 mm thick thermal insulation. In the inlet duct in front of the LHS unit there is the axial fan with nominal power of 30 W. Fan is connected to the speed regulator which regulates the fan speed according to five different speed settings. The speed regulator is controlled with a programmable time switch. At the same time, the motor hatch is controlled with a programmable time switch. The motor switching hatch is installed in the outlet duct of the LHS unit and it enables to redirect the air flow outdoors or in the office.

Operation of investigated system

The investigated ventilation system operated in the heating mode for continuous ventilation of the office. Operation of the investigated system consists of charging period (Figure 3a) and discharging period (Figure 3b).

In the charging period, the heat is stored in the LHS unit during presence of solar radiation which heats the cold outside air in the air solar collector. Heated air transfers the heat to the PCM in the LHS unit and is then supplied to the office (Figure 3a). The discharging period is taking place when air is not heated in the air solar collector due to insufficient solar radiation. The cold outside air is then heated in the LHS unit, where accumulated heat in the PCM releases and transfers to the passing air flow which is supplied to the office with the suitable temperature level for thermal comfort (Figure 3b).

Figure 3. Ventilation of the office in the heating mode in the charging period (a) and in the discharging period (b).

Experiment

The experiment was performed with constant air flow rate through the LHS unit. The LHS unit stored heat during daytime and released it during night-time by transporting it outdoors. Therefore, the fan operated the whole time and the switching hatch directed air to the outlet duct of the system which led outdoors. As a result, we achieved maximum heat flows because the LHS unit completely released the heat during night time and accumulated maximum capacity of the heat during daytime when solar radiation was present. The measurements were performed over six sunny days at the end of March. Figure 4 shows the measured air temperatures for the mentioned time period: the room temperature (relevant for calculating heat losses of LHS unit), inlet air temperature of the air solar collector, the air temperature at the inlet of the LHS unit (which is essentially the outlet temperature from the air solar collector) and the outlet air temperature of the LHS unit.

The collector inlet air temperature was mainly between 6°C and 34°C with the average temperature of 18°C (Figure 4). On March 28, the air temperature increased up to 45°C. The reason for such high amplitudes was the measurements of the air temperatures near the south side of building envelope (facade), where the air solar collector is installed. The building envelope (facade) was heated by solar radiation and thereby the radiation from the building envelope heated the surrounding air. Higher temperatures near building envelope are favourable from the energy performance point of view as the air temperature close to the facade tends to be higher (sometimes significantly) than the outdoor air temperature.

Figure 4. Measured air temperatures.

During the system operation the peaks of the outlet air temperature from the air solar collector were around 70°C, as we can see from Figure 4. The maximum air temperature reached 74°C while the minimum air temperature was 14°C. The maximum outlet air temperature from the LHS unit was 65°C, at which the temperature level became too high for direct ventilation of the office and we had to reject the excess heat. The minimum air temperature from the LHS unit was 21°C, which was suitable for direct ventilation of the office.

Outlet air temperatures are presented in the organized diagram (Figure 5), where a comparison between different ventilation systems is shown. The comparison is made between ventilation system with no additional system (direct supply of the outside air to the office), ventilation system with the air solar collector and ventilation system with the air solar collector including the LHS unit. The interest of the presented experiment was the comparison between the air solar collector ventilation system with LHS unit and without LHS unit. Difference between outlet air temperatures of the air solar collector and the LHS unit (Figure 5) presents accumulated or released heat in the LHS unit. When the curve of the outlet air temperature from the air solar collector is higher than the curve of the outlet air temperature from the LHS unit, the heat is accumulated in the LHS unit. When the curve of the air solar collector is lower than the curve of the LHS unit, the heat from the LHS unit is released.

Figure 5. Organized diagram of outlet air temperatures.

Conclusion

In this experiment the investigated ventilation system with the air solar collector which includes the LHS unit increased average air temperature of the office ventilation in the temperature range up to 23°C. This statement implies a good potential for the application of LHS in the ventilation system. Outlet temperature from the LHS unit (supply temperature for the office ventilation) with value from 20°C to 23°C appeared in 43% of the total ventilation time which presents the positive impact of the LHS unit on the indoor thermal comfort.

Average daily energy savings of the ventilation system without LHS in the period of the experiment (6 days at the end of the March) were 89%, according to the required heat for covering total ventilation losses. In case of the integrated LHS unit in the ventilation system, maximum energy savings (100%) were achieved in the same time period. This means that ventilation losses were completely covered and no auxiliary heating was required. In the ventilation system without the LHS unit, average energy savings of the ventilation system with the LHS unit were 11% for the period of the experiment, while maximum energy savings were 21% and minimum were 7%. For analysis of the whole heating season it is recommended to consult work of Stritih et al. [5], where analysis on monthly basis was made through numerical simulation of the investigated system.

The availability of energy from renewable energy sources presents a problem in energy supply, when the energy from renewable source is not available at the best possible conditions. That is why the advantage of the short-term energy storage in PCM is in levelling the mismatch between energy supply and demand. PCM can be used in different kind of applications of active ventilation system where energy is stored at the time when the source of energy has the highest potential, and is released when energy demand occurs. Since the indoor thermal comfort is of significant importance, the melting temperature of PCM used in ventilation system is selected at the temperature of indoor thermal comfort because PCM releases heat at almost constant temperature in the latent region of heat.

Acknowledgements

This study was financially supported by the Slovenian Research Agency through the research program P2-0223.

References

[1]     E. Osterman. "Analysis of a latent heat storage unit for heating and cooling of building's space." PhD thesis, Ljubljana (2015).

[2]     SolAir. "Air solar collectors." Solaird.o.o., Celje (2016). Accessible on: http://www.solair.eu/.

[3]     Fraunhofer ISE. "Test report according to EN 12975-1:2006 + A1:2010/EN ISO 9806:2013." Fraunhofer Institute for Solar Energy Systems ISE, Freiburg (2016).

[4]     Rubitherm. "Data sheet." Rubitherm Technologies GmbH, Berlin (2016). Accessible on: https://www.rubitherm.eu/media/products/datasheets/Techdata_-RT22HC_EN_29062016.PDF.

[5]     U. Stritih, P. Charvat, R. Koželj, L. Klimes, E. Osterman, M. Ostry, V. Butala: "Experimental and numerical investigations of PCM thermal storage system for heating and cooling of buildings." Prepared for publication in Special Issue on Energy Storage with Energy Efficient Buildings and Districts in Sustainable Cities and Society (2017).