PASSIVE SOLAR WALL INTEGRATED WITH A LATENT STORAGE LAYER

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1 PASSIVE SOLAR WALL INTEGRATED WITH A LATENT STORAGE LAYER Paolo PRINCIPI Prof. Eng. 1 Roberto FIORETTI Dr. Eng. 2 Department of Energetica, Università Politecnica delle Marche, Ancona, Italy 1 p.principi@univpm.it, 2 r.fioretti@univpm.it Keywords: Phase change material (PCM), solar system, thermal mass, low energy consumption, responsive building elements Summary Active and passive solar systems are possible solutions to exploiting the solar energy and to reduce the consumption for the heating of internal environment. A kind of light wall create with inserting of a PCM layer under a glass transparent layer, permit to absorb and store the energy due to solar radiation incident on the external surface, during cool seasons. When the façade is directly irradiate the great part of the energy crossing through the glass is stored by the PCM layer. Into the air space between the PCM layer and the glazing the air temperature rises thank to greenhouse effect. Experimental analysis in building, carried out during winter season in the outdoor laboratory of Polytechnic University of Ancona, demonstrate the validity of this solution for increase the heat flow in this part of building. 1. Introduction Since 7 8 years the interest in the exploiting of the solar energy to increase the energy performance in building are registred., and at this moment, 3 years after the importance of this fields is still an item of primary importance, for the energy performance in the building sector. The objectives to apply the solar systems in a building is to reduce the energy consumption by fossil fuel, exploiting the natural energy sources, transforming it in heat or in electric energy. The are basically two distinct approaches to the solar heating of building active and passive. Usually, active system employ hardware and mechanical equipment to collect and transport heat, while passive system collect and transport heat by non mechanical means. There are two basic elements in passive solar heating system: a south-facing glass for solar collection and thermal mass for heat, absorption, storage and distribution. A well-known example a passive solar heating system is the Trombe wall that is composed of glazing area and thermal mass located 1 cm or more directly behind the glass, which serves for heat storage and distribution. This mass that works as an heat accumulator, as well as store energy allow to delaying the heat gain, releasing the energy several hours after the accumulation, in the times as the night when there is more necessity of heat to keeping comfortable the internal environments. The wall works by absorbing sunlight on its outer face and than transferring this heat through the wall by conduction. The outside surface is usually painted with a dark color to increase absorption of sunlight. After the heat conduction through the wall it is distributed to the internal space by radiation and to some degree by convention from the inner face. 23

Figure 1 Night and day behaviour of a solar passive wall.

If we add vents to the wall in the higher and lower part of the wall, the distribution of heat by natural convention from the exterior air space is also possible to increase

Night and day behaviour of a solar passive wall with opening.

2 Figure 1 Night and day behaviour of a solar passive wall. The thermal inertia of the massive part of this wall fix the storing capacity of this system, the delaying and the suitable of this technology to increase the heat gain. If we add vents to the wall in the higher and lower part of the wall, the distribution of heat by natural convention from the exterior air space is also possible to increase the amount of heat introduce in the living space. This phenomena it s possible only during daytime and early evening. Figure 2 2. Night and day behaviour of a solar passive wall with opening. Phase change materials for building Introduction of phase change materials into the building components can considerably increase the thermal mass of those, without substantial increase the weight. Thank to their latent fusion heat, and in smaller part, their specific heat, these materials act as heat accumulators; absorbing and discharging heat, keeping their temperature unaltered and thus avoiding the overheating of the elements they are contained in. 236

3 Figure 3 Graphic temperature-energy of a PCM material. The advantage given by the use of PCM in matter of energetic behaviour, instead of traditional systems is in their heat accumulation ability, with lower value of weight and thickness. PCMs are categorized as organic, inorganic and eutectic materials, with a great selection of material showing several characteristics as melting point temperature, latent heat, specific heat and chemical properties. The principal applications in the building field are; - underfloor system, to control the temperature and store energy in the winter day, and to store energy and to keep the thermal comfort during the summer; - in walls to increase the thermal comfort, reducing the energy consumption for cooling, and to exploit the solar energy, - in air exchange system to stabilize the internal temperature, storing in the warm hours and releasing energy during the night, combined with the night ventilation; - heat store unit in cooling and heating systems. Normally the passive solar walls are characterized by a layer builds with high inertia material as concrete, masonry, stone, necessary to store the heat; to allow a right behaviour of this wall to the least it should be a weight of 3-4 kg/m 2 for a thick of,3-,3 cm. This great quantity of weight increase the cost for the structure and it isn t a right solution for the fabric build with light weight components concept based on the lightness of whole the elements. The thermal mass is due just to sensible heat of those materials. This weighty and thick layer could be replace with a layer made with PCM that allow to safe weight and space, without reduce the storing capacity. Thus the PCM should be put in the external part of the opaque wall, behind the air layer to permit to store the solar radiation. Moreover this layer should be put before an insulating layer giving to this wall a suitable thermal resistance, required for control the heat loss. 3. Experimental analysis In this research field in the department of Energetica of the Polytechnic University of Marche it was developed a passive storage wall, utilizing PCM as thermal mass and glass layer to transmit and absorb the solar radiation during winter season. The energetic evaluation for behaviour of this solution undergoing actual actions was carried out in experimental facilities located in the Energy renewable laboratory of the department of Energetica. They were evaluated during all the year in terms of thermal behaviour comparing these results of this test with those characterized the traditional walls. The experimental buildings consist in eight cubic shape boxes measuring 3x3x3m, built using panels and,22 m thick polyurethanic foam layer. The south facing walls, on the other hand, were built using different technology box by box in order to evaluate their thermal performance. 237

Figure 4 Experimental box with passive solar wall.

temperature of multilayer wall. The temperatures value recorded inside each box were harvested with the Datataker DT datalogger.

The external environment conditions were recorded using an LSI meteorological station placed on site. Figure Monitoring system (a)thermocouples T, (b) RTD 1.

4 Figure 4 Experimental box with passive solar wall. For this purpose a temperature monitoring system was carried out using a 9 T type thermocouple series monitoring external air temperature and 1 RTD thermoresistance series recording interfaces temperature of multilayer wall. The temperatures value recorded inside each box were harvested with the Datataker DT datalogger. The air temperature within the box was set at 2 C and checked using a thermal control system designed and tested for these boxes. The external environment conditions were recorded using an LSI meteorological station placed on site. Figure Monitoring system (a)thermocouples T, (b) RTD 1. The research focalized the attention on two specific test walls build with a different stratification. The passive solar wall with PCM (melting point 32 C) was compared with a similar façade but without this layer, all built with the same geometric and physical characteristics for the first part: -.2 m thick plasterboard panel; -.22 m thick mineral wool insulation; -.2 m thick wooden panel covered by an outside vapour barrier. The second exterior facing layers is made by a 2 cm commercial cement board and putty layers, for the comparison box and 3 cm thick PCM layer, cm an air chamber, and a 4 mm thick glass for box with the passive wall. The air chamber is equipped with variable openings in upper and lower parts, those was closed during the winter and opened in the summer to evacuate the hot air and avoid the overheating of the phase change material layer. 238

Figure 6 South façade stratification for the test and comparison walls. The energetic behaviour of this façade was tested during the 24 winter.

The functioning of the passive wall is directly related to the solar radiation that irradiates its external surface inducing the PCM layer to change its phase only absorbing a strong

The external air temperature doesn t contribute significantly to the heat storing process, but it s important in the PCM solidification process, during the night.

During the day characterized by a lower external air temperature and a strong solar radiation, the PCM began to store thermal energy during the melting phase, and releasing it during the

The integral of this difference (marked with orange), fig. 8, is the energy saved (not required for keeping thermal comfort conditions).

temperature, direct radiation, global radiation. Figure 8 Heat fluxes incoming through south façades. 239 16/3/2. 1/3/2 23.1 1/3/2 22.3 1/3/2 21.4 1/3/2 21. 1/3/2 2.1 1/3/2 19.3 1/3/2 18.

5 Figure 6 South façade stratification for the test and comparison walls. The energetic behaviour of this façade was tested during the 24 winter. The figures show the whether parameter data for the period of two days, the heat flux through the walls, the air chamber and the PCM layer temperature, respectively. The functioning of the passive wall is directly related to the solar radiation that irradiates its external surface inducing the PCM layer to change its phase only absorbing a strong amount of solar radiation. The external air temperature doesn t contribute significantly to the heat storing process, but it s important in the PCM solidification process, during the night. In the test period, the temperature in the air space was higher than the air external temperature (Figure 9). During the day characterized by a lower external air temperature and a strong solar radiation, the PCM began to store thermal energy during the melting phase, and releasing it during the night keeping its temperature higher, producing a thermal load inside the box (Figure 1). The heat flux in passive wall resulted higher than the comparison wall for great part of the day. The integral of this difference (marked with orange), fig. 8, is the energy saved (not required for keeping thermal comfort conditions) Temp 9 rad glob [W/m2] rad dir [W/m2] 8 rad dir [W/m2] Weather data: Dry bulb temperature, direct radiation, global radiation. Figure 8 Heat fluxes incoming through south façades /3/2. 1/3/ /3/ /3/ /3/ /3/ /3/ /3/ /3/ /3/ /3/ /3/ /3/2 1. 1/3/ /3/ /3/ /3/ /3/ /3/ /3/2 9. 1/3/ /3/ /3/ /3/ /3/2 6. 1/3/2.1 1/3/ /3/ /3/ / Da 14 3/2 ta/o / ra 14 3/2 / /2 1 / /2 2 / /2 3 / /2 4 /. 14 3/2 4 /. 14 3/2 / /2 6 / /2 7 / /2 8 / / / /2 / 9. /.4 / 1.3 / 2.2 / 3.1 / 4. / 4. /.4 / 6.3 / 7.2 / 8.1 / 9. / /2 2 / /2 2 / 1.3 3/ Figure 7 1/3/ /3/ /3/ /3/2. 1 Temp rad glob [W/m2]

6 T interspace T outdoor 2 T interspace T outdoor /3/14 :: 24/3/14 :4: 24/3/14 1:3: 24/3/14 2:1: 24/3/14 3:: 24/3/14 3:4: 24/3/14 4:3: 24/3/14 :1: 24/3/14 6:: 24/3/14 6:4: 24/3/14 7:3: 24/3/14 8:1: 24/3/14 9:: 24/3/14 9:4: 24/3/14 1:3: 24/3/14 11:1: 24/3/14 12:: 24/3/14 12:4: 24/3/14 13:3: 24/3/14 14:1: 24/3/14 1:: 24/3/14 1:4: 24/3/14 16:3: 24/3/14 17:1: 24/3/14 18:: 24/3/14 18:4: 24/3/14 19:3: 24/3/14 2:1: 24/3/14 21:: 24/3/14 21:4: 24/3/14 22:3: 24/3/14 23:1: 24/3/1 :: 24/3/1 :4: 24/3/1 1:3: 24/3/1 2:1: 24/3/1 3:: 24/3/1 3:4: 24/3/1 4:3: 24/3/1 :1: 24/3/1 6:: 24/3/1 6:4: 24/3/1 7:3: 24/3/1 8:1: 24/3/1 9:: 24/3/1 9:4: 24/3/1 1:3: 24/3/1 11:1: 24/3/1 12:: 24/3/1 12:4: 24/3/1 13:3: 24/3/1 14:1: 24/3/1 1:: 24/3/1 1:4: 24/3/1 16:3: 24/3/1 17:1: 24/3/1 18:: 24/3/1 18:4: 24/3/1 19:3: 24/3/1 2:1: 24/3/1 21:: 24/3/1 21:4: 24/3/1 22:3: 24/3/1 23:1: Figure 9 Air interspace and outdoor dry bulb temperature /3/14 :: 24/3/14 1:: 24/3/14 2:: 24/3/14 3:: 24/3/14 4:: 24/3/14 :: 24/3/14 6:: 24/3/14 7:: 24/3/14 8:: 24/3/14 9:: 24/3/14 1:: 24/3/14 11:: 24/3/14 12:: 24/3/14 13:: 24/3/14 14:: 24/3/14 1:: 24/3/14 16:: 24/3/14 17:: 24/3/14 18:: 24/3/14 19:: 24/3/14 2:: 24/3/14 21:: 24/3/14 22:: 24/3/14 23:: 24/3/1 :: 24/3/1 1:: 24/3/1 2:: 24/3/1 3:: 24/3/1 4:: 24/3/1 :: 24/3/1 6:: 24/3/1 7:: 24/3/1 8:: 24/3/1 9:: 24/3/1 1:: 24/3/1 11:: 24/3/1 12:: 24/3/1 13:: 24/3/1 14:: 24/3/1 1:: 24/3/1 16:: 24/3/1 17:: 24/3/1 18:: 24/3/1 19:: 24/3/1 2:: 24/3/1 21:: 24/3/1 22:: 24/3/1 23:: Figure 1 Temperature behind the PCM layer. 4. Conclusions The passive solar system is a right solution for reduce the energy consumption of the building for the heating by fossil fuel. The wall could be integrated in south façade, and utilised without the necessity of further space, and coupling with warming equipment. In this technology, the PCM layer is able to replace the heavy layer made with traditional material as concrete, and it s suite for applying in fabric built with light dry elements. The results demonstrated the ability of this kind of solar wall to exploit the solar energy, and to bring the energy stored during the sunny hours inside the living space to reduce the energy consumption and the energy lost by this façade. Although the high thermal resistance, due in particular to the layer of mineral wool, the energy is released both toward the internal and external environments. The PCM layer can store the energy during the day and keeping high its temperature for the rest of the night, coming back every day in the solid phase. References A. Carbonari, M. De Grassi, C. Di Perna and P. Principi 26,Numerical and experimental analysis of PCM containing sandwich panels for prefabricated walls, Energy and Buildings, Volume 38, Issue, pp Mario De Grassi, Alessandro Carbonari and Giulio Palomba 26 A statistical approach for the evaluation of the thermal behavior of dry assembled PCM containing walls Building and Environment, Volume 41, Issue 4, pp K. Darkwa and P.W. O Callaghan 26, Simulation of phase change drywalls in a passive solar building,applied Thermal Engineering, Volume 26, Issues 8-9, pp Lemma M., Fioretti R., Imperadori M. 2, Development of a technology for inserting a PCM layer in building envelopes, proceeding of XXXIII IAHS World Congress Housing, Process & Product, Pretoria 2. Belén Zalba, José M Marín, Luisa F. Cabeza and Harald Mehling 23, Review on thermal energy storage with phase change: materials, heat transfer analysis and applications, Applied Thermal Engineering, Volume 23, Issue 3, pp

Thermal efficiency of a modified thermal storage wall containing phase change material in comparative test periods

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