SMART GREEN ROOFS: COOLING WITH VARIABLE INSULATION

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1 SMART GREEN ROOFS: COOLING WITH VARIABLE INSULATION Pablo La Roche California State Polytechnic University, Pomona / HMC Architects / 3801 West Temple Avenue Pomona, CA pablo.laroche@hmcarchitects.com Eric Carbonnier HMC Architects 3546 Concours, Ontario CA eric.carbonnier@hmcarchitects.com Christina Halstead California State Polytechnic University, Pomona ABSTRACT Conventional uses of green roofs aim at improving the heat island effect, stormwater management, air quality, and energy conservation. This paper builds on these strengths and specifically examines passive cooling potential by selectively coupling the green roof s soil mass with the inside environment to improve indoor temperatures and energy conservation. Previous experiments on green roofs indicate that there is potential for indoor temperature performance enhancements of non-insulated green roofs compared to insulated green roofs and conventional non-green roofs (La Roche, 2006, 2009). However, insulation is needed to keep heat out when it is too hot outdoors or to keep heat inside when it is too cold outdoors. A smart ventilation system that improves thermal performance by coupling or uncoupling the thermal mass as necessary is proposed. To achieve this the system has an insulated plenum in which a fan is activated by temperature based rules. When the fan is on the plenum is ventilated and when it is turned off the ceiling acts as an insulator. This paper compares four test cells: a traditional green roof with insulation underneath; a non-insulated green roof; a green roof with an insulated plenum and a fan referred to as the smart green roof; and a baseline test cell with a conventional insulated roof. Green roof testing was conducted at the Lyle Center for Regenerative Studies on the California State Polytechnic University, Pomona campus situated in the hot/arid climate of Southern California. Results as indicated by the plenum sensor are very good, because the lowest values when heating is required are always achieved in this location and the temperature is very stable and cool even in very warm days. However the fan needs to be more powerful so that this effect is transferred to the rest of the cell. INTRODUCTION A living or green roof is a roof that is substantially covered with vegetation. Green roofs have been proven to have positive effects on buildings by reducing the stress on the roof surface, improving thermal comfort inside the building, reducing noise transmission into the building, reducing the urban heat island effect by reducing hot surfaces facing the sky, reducing storm water runoff, reoxygenating the air and removing airborne toxins, recycling nutrients and providing habitat for living organisms, all of this while creating peaceful environments. The positive thermal effects of green roofs are usually described by the reduction of the external surface temperature due to the effect of vegetation, and the reduction of the thermal transmittance of the assembly, mostly due to the effects of insulation, usually placed between the sustaining material and the interior space of the building. A common misconception is to think of green roofs as insulators. They are insulators when they have insulation as part of the assembly and studies have demonstrated that a well planned and managed green roof with insulation- acts as a high quality insulation device in the summer (Theodosiou T., 2003). However, little has been done to take advantage of the mass of the green roof as a heat sink in temperate or hot climates. By acting as heat sinks, they can contribute to cooling of spaces if the mass of the soil is cooled, by night ventilation for example. There has been some research in this direction that indicates potential to reduce the cooling loads inside buildings (Eumorfopoulou E, Aravantinos D., 1998) (La Roche, P., 2006, 2009). Apart from providing protection against overheating, a green roof can also provide some cooling through the evaporative process in the plants. 1

2 2. CLIMATE DATA This paper discusses some ongoing tests with the green roofs beginningg in the summer of These green roofs have been built and are being tested at the Lyle Center for Regenerative Studies in California State Polytechnicc University Pomona, located in a hot and dry climate with mild winters about 30 miles east of Los Angeles in southern California. Figure 1 shows temperature and radiation data from the nearby Chino airport weather station plotted in Climate Consultant. Fig. 1: Climate Data compliant insulated roof, with a U value of W /m2 K, painted white (Fig 2). The three green roofs have different conditions: a non-insulated green roof (Fig 3); a greenn roof with insulation underneath (Fig 4); and an insulated plenum green roof referred to as a smart variable insulationn green roof (Fig 5). The planting material and partss of the roof were substituted several times, the last timee being in the summer of 2011 for this series. The growth medium in the non insulated green roof is thermally coupled with the interior via a metal plate, while in thee other green roof there are 10 cm of matt insulation between the space and the soil Night ventilation iss provided with a fan and all of the cells are equipped with dimmers and timers to adjust the ventilation rate and start/end times. However it is possible to control any of the fans also using a smart ventilation system that has been used for other experiments. In previous papers a smart controller that optimizes the use of forced ventilation for structure cooling in a building was tested (La Roche, Milne 2001), (La Roche, Milne 2002). This controller used a set of decision rules to control a fan to maximize indoor thermal comfort and minimize cooling energy costs using outdoor air, the greatest potential source of free cooling energy in many temperate climates. The controller can be programmed usingg different logical rules that turn the fan on and off to cool down the building's interior mass so that it can 'coast' comfortably through the next day. 3. EXPERIMENTAL SYSTEM The experimental system consists of a microprocessor r controller connected to thermistors that measuree temperature, a laptop computer connected to it whichh contains the control programs and collects and storess experimental data, four test cells and an active ventilationn system, which consists of a 4-inch inlet and on the outlett side a fan that turns on or off by a signal from the microcomputer. Four test cells were used, three of them had green roofs and another one had a conventional roof. All test cells have the same dimension, 1.2 x 1.2 x 1.2 meters and weree built using 2 by 4 inch stud wall construction with drywalll on the inside, plywood on the outside and batt insulationn in between for a U value of 0.12 W /m2 K. Exterior is white and they have 0.61 m by 0.61 m (2 x 2 ) double glazed windows and 3.8 cm thick concretee pavers as the slab. Fig. 2: Control Test Cell The roof is the only difference between the cells so that it is easy to compare the performance of different types of roofs. The first cell is the control cell and has a code 2

3 temperature rise. During overheated periods the fan is turned off because the sensorss indicate thatt it is not possible to cool the space with outdoor air. must be closed to avoid heat gains by convection. Nocturnal ventilative cooling is a well known strategy that has been used for many years, mostly in warm and dry climates (La Roche, P. Milne, M., 2004) (Grondzik, Kwok, Stein, Reynolds, 2010). Fig 3: Cell with non-insulated Green Roof Manyy series were performed beginning at the end of the summer of 2011, of which only a selection are presented in this paper Series 1 In this series beginning in September 8, 2011, the cell are not ventilated and are unshaded. This series establishes a baseline understanding of the thermal behavior of the cells without any cooling by ventilation. Thermal gains by direct solar radiation are intense because of the large window to floor ratio 1:4. Fig 4: Test Celll with Insulated Green Roof Fig 6: Two days in series 1 Fig 5: Test Celll with Smart Green Roof 4. EXPERIMENTAL RESULTS As explained in the introduction the cooling with thesee green roofs is provided by ventilation which mostly occurs at night when the thermal mass of the green roof is cooled by forced convectionn that pushess air from the inside, bypassing the thermal resistance of the envelope. During the daytime, the cool mass of the green roof acts as a heat sink, absorbing the heat penetrating into and generated inside the building, reducing the rate of indoor Maximum temperatures are a good indicator of the cooling effectiveness of a system. The higher the difference between the maximumm averages in both cells, the better the performance of the experimental cell. In this series the best performing test cell is the non-insulated greenn roof, probably because of the thermal mass. The smart green roof is not performing well, and with the highest maximum temperatures it is the worst performing of the cells. However the plenum temperature of this cell is thee most stable temperature with the lowest maximum temperatures. The plenum fans are not turned on at any point. If they were activated then the maximum temperature inside the cell would be lower because more heat would be absorbed by the thermal mass of the green roof inside the plenum. Because the windows are not shaded the outdoor temperature is lower than all of the 3

4 temperatures inside the test cells. There is a bit of thermal lag and the maximum temperature inside the cells occurs a couple of hours after the maximum outside temperature Series 2 This series starts on October 14 and compares the performance of the smart variable insulationn roof with the other cells without ventilation. All of the cells are shaded. This series is similar to comparing a building with a variable insulation green roof compared to other buildings that are not ventilated and that have non insulated and insulated green roofs, and insulated roofs. It is assumed that to have variable insulation it must have possibility to ventilate according to the rules set up by the programmer. If t o < t i and t i > Cf low and t i <Cf_high then fan ON else fan OFF. Where: t o is the temperaturee outside; t i is the temperaturee inside ; C f_low w, is comfort low at C (65 F); C f_hig gh is Comfort high at C (78 F). In this series the smart green roof works very well and has very low maximumm values, similar to the values inside the uninsulated green roof. The lowest maximum values are inside the plenum but they are still quite close to those inside the non insulated green roof and smart green roof. Whenn the fans in the smart green (plenum and exterior air change fan) roof are on they both track each other quite well,, however when the fans are turned off because it is not necessary to cool from the exterior then the plenum temperature decouples from the space temperature and stayss cooler than the interior space. This indicates that it would probably be more effectivee if the plenum fan would continue working, coupling these two and mixing the air of the space with the thermal mass of the green roof Series 3 Fig 7: Two days in series 2 The variable insulation green roof is insulated according to the rules that have been programmed and the interiorr and exterior conditions. First the program compares the values of the internal temperature sensor with the external values. The variable insulation green roof has a layer of insulation under the planting material with an air space in between. A fan is located in the insulation material that is activated when needed according to the rules, so that it can move air from the test cell space through the plenum and in contactt with the green roof. When no coupling with the thermal mass is desired then the fan is turned off. This is the case of a winter night in which the heat inside the space would be lost to the exterior through the green roof. Previous papers evaluated various control strategies with different relationships between variables such as air change rates and values for comfort low and comfort highh (La Roche, Milne, 2003) The rule that achieved the most hours in comfort and the lowest maximumm temperatures in the summer is used to control the plenum fan: In this series that begins Feb 24, 2012 all test cells are nightt ventilated and are not shaded. The variable insulation roof has the fan operating according to the samee rules stated in series two while the rest of the cells operate with a fan from 9pm to 6am. Fig 8: Two days in series 3 All of the temperatures in the cells are still above the outdoor temperature because of the unshaded windows. In this series the nonn insulated control cell is the warmest 4

5 followed by the insulated green roof. The lowest maximum temperature is inside the plenum of the uninsulated green roof followed by the non insulated green roof. This demonstrates the effectiveness of combining thermal mass with night ventilation; the two cells that have the lowest maximum temperatures are those that are coupled with the thermal mass Series 4 In this series beginning in March 26, 2012, all fans are turned on in alll cells from 8pm to 3am. The windows are unshaded. All of the cells are ventilated at the same time and the plenum fan is not turned on in any moment. Again the plenum temperature in the smart roof is the lowest and best value during the daytime. The nonn insulated green roof is also very effective. The smart green roof, the insulated green roof and insulated roof are not very effective because they have the insulation and no contact with the thermal mass. The smart green roof s maximum temperature is not low because the plenum fan is not turned on. We will include in the presentation a series that compares them all with the night ventilationn system working but also with the plenum fan working. Green roofs, when combined with night ventilation can lead to more comfortable conditions inside buildings, with increased energy efficiency. When the plenum fan was turned off the variable insulation cell was also the warmest temperature indicating that the variable insulation roof can also work as a non insulated cell and as ann insulated cell. This should give them added value increasing their applicability. 6. REFERENCES Del Barrio, E. (1998) Analysis of the green roofs cooling potential in buildings. Energy and Buildings, 27: p Eumorfopoulou E, Aravantinos D. (1998) The contribution of a planted roof to the thermal protection of buildings in Greece. Energy and Buildings: 27, Grondzik, Kwok, Stein, Reynolds, Mechanical and Electrical Equipment for Buildings, 2010 La Roche, P. Milne, M. (2004), Effects of Window Size and Mass on Thermal Comfort using an Intelligent Ventilation Controller. Solar Energy: Number 77 p La Roche, P. (2006). Green Cooling: Combining Vegetated Roofs with Night Ventilation, American Solar Energy Society ASES 2006 Conference, Denver, USA. La Roche P. (2009) Low Cost Green Roofs for Cooling: Experimental Series in a Hot and Dry Climate. Passive Low Energy Conference, PLEA 2009, Quebec Canada. Theodosiou T. (2003) Summer period analysis of the performance of a planted roof as a passive cooling technique. Energy and Buildings, 35: p Fig 9: Two days in series 4 5. CONCLUSION All of the options with night ventilation and thermal masss work better to keep the space cool. Consistently the lowest maximum temperature is in the plenum of the smart roof. This occurs in both the series with night ventilation and the series without night ventilation. Highest temperatures are also lower in the non insulated green roof and the variable insulation green roof when the plenum fan is turned on. 5