The Net-Zero building Solar 2002

Transcription

1 The Net-Zero building Solar 2002 Learning from an early example Koen Claes (1), Ralf Klein (3), Guido Henri De Couvreur (2), Alexis Versele (3), Hilde Breesch (3) (1) Hogeschool voor Wetenschap & Kunst, campus DE NAYER, Jan De Nayerlaan 5, BE-2860 Sint-Katelijne-Waver (2) MONDO VZW, Bastijnstraat 85, BE-2590 Berlaar (3) Onderzoeksgroep Duurzaam Bouwen, KaHo Sint-Lieven, Technologiecampus Gent, Gebroeders Desmetstraat 1, BE-9000 Gent, Tel. +32 (0) , Abstract This paper describes the Solar 2002 building in Berlaar (Belgium) that was constructed by the Flemish non-profit organization mondo vzw. The aim of this retro-fit of an existing single family dwelling was the creation of an example project, demonstrating the potential of solar energy for the energy needs of buildings in a moderate, rather cloudy climate. This has led to a Net-Zero Energy building, that finally supplies more energy to the grid, then it consumes on an annual basis. While the approach of the project aims at sustainability in a much broader sense, including material and water use, this paper will focus on the energy-aspect for the building in use. Technical details of the building and the employed renewable energy technologies are given. Introduction The Solar 2002 project is an early example of a NZESB. An important issue with this type of buildings is how to classify them as Net Zero Energy (Torcellini P. et al, 2006). First of all detailed simulations have to be performed during the design stage to ensure feasibility of the required energy target. Therefore simulation tools with extended capabilities are necessary. In addition, a detailed building-inuse assessment is required to confirm the predicted energy performance and comfort level. Comparing simulation results and monitoring data for extended periods can contribute to the further development of tools adapted to the different stages of the design process. This will also help to get a better picture of the reliability of simplified methods in the context of NZESB. Description of the building Solar 2002 is a renovation project of a single-family detached home. It consists of two storeys with a total heated floor area of 168m². All the living areas are located at the ground floor. The first floor contains a toilet, a polyvalent room and an office room, and is only heated when visitors are present. There is a garage attached to the east facade, which is accessible from within the house. Below the house is a crawlspace that contains an open water storage reservoir and below the garage is a basement that serves as a technical space. For the selection of a suitable existing building for the renovation project, one of the selection criteria was the orientation and slope of the pitched roof. The roof of the selected building is oriented south and has two different slopes (30 and 60 ) for optimal performance of the solar energy conversion systems. The living quarters are oriented south-west. The first step in achieving Net-Zero is minimizing the energy demand of the building for heating and cooling. Therefore good thermal performance of the building envelope is essential. Insulation was

2 added to the existing structure to reduce the transmission losses, achieving U-values as listed in Table 1. The windows have double-glazing and a reflective film, together with a low-e coating and krypton filling resulting in a solar energy transmittance of Attention was also paid to the infiltration losses by placing an airtight layer at the inner surface of each element and their connection. Table 1: U value building elements Construction Floor construction External wall construction Roof construction Glazing U-value 0.25 W/m²K 0.21 W/m²K 0.21 W/m²K 0.5 W/m²K To ensure comfortable indoor conditions during summer without air conditioning, solar gains are minimised with overhangs and shutters on the south and west facing windows. The shutters on the ground floor are controlled manually. The ones protecting the roof windows are controlled automatically based on the difference between inside and outside temperature. The external face of the floor slab of the ground floor was insulated to prevent heat losses, while increasing the thermal capacity of the building. During summer periods natural ventilation by opening windows on both floors is used to cool down the thermal mass during the night. To prevent overheating in the summer the intake air passes above the free water surface of a permanently filled reservoir in the crawl space. This provides cooling through heat exchange between air and water and evaporation of the water. Material and water use This project not only focuses on the energy-aspect of the building but at sustainability in a much broader sense. It aims at living in harmony with the environment as much as possible. Therefore a lot of attention was paid to material use during renovation. While no systematic LCA analysis was performed, based on existing literature and experience every material was selected to have a minimum impact on the environment, considering the entire life cycle. For all parts of the structure timber from local production or recovered bricks were used. In Table 2 an overview of the used insulation materials is given. Finishes consist of cork or wooden fibreboard. Table 2: Used insulating materials in renovation Building elements Floor construction External wall construction Roof construction Internal wall construction Materials Vermiculite Mineral wool Recycled cellulose Mineral wool Another reached target is Zero Water : no water from the water supply system of a public utility is consumed. All rainwater from the roofs is stored in large tanks (9000l) and purified to drinkable standards. This covers the entire water use of the occupants. The wastewater is treated in a local installation and sent to a pond in the garden to close the natural cycle.

3 Description of the installation For the production of domestic hot water, as well as space heating, solar energy is collected as much as possible. Therefore, a solar hot air collector of 36m² is installed at a roof slope of 60 oriented south, as shown in Figure 1. This so-called Couvreur-solarroof functions like a Trombe wall, providing a dynamic insulation system for a great part of the year. Through an air-water heat exchanger this collector is coupled with a storage tank (850 litres) in the basement. This tank is heated by the collectors starting from a temperature difference of 15 K between the bottom of the tank and the air outlet of the air collectors. During the colder parts of the year the storage tank is heated by a heat pump which uses the water reservoir in the crawl space as heat source. Also a solar heatpipe collector of 2m² is mounted on the roof with an integrated boiler, as shown in Figure 1. This boiler and the storage tank are connected so that, during sunny days this provides heat at a high temperature to the storage tank in the basement. Figure 1: The south facing roofs with different types of collectors Space heating is separated for the two storeys. Radiant floor heating ensures a comfortable temperature for the living quarters at the ground floor. The first floor is heated by ventilation air, providing a quick response to varying demand due to changes in occupation of the meeting room. The fresh air passes through an air gap on the backside of the PV panels, providing cooling and thereby increased efficiency for the panels and pre-heating for the ventilation air. During periods with low exterior temperatures, when this preheating system is not sufficient, a heat-exchanger in the intake air duct provides additional heat from the water storage tank. To ensure good indoor air quality and to limit heat-losses a mechanical ventilation system was installed, including a heat recovery unit. Ventilation rates are controlled by a CO 2 sensor. As earlier stated the intake air can be preheated by circulating it behind the PV panels or pre-cooled by the water in the crawl space. Electricity was produced initially by a photovoltaic array of 5,6 kwp peak performance. These 36m² of photovoltaic modules were integrated in the south oriented roof with a slope of 30, as shown in Figure 1. A few years ago extra photovoltaic modules were placed on the south facing roof overhang. With this new system, the annual average electricity production over the past five years is 4374 kwh. With a monitored average energy consumption of 4243 kwh, during this period more energy was supplied to the grid then consumed in the building.

4 Energy performance of the building In order to estimate comfort and energy performance of the building, dynamic simulations are performed with EnergyPlus (Crawley et al, 2004), using DesignBuilder (DesignBuilder, 2010) as a graphical interface. Due to the lack of measured weather data during the monitoring period, the TMY2 weather file for Brussels 1 (Belgium) is used. This weather file is available from the U.S. Department of Energy EnergyPlus climate file database (US-DOE, 2010). This simplification does not compromise the simulation accuracy (Wasilowski et al, 2009). Another parameter that influences the results for energy demand is the indoor temperature. From available monitoring results over relative short periods of time during the monitoring period an average indoor temperature of 18 C could be assumed for the ground floor. For the first floor no accurate average temperature could be determined due to the strong variation in occupancy. In this first step the same average temperature is assumed for the complete building. The model is based on the construction data given earlier in this paper. Reliable monitoring data is not available for the heat fluxes from preheating of the ventilation air through the photovoltaic array and the heat exchanger in the ventilation unit. Consequently these gains or losses are neglected in this first analysis. Further investigation is needed to estimate the influence of these effects on the total heat balance of the building. For estimating internal gains inside the heated volume, these gains are divided into two categories, d.i. occupancy and appliances. For occupancy 2 persons are assumed to be continuously present in the house. Appliances are assumed to produce 2 W/m² from 16:00 to 22:00. These values seem to provide acceptable results concerning annual energy demand. Results show that the required heating power to maintain an inside air temperature of 18 C amounts to 2 kw or 11,9 W/m². This is comparable to the energy demand for high performance buildings, e.g. passive houses where the peak heat load should not exceed 10 W/m² treated floor area. Assuming a cooling set-point of 25 C, the peak cooling load also is 2kW or 11,9 W/m² treated floor area. When the energy need for space heating on an annual basis is simulated, the total heat gains and losses are divided as shown in Figure 2. The annual heating demand is 13,7 kwh/m² treated floor area, the cooling load 1,25 kwh/m² treated floor area. Several other parameters, like usage of shutters during the day, will also affect the heat balance and need further investigation. In addition, the effect of cooling the ventilation air with the water in the crawl space should be simulated. Figure 2: Annual heat gains and losses Annual heat gains and losses (kwh) Transmission Ventilation Internal gains Solar heat gains Heating demand Cooling demand 1 About 50 km from the Solar 2002 building in Berlaar (Belgium)

5 Simulation versus monitoring After completion of the renovation works numerous measurement points were installed. The initial aim was to measure all heat flows in the building and installation. However, due to different problems not all parameters could be monitored and complete data over several years is not available. Therefore it is not possible to perform a detailed analysis of the energy balance of the building on an annual basis. In this paper shorter periods of measured data were taken into account to analyze the indoor climate in the building. From the available monitoring data a summer week in June 2006 was selected for analysis. The curves of air temperatures for the ground and first floor and the outside dry-bulb temperature are shown in Figure 3. Despite the rising outside temperature, the temperature on the ground floor stays comfortable and relatively constant. It only rises gradually and stays under the 25 C during the entire week, while outside temperatures reach 30 C and more. Transmission losses and gains are limited due to the high insulation levels of the building fabric. Also the thermal mass of the ground floor contributes to a more stable indoor temperature. Based on measured intake and exhaust temperatures of the ventilation unit (which seem to remain equal to respectively outdoor and indoor temperatures) it is concluded that mechanical ventilation was switched off during this period. In addition there seems to be only very little internal heat gains on the ground floor. During the last two nights of the week, ground floor temperatures decrease rapidly during night time. Probably some windows where opened for night time ventilation. Temperatures on the first floor are gradually rising during the week and follow much closer the outside temperatures. These rooms where not in use during the considered period. Due to the monitoring equipment on the first floor there certainly are continuous internal heat gains. Unfortunately the extent of these gains was not measured. Figure 3: Measured air temperatures Temperature ( C) /06 8/06 9/06 10/06 11/06 12/06 13/06 14/06 Outside temperature Temperature first floor Temperature ground floor To compare the measured temperature curves with simulation results first of all appropriate weather data has to be selected. A lot of climate conditions, like several solar radiation parameters, wind

6 speed and direction etc. are lacking from the monitoring results. Therefore the TMY2 weather file of Brussels was used. Within this file an appropriate summer week was selected based on outside drybulb temperature analysis. For internal heat gains no occupation or heat production was assumed on both floors, except for the monitoring equipment on the first floor that produces a constant amount of internal heat gains. Simulation results for operative temperatures on the ground floor are shown in Figure 4. Different scenarios for the applied ventilation strategy were compared. Figure 4: Operative temperatures on the ground floor for different ventilation strategies Temperature ( C) /08 6/08 7/08 8/08 9/08 10/08 11/08 12/08 Outside Dry Bulb Temperature Ventilation switched on Ventilation switched off Night time ventilation Figure 4 shows that the inside temperature stays relatively high and constant during the whole week when ventilation is switched off (well insulated and no internal gains). A constant ventilation rate of 1 ac/h results in a lower temperature during the day and the night. The temperature stays between 20 C and 25 C, except short peak periods during the hottest days. The temperature variation and the peak values increase when the ventilation rates are increased. These temperature curves seem to be more in accordance with the measured temperatures shown in Figure 3. This could indicate the use of natural ventilation, unfortunately no information about window operation is available to confirm this assumption. As expected, application of night-time ventilation yields comfortable indoor temperatures even for the hottest days of this week under the assumed conditions. It is clear that during periods with a higher internal heat gains extra measures are necessary to guaranty a comfortable indoor climate. This could be achieved by the effect of cool ventilation air taken in through the crawl space. Also other parameters like usage of shutters during the day will affect these temperature curves. Further investigation is needed to confirm these assumptions and to decide on optimal control strategies.

7 Conclusions Despite the early realization of the Solar 2002 project it achieves very good results concerning the energy performance as well as the environmental impact of the building. The project shows that ambitious goals can be achieved for the energy target of the building together with comfortable indoor conditions. Due to the combination of several innovative technologies the Net Zero Energy Building target is attainable with a minimum impact on the environment. While the combination of all the technologies in this building is a real challenge for optimal control strategies and might be too complex for general use, it offers a very interesting environment to test and compare different options. While providing very useful experience, detailed analysis of the performance of the building remains difficult, due to the lack of some essential information. This underlines the importance of a detailed planning of the monitoring assisted by simulations from the very beginning of the operation phase of the building. Also the aims of monitoring have to be well defined in advance and continuous follow-up and documentation is needed. References Torcellini, P., et al Zero energy buildings: a critical look at the definition, Proceedings of the 2006 ACEEE Summer Study on Energy Efficiency in Buildings. Crawley, D. B., et al. EnergyPlus: An Update. Proceedings of SimBuild 2004 in Boulder,Colorado; International Building Performance Simulation Association. August 4-6, 2004, Boulder, CO, USA. US Department of Energy (US-DOE). (Last accessed May 2010). EnergyPlus Version DLL default version embedded in DesignBuilder, DesignBuilder version (Last accessed May 2010). Wasilowski H., Reinhart C., Modelling an existing building in DesignBuilder/E+: Custom versus default inputs, Proceedings of Building Simulation 2009 Conference, Glasgow, Scotland, July 2009