A Systematic Approach for Energy Design of Advanced Solar Houses

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advanced solar house. Solar energy; net-zero energy solar building; passive solar design; load managemen; thermal energy storage; predictive control.
A Systematic Approach for Energy Design of Advanced Solar Houses José A. Candanedo (Author)

Andreas K. Athienitis (Author)

Dept. of Building, Civil and Environmental Engineering Concordia University Montréal, Canada [email protected]

Dept. of Building, Civil and Environmental Engineering Concordia University Montréal, Canada [email protected]

Abstract—Designing a net-zero energy solar house (defined here as a house that generates on site as much energy as it consumes over an average year) requires an all-inclusive methodology exploiting synergies between passive solar design and the operation of renewable energy systems. By following this method, it is also possible to build a house whose energy production substantially exceeds its consumption, so that the additional available power can be used for other purposes (e.g., for a plug-in electric vehicle or in a greenhouse). This paper describes a general methodology for the design of such an advanced solar house. Solar energy; net-zero energy solar building; passive solar design; load managemen; thermal energy storage; predictive control

I.

INTRODUCTION

A net-zero energy building (NZEB) is defined here as a building that generates on-site as much energy as it consumes over the course of a year, although alternative definitions exist [1]. Since this objective nearly always requires the use of solar energy, these buildings are also called “net-zero energy solar buildings” (NZESB). A renewable energy system, usually a building-integrated photovoltaic (BIPV) installation, is used to generate electric energy. When the generation exceeds the consumption of the house, the additional power is delivered to the utility grid; when the generation is absent or insufficient for the building needs, electric power is taken from the grid. A conventional approach to building a house consists of a sequential process: a design is presented by an architect, followed by the structural engineer’s contributions and finally by the mechanical and electrical blueprints. With this strategy, from the point of view of the professionals designing the HVAC system and the electrical systems, the house is seen solely as a system that receives power and resources from the electric, gas and water distribution networks. In contrast, building a net-zero energy solar house (NZESH) requires a substantially different approach. The creation of a NZESH implies collaboration between all the professionals involved from an early stage, since the interaction between different systems is essential for solutions that are technically rational and financially sensible. For example, efficient use of available space is a must, and therefore components should play more than one function. Iterations and

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revisions are necessary. Designing a NZESH is a multidisciplinary task. Control and operation of the building will also benefit from an integrated analysis. The design of renewable energy systems such as PV panels and solar thermal collectors is intimately linked to key geometric parameters (e.g., exposed surface area at a given orientation). The systems for generating thermal or electric energy are often enough to supply most or all the building needs. Since this frequently coincides with the periods of peak demand experienced by electric utilities, advanced houses can significantly contribute to lessen the pressure on transmission and distribution power lines. Load management (both at the building scale and at the utility scale) can benefit from the implementation of two complementary techniques: thermal energy storage and predictive control strategies. II.

PASSIVE SOLAR DESIGN AND BUILDING SIMULATION

A. Basic Steps and Key Parameters The first step when building a NZESH is the application of passive solar design, a technique whose basic principles have been known since antiquity [2], but which requires careful planning and quantitative analysis. Passive solar design is an efficient, practical and inexpensive way to use solar energy in a building. Passive solar design is based on: •

Building envelope with high insulation levels selected mainly based on the local climate to minimize heat transfer with the outdoor environment. Although adding more insulation is beneficial in general, diminishing returns quickly emerge as other factors (mainly infiltration and ventilation) become dominant.

•

Adequate sizing of fenestration. In cold climates, this includes favoring near equatorial orientations (i.e., due South in the northern hemisphere, and due North in the southern hemisphere). Large fenestration areas are essential in capturing passive solar heat gains. Given than solar radiation values can easily exceed 1000 W/m2 on a vertical surface on a winter sunny day, it is possible to have heat gains in the order of tens of kW. This value will typically be higher than the power delivered by a heating system. Advances in fenestration technologies in the last decades (e.g. low

emissivity coatings), have improvedd their insulation values while keeping solar transmittannce high and have thus permitted increasing window/walll ratios. •

Higher levels of thermal mass in inteerior surfaces, for instance, by having concrete floors annd masonry walls or integrated phase change materials. The heat storage capacity of these materials accomplishhes two purposes: (a) storing solar heat gains so that th they can be used over longer periods, (b) as a result, reeducing excessive temperature fluctuations. The thermall mass should be as distributed as possible throughout th the interior space. The use of the building’s thermal masss for storing solar thermal energy is also called “passive thermal storage”. Traditional Canadian wood-frame cconstruction has relatively low thermal mass.

•

Properly sized overhangs must be installed on the façades with large fenestration areas iin order to avoid overheating in the summer. Motoorized blinds or curtains can contribute to reduce excesssive heat gains.

•

Measures to facilitate natural ventilation in the cooling season, for instance, by opening winddows in opposite sides of the building that are more or less aligned with the direction of the prevailing winds.

•

Reduction of infiltration, which has iimportant effects on energy consumption.

•

Installation of a heat recovery ventilatoor (HRV).

The performance of the building is determiined by a number of geometric and physical parameters (Fig. 11), which are not completely independent from each other. Some of these parameters are the ratio of heated volume to exposed surface area, the level of insulation, optical prroperties of the fenestration, the aspect ratio (length/width raatio), the azimuth angle of the building main façades, the area occcupied by a roof BIPV system, by solar collectors or by anyy other buildingintegrated system, the slope angle of the roof.

B. Calculation of Heating and Coo oling Loads The calculation of the heating g and cooling needs of a building requires as inputs climatic variables (e.g. temperature and solar irradiance), characteristics of the building envelope, internal heat gains and other varriables. Software tools are highly desirable for this complex taask in order to compare the effect of varying the aforementioneed parameters in a building. Modeling heat transfer phenomenaa can be addressed through many different methods, which may y significantly differ in level of detail and complexity. One off the simplest is the “binmethod” (e.g. HOT2000 [3]), whicch consists of dividing the 8,760 hours of the year in categories according to the air temperature experienced, and calcullating the heating load with the exposed area and insulation lev vels. Other simulation tools (Energy+, TRNSYS) use transferr functions to address the complex calculations of heat con nduction through massive, well-insulated walls [4]. Tools succh as ESP-r [5] handle this problem by dividing the walls into several layers and carrying out control volume finite difference modeling. This latter strategy is computationally demand ding, but highly flexible and accurate. C. Thermal Network Analysis Thermal network analysis proviides a useful representation of the heat transfer exchanges in a building. It is based on electric circuit analogies (i.e., therm mal resistances are akin to electrical resistors, thermal masses to t capacitors, temperature to voltage, and heat flux to electric currrent). Many of the methods of network analysis familiar to eleectrical engineers are used: Thévenin and Norton equivalents, two-port network analysis, frequency response techniques, etcc. The application of these methods relies on the assumption of a linear time-invariant (LTI) system. Although some heat trransfer phenomena are nonlinear (notably, radiative heat transffer), the inaccuracies due to this assumption are usually acceptab ble. The concept of thermal adm mittance, derived from the thermal network analysis, is a pow werful tool in passive solar design. Expressed in units of W/K K, thermal admittance is the ratio between heat flux and temperrature difference, including information about the phase angle between b them when both are modeled as sinusoidal waves. Fo or example, “wall thermal admittances” are used as parameters in the two-port network representation of a wall with seeveral layers of materials. Thermal admittance is a functio on of the frequency: the fundamental frequency is usually baased on a daily cycle (24 hr period). The “self admittance” of a mateerial which is well-insulated on one side gives an idea of itss capacity to absorb solar radiation. Interestingly, self-admittance does not increase indefinitely with the thickness of a material. In the case of concrete walls or floors, maximu um thermal admittance is internal layers of a wall

Figure 1. Typical features of a NZESH: (1) south-faccing fenestration; (2) significant thermal mass in floors; (3) properly sized overhangs; (4) solar thermal collector on façade ; (5) TES tank; (5) DHW taank; (7) solar-assisted heat pump; (8) radiant floor heating system. Some important parameters are shown: BIPV/T length; BIPV/T tilt angle (β), the house’s aaspect ratio (b/c).

Solar radiation

Ext. Temp.

Room interior

Figure 2. Typical thermal network rep presentation of an external wall.

achieved near 20 cm [2]. A “room thermal admittance” can be defined to model the response of the room air temperature to incoming solar heat gains. Room thermal admittance has been used in the characterization of buildings in the U.K. [6]. Transfer functions (TFs) relating the interior temperature response to external impulses can be obtained from more complex building models. The solar radiation/interior temperature TF, which has units of admittance, is usually the dominant factor in a passive solar building. TFs for a particular building: (a) by solving the equations associated with the thermal network representation; (b) with system identification techniques based on the output of a complex model created with a building simulation software tool; and (c) by system identification of experimentally measured inputs and outputs in an actual building. Ongoing efforts of the Concordia Solar Laboratory are focused on the determination of transfer functions that could play the role of a simplified model for a given building, and could be useful in the implementation of advanced control strategies (Fig. 3) [7]. III.

ENERGY EFFICIENCY MEASURES FOR CONSERVATION OF ELECTRIC ENERGY

The use of energy-efficient appliances (user loads) is essential for a successful NZESH design. All the appliances selected (stove, dryer, clothes washer, etc.) should be recognized energy-efficient models. This can be certified by using appliances including with an official label (e.g., EnergyStar®) [8]. Efficient lighting systems should be selected, with appropriate controls (e.g. occupancy sensors). IV.

BUILDING-INTEGRATED PHOTOVOLTAIC SYSTEMS

Photovoltaic panels are well suited for local electricity generation in buildings. Solar radiation is available everywhere. No moving parts are involved and maintenance requirements are minimal or nonexistent. PV panels used as essential components of the outermost layer of the building envelope (i.e. roofs or façades) are usually called “buildingintegrated PV” or BIPV systems. This approach has several advantages: no additional space is needed; costs are significantly reduced as PV panels replace roof shingles or wall cladding; and PV panels may even contribute to the aesthetics of the building. Solar radiation

TF 1

Exterior Temp.

TF 2

Others

TF 3

Reference(t)

Control System

TF

A. BIPV/T Systems Since higher temperatures are detrimental to the operation of PV panels, it is not recommended to mount them directly on top of other materials [9]. An air gap is usually left under the PV arrays to facilitate cooling. Given that most of the incident solar radiation is lost as thermal energy to the surroundings, and considering that PV arrays occupy large exterior surface areas, heat extraction (either using air or another fluid) is an attractive option. BIPV systems with the additional function of thermal energy recovery are called “BIPV/thermal” or BIPV/T systems (Fig. 4) [10]. This is usually achieved with a variable speed fan drawing outdoor air under the PV modules and conducting it to where it is needed. The final air temperature depends on factors such as solar radiation, exterior air temperature, wind speed, and the length of the air channel. With this configuration, it is not unusual to reach temperatures 30 ºC higher than the exterior air temperature. Higher air speeds are usually more efficient in removing thermal energy from the PV panels, but this has two disadvantages: (a) the exit air temperature is lower and (b) much higher fan power consumption, which varies approximately as the cube of the fan speed. The latter problem is partly compensated because the lower PV temperatures entail higher electric efficiencies and consequently more power generation. However, when the fan power consumption starts to exceed the additional generation, the decision to increase or not the fan speed will depend on the need for heat extraction from the roof. Fan power consumption can be reduced by designing better ducting systems and through the selection of high-efficiency fans and motors. B. Geometry of the BIPV/T - Electrical/Thermal Interaction Changing the geometry of a building affects the area available for BIPV or BIPV/T installations, and consequently has an impact on the peak electrical generation capacity and the availability of thermal energy. It is recommended that BIPV systems occupy the totality of the roof or façade on which they are installed. The use of different materials could cause leakage of water through the boundaries and is not aesthetically pleasant (an important factor in residential applications). Unlike electrical energy, which may be used for practically any purpose, the applicability of the thermal energy extracted from the roof will depend on the air temperature, on the intended final use, and on the intermediate steps. Quantification of the energy “quality” is a complex task. Solar radiation

4

Output

GL AI

Sensor PV

Figure 3. Use of system-identified transfer functions (from models or experiments) for control applications. The output is typically the room-air temperature, or the “operative temperature” (which considers the temperature of the floor and walls). In this case, TF1 has units of thermal admittance (W/K); in a passive solar house, solar radiation is the dominant factor in the indoor temperature fluctuations.

AI

R

FL

SE

CT

IO

N AI

R

I AZ

R

FL

NG

SE

CT

IO

N

Tair-out

OW

OW FL

OW

Tair-in

Figure 4. Schematic of an air-based BIPV/T roof. This example has a glazing section for higher final air temperatures.

V.

SOLAR THERMAL COLLECTORS FOR HOT WATER

Domestic water heating accounts for about 18% of the energy consumed in Canadian homes [11]. Just as in the case of electric user loads, the first measure should be to reduce domestic hot water consumption (faucets with aerators, lowflow showerheads, low-water consumption appliances). A heat exchanger installed in shower drains can be used to recover a significant fraction of the thermal energy. From the energy supply side, a BIPV/T system could also provide a substantial portion of the domestic hot water energy needs. However, a solar thermal collector is more suitable for domestic hot water (DHW) because of the temperature ranges involved. While space heating requires temperatures of 25-40 ºC, DHW is typically kept at 55-60 ºC. VI.

THERMAL ENERGY STORAGE, CONTROL AND LOAD MANAGEMENT

A. Utilization of BIPV/T Thermal Energy and TES Devices In air-based BIPV/T systems, the heat removed from the roof can be used to heat up a structural element of the house with a large thermal mass, typically a hollow-core concrete floor slab [12]. Using the “passive” thermal storage element permits delaying the release of the heat to the indoor air space, thus contributing to extend the use of the collected solar thermal energy. Another alternative is to use the air from the BIPV/T roof to charge an active thermal energy storage system (TES). Several alternatives exist for active TES systems (PCM materials, thermochemical storage) [13], but the most common one in solar systems is a large water reservoir. Water has several advantages, including its high specific heat, widespread availability and the fact that plumbers and installers are familiar with its use [13, 14]. The active TES may be charged either with an air-to-water heat exchanger (when the air temperatures are high enough) or with a heat pump (HP), which requires supplying electrical power to a compressor). Heat may be then supplied to the space with a radiant floor heating system or with a coil in an air-handling unit. Direct injection of the heat from the BIPV/T system is discouraged as overheating could easily occur. B. Energy Storage and Control -Electric Load Management Due to the variability of solar radiation, storage and control of the solar energy resource is a central topic in solar energy engineering. Since a NZESH exchanges electric power with the utility grid, the grid plays the role of a reservoir for the house’s electric generation. An intelligently planned NZESH not only consumes less energy: it can also have a significant impact on peak load shedding for the grid. Load management is facilitated in several ways: • Reduced demand due to the use of high-efficiency appliances and lighting. • Electric power generation with PV panels. • Collection and storage of solar thermal energy. This can be accomplished by: (a) storing passive solar heat gains in the building’s thermal mass and (b) storing heat in active TES systems and DHW tanks. This is especially

relevant for locations, such as Québec, where electricity is intensively used for space and DHW and winter peaks are an issue. A record winter peak (37,230 MW) in Québec was recently registered on January 16th, 2009, at 8:00 a.m [15]. • Use of heat pumps instead of baseboard heaters when electricity is used for heating. • Advanced control of appliances and systems (e.g. occupancy sensors for lights and variable speed controllers for pumps and fans). • A NZESH and a utility grid with “smart” features can be mutually beneficial. For example, a smart meter could automatically switch on the clothes dryer at offpeak times (demand response strategies). This smart meter could communicate with the occupants through a human-machine interface (HMI) display to suggest a schedule for energy-intensive domestic chores (for example, clothes washing, clothes drying, cooking), and for charging an electric vehicle. HMI devices could also display energy and power consumed to raise awareness in the house’s occupants. Strategies for displacing thermal loads to off-peak periods have often been applied in commercial buildings, where timeof-use rates and demand charges provide incentives for their implementation. Commercial buildings tend to require cooling more often than heating. Consequently, ice storage systems are sometimes used to displace cooling loads to off-peak periods [16]. Commercial buildings have also been the focus of research on optimal control of passive storage (i.e., in the building’s thermal mass) and active storage devices [16, 17]. High temperature electric thermal storage (ETS) devices have also been used for displacing heating loads in commercial buildings and have been studied for residential buildings [18]. Zero-peak houses and communities (i.e., that will not drain power from the grid at on-peak periods) are possible and currently are subject of research [19, 20]. Apart from their impact on daily load fluctuations, NZESHs can also have a significant effect on yearly load profiles. This impact will of course be a strong function of location (climate, conditions of the electric grid, electric energy sources, etc.). C. Energy Storage and Control Suitable control is required to manage the collection and delivery of solar thermal energy. Control systems have to manage passive energy storage and active TES devices in order to minimize the need for a backup heat source. The strong linkage between thermal and electric loads also implies that management of TES devices (including DHW tanks) is also a way of implementing load management. A backup heat source remains necessary in most applications, since space restrictions in a single-family house impose a limit on the size of the active TES system that can be installed. Recommended alternatives for the backup source are a ground-source loop or a wood-pellet boiler. In larger installations –for example, at a community scale– the use of large TES tanks is more easily justifiable, offering the opportunity of long-time (seasonal) thermal energy storage.

D. Predictive Control Algorithms The peak power generation of a NZESH usually occurs at solar noon: this usually does not coincide with the peak heating and DHW loads. Predictive control can be used to correct the mismatch between the times of energy collection and its use. Weather and load forecast are valuable tools for the implementation of predictive control. Online weather forecasts with abundant and detailed information (e.g., beam and diffuse horizontal radiation, temperature, humidity, precipitation, wind speed and direction) are currently available [21]. Predictive control can be useful to establish the desired house temperature and state of charge of the TES over time. For example, if cold conditions are expected on the next day or during the following hours, it is advisable to raise both the house temperature and the temperatures in a storage TES tank. The house is allowed to coast during for a certain period. Afterwards, thermal energy is removed from the TES device(s) and delivered to the house as required. In contrast, if a mild sunny day is expected – particularly in the intermediate seasons– then the house temperature can be lowered and motorized blinds can be closed, totally or partially, to prevent overheating [22]. Predictive control algorithms can be used to anticipate the time delay of a radiant floor heating system, which is of the order of hours. The HVAC system needs to act with anticipation to account for this delay and avoid occupant discomfort. Transfer functions, obtained from system identification of experiments or numerical models, are useful tools for this task. VII. OTHER POTENTIAL USES FOR THE ENERGY COLLECTED The BIPV system can be planned to have additional energy for a plug-in electric vehicle. The batteries of the electric vehicle can also serve as an energy supply in the case of a grid blackout for basic loads (V2H scheme). Since the number of PV modules affects the thermal performance of a BIPV/T system, the selection of the PV installed capacity should consider this. Energy exchange (both electrical and thermal) can also take place between the house and an annex, for example, a greenhouse or a storage shed. Each of these peripheral buildings should contribute to their energy consumption. The design of a greenhouse offers interesting possibilities for solar energy use. VIII. CASE STUDY: THE ALSTONVALE NET ZERO HOUSE The Alstonvale Net Zero House (ANZH) is a net-zero energy solar house currently under construction in Hudson (Québec), near Montréal [23, 24]. The ANZH is one of the winners of the Equilibrium Initiative, a Canadian competition of advanced houses that took place in 2006-2007. The ANZH illustrates many of the design strategies and systems discussed in this paper. This 230-m2 house has a well insulated building envelope, a 20-cm concrete floor in its main level, and a 6-cm concrete floor on the upper level. About 42% of its south façade is covered with advanced fenestration (triple-glazed with low emissivity coatings).

Solar heat gains will be the most important contributor to covering the heating energy of the building. During sunny days, even the coldest days in winter (which can be below -20 °C in Montréal), there will be no need for mechanical heating. The energy stored in the thermal mass should maintain comfortable conditions for about 24 hours. The ANZH will include a BIPV/T roof, installed with a due South orientation, at a 45° slope (nearly identical to the latitude of the site). The BIPV/T system will consist of 3 rows of 16 panels, each with a rated power of 175 W at standard test conditions, adding up to 8.4 kW. A fourth row consisting of 16 glass rectangles (see Fig. 4) will be located near the top of the roof. In summary, 75% of the roof will be covered with PV panels, and 25% with a glazing section. A dark absorber surface with low emissivity (to reduce heat losses) will receive the solar radiation passing through the glazing. This last section will help increase the temperature of the air moving upwards through the channel. Up to 1000 L/s of exterior air will be driven with a variable speed fan under the PV panels and the glazing. This system can increase the air temperature more than 30°C, depending on solar radiation, exterior temperature and wind speed. Apart from the 8.4 kW of electric power, more than 22 kW of thermal energy can be recovered with two heat pumps, and stored in a 4,000 L water tank. This thermal energy can be used during cloudy days or when thermal mass of the house has been discharged (i.e., its temperature has dropped). The originally planned electrical generation capacity was 5.6 kW, corresponding to 32 panels or 50% of the roof. According to simulations with RETScreen [25], 5.6 kW of PV generates about 7,000 kWh/year, which should be enough for the electrical loads of the house (appliances, lighting and mechanical equipment). The installed capacity was increased, first to 42 panels (7.3 kW) to generate additional power for an electric vehicle (1,500 – 1,800 kWh/year) and then to the current 8.4 kW, for several reasons: to improve aesthetics, to facilitate the wiring of PV panels with similar temperatures, and to account for other factors affecting PV generation (snow, equipment downtime, etc). It is estimated, however, that the annual generation (about 10,000 kWh) will still significantly exceed the energy consumed by the house and the electric car. Adding more PV panels implies reducing the glazing area on top of the roof, which results in lower final air temperatures. This means that the thermal energy output of the roof is reduced when additional PV capacity is added. To partially compensate for the effect of more PV panels, an extremely low emissivity material was selected for the absorber plate under the glazing. The backup thermal energy source consists of a ground loop. It is has been estimated that this ground source will be used about 1/3 of the time. Predictive control can help in using more effectively the BIPV/T roof and in reducing the dependence on the ground source [24]. The energy flux (Sankey) diagram shown in Fig. 5 illustrates the annual energy balances. The contribution of the passive solar heat gains is significant for covering the gross heating load.

[4]

[5] [6] [7] [8] [9]

Figure 5. Sankey diagram of energy fluxes in the ANZ ZH: A - Electric car (1,600 kWh); B – Fans and pumps (1,000 kWh); C – Lighting and appliances (4,000 kWh); D – Heat pumps (2, 400 kWhh); E – Internal heat gains (3,400 kWh); F – 1,600 kWh.

IX.

CONCLUSIONS

Designing the energy system of a NZ ZESH should be undertaken in a comprehensive manner, takking into account interactions between the electric energy generation and consumption, heating and cooling loads and thhe performance of the mechanical system. This requires a new appproach from the professionals involved, and a deeper insight into the physical phenomena involved. Further investigations are needed on aadvanced control algorithms to manage the exchange of electricc energy between the house and the grid, and to manage energyy fluxes (electric and thermal) within the house’s systems, andd with peripheral devices or installations (such a sheds, greennhouses, electric vehicles and energy storage systems). These ccontrol strategies could be instrumental for the optimal expploitation of the widespread installation of smart meters and other “smart grid” features. As mentioned above, most investigaations have dealt with commercial buildings, and solutions tailoored to the needs of residential buildings are still required.

[10]

[11] [12]

[13] [14]

[15] [16]

[17]

[18]

[19]

ACKNOWLEDGMENT Funding and technical support have been provided by the Canadian Solar Buildings Research Netw work (SBRN) a strategic network of the Natural Sciences and Engineering Research Council of Canada (NSER RC), and by CanmetENERGY Varennes (Natural Ressources Canada) through the TEAM (Technology Early A Action Measures) initiative. Technical and financial support off Hydro Québec, CMHC and l’Agence de l’Efficacité Énergétiqque du Québec to the Alstonvale project is gratefully acknowlledged. The first author would like to thank NSERC for ffinancial support through a CGS Alexander Graham Bell Graduaate Scholarship.

[20]

[21] [22]

[23]

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