Chapter 7 Solar Energy for Buildings: Approaching Zero Net Energy

• The sun plays an important role in energy-efficient buildings

• Windows can provide natural daylighting but can also increase cooling loads

• Solar thermal collectors and solar photovoltaics (PVs) can be used for heating, cooling, water heating, and cooking

• Goal is to have all-electric systems offset by rooftop PVs

7.1 The Solar Resource

• Understanding the sun's position can help with heating and cooling

• Quantitative evaluation of its impact on building energy demand

7.1.1 Solar Angles to Help Us Design Overhangs

• Sun rises in the east, reaches maximum height at solar noon, and sets in the west

• Altitude angle (b) and azimuth angle (f) are key angles of interest

• Altitude angle at solar noon varies with seasons

• Relationship: bN = 90 - L + δ, where bN is the altitude angle, L is the local latitude, and δ is the solar declination

• Solar declination varies sinusoidally between ±23.45°

• Equations for solar declination: δ = 23.5 sin [ 360° (n - 81)] / 365°

• Overhangs can be designed based on solar noon altitude angles

• Equation for shadow distance: y = P tanβN

7.1.2 Sun Path Diagrams

• Equations for locating the sun at any time of day and any day of the year are cumbersome

• Sun path diagrams available on the web can show the sun's location

• Useful for site analysis and determining obstructions

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• Example of designing an overhang for Palo Alto, California

• Altitude angle of the sun at solar noon on June 21: bN (June 21) = 76°

• Altitude angle of the sun at solar noon on December 21: bN (December 21) = 29°

• Designing overhangs for solar noon provides shading in summer and exposure to sun in winter

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• Figures illustrating the location of the sun and altitude angles at solar noon

• Example of designing an overhang for Palo Alto, California

• 2-foot overhang shades glass doors at noon in June and exposes windows to sun at noon in December

• Designing overhangs for solar noon provides shading or exposure to sun throughout the day

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• Importance of measuring altitude and azimuth angles of potential obstructions

  • Can be measured using a protractor, plumb bob, and compass

• Sketching obstructions on a sun path diagram helps determine monthly hour-by-hour shading problems at a site

• Example of a site diagrammed in Figure 7.5

  • Full sun from February through October

  • Shaded from roughly 8:30 to 10:00 a.m. and after about 3:00 p.m. from November through January

• Shadow diagrams are useful for determining the distance and direction of shadows cast by obstacles

• Applications of shadow diagrams

  • Spacing buildings in a housing development to ensure good solar access for each unit

  • Designing a rooftop PV array to avoid shading between rows of collectors

• Shadow diagrams enable quick and easy analysis for decision making

• Spacing of grid lines on the shadow diagram should be the same as the height of the imaginary peg

• Shading diagrams are useful for designing solar collectors, as shading can significantly reduce power output

• Shadow diagrams can be used for architectural modeling and heliodon testing of building models

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• Figure 7.5 shows a sun path diagram for 40°N latitude, indicating shading from November through January in the morning and late afternoon

• Figure 7.6 illustrates the origins of a shadow diagram

• Figure 7.7 shows a shadow diagram drawn for 40°N latitude, indicating the shadow line at 8 a.m.

• Figure 7.8 demonstrates the rooftop spacing of solar collectors to avoid shading

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• Solution Box 7.2 provides an example of rooftop solar collector spacing

• The design calls for rack-mounted photovoltaics on a south-facing commercial building at 40°N latitude

• The goal is to ensure no shading between 8 a.m. and 4 p.m. throughout the year

• The shadow diagram in Figure 7.7 is used to determine the worst time of the year for shading (December)

• The spacing between collector rows is calculated based on the height of the back of a row of collectors (3 ft)

• Tradeoffs and alternative options for spacing are considered

• Figure 7.10 shows a simple heliodon with a shadow diagram for testing shading

• Figure 7.9 demonstrates the use of a shadow diagram with a physical model to predict shading problems, specifically on rooftop photovoltaics

Page 7: Energy-Efficient Building Design Strategies

• Certain fundamental building design guidelines can improve energy efficiency and occupancy comfort.

• Building orientation plays a crucial role in achieving energy efficiency.

  • South-facing windows are ideal for solar gains and can be controlled with overhangs.

  • East- and west-facing windows are exposed to horizontal solar radiation and can cause overheating in the summer.

• Orienting the building along the east-west direction maximizes controlled solar gains and minimizes exposure to morning and afternoon summer heating.

Page 8: South-Facing Windows for Solar Gains

• South-facing windows can bring in a lot of solar energy but also lose energy all day and night during winter.

• The solar heat gain coefficient (SHGC) measures the fraction of solar energy transmitted into the building.

• The National Fenestration Rating Council (NFRC) window labels provide information on U-factor, SHGC, visible wavelengths transmission, and air leakage.

• Simple, double-glazed, south-facing windows provide net positive heat flow in the winter, exceeding thermal losses.

Page 9: Cooling Loads

• Cooling calculations are more complex than heating calculations.

• Cooling loads include UA heat gains, unwanted solar gains, infiltration and ventilation air, and internal gains.

• The best way to reduce air conditioning loads is to focus on avoiding the need for cooling.

• Building orientation and taking advantage of prevailing winds can help reduce cooling loads.

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• Ventilation efficiency is improved when breezes approach at an angle

  • Locating heat and humidity-producing rooms on the leeward side of the house helps prevent their effects on other areas

  • Attached garages should also be placed on the leeward side to maintain airflow

• Sun beating down on rooftops increases cooling loads and contributes to the heat island effect

  • Attic temperatures can be reduced by using continuous ridge and soffit vents with radiant barriers

  • Dark-colored roofing tiles increase cooling loads

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• Cooling loads are more complicated than just reducing solar gains

• Trade winds can be utilized for natural cooling

• Reducing roof cooling loads can be achieved through shading, roofing, vents, and radiant barriers

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• Green roofs can reduce rooftop temperatures and provide stormwater management services

• Increasing roof surface emissivity in the far-infrared region can encourage radiative cooling

• Proper building orientation, glass selection, shading, and vegetation placement can reduce air conditioning loads in commercial buildings

• Better windows can reduce cooling and lighting loads

  • Window orientation is important to avoid solar gains

  • East- and west-facing windows receive more solar radiation in the summer, increasing air conditioning loads

  • Tinted windows absorb solar radiation, resulting in hot glass and discomfort

  • Reflective windows block all wavelengths, reducing natural daylight and increasing the need for artificial lighting

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• Spectrally selective, low-emissivity glass is a modern approach to controlling solar gains in buildings.

  • Solar spectrum: 7% UV, 47% visible, 46% infrared.

  • Glazing with low solar heat gain coefficient (SHGC) and high visible light transmittance (VT) can reduce air conditioning loads and maximize daylight.

  • Tradeoff between SHGC and VT can be evaluated using the light-to-solar-gain (LSG) ratio.

• High-emissivity surfaces in rooftops can increase surface reflectance and cooling.

• Spectrally selective, low-e windows with automatic dimming lighting systems can reduce lighting and cooling loads in commercial buildings.

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• Low-e windows have lower SHGC, higher VT, and higher R-value compared to traditional windows.

• Cooling loads can be calculated by analyzing building envelope gains, solar gains, and dehumidification needs.

• Heat gains due to temperature differences can be calculated using the (UA)-value analysis.

• Cooling degree-days (CDD) can be used to estimate annual cooling loads.

• Annual cost of cooling can be estimated by accounting for the efficiency of the air conditioner and duct inefficiencies.

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• Annual electricity input for cooling can be estimated using the SEER and duct efficiency.

• Clear-sky irradiance estimates can be used to calculate cooling driven by solar gain loads.

• Example calculation provided for cooling load in Houston due to envelope and west-facing window gains.

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• Solar gain through west-facing windows is about 1100 Btu/ft2 per day of solar irradiance.

  • This value is similar for a wide range of latitudes.

• The equation for calculating solar gain is: Solar gain = 1100 Btu/ft2/day × N (days/yr) × SHGC.

• Dehumidification is a major part of the cooling load in many parts of the country.

• It takes about 1060 Btu to evaporate 1 pound of water.

• To dehumidify moist air, 1060 Btu of latent heat needs to be removed for every pound of water removed.

• A psychrometric chart shows the moisture content of air versus dry bulb temperature.

• Relative humidity (rH) is the fraction of the maximum moisture that air can hold at a given dry-bulb temperature.

• The process of dehumidification is illustrated on the psychrometric chart.

• The examples in Solution Boxes 7.4 and 7.5 demonstrate the sources of cooling loads for buildings: heat transfer through the building envelope, solar gains through windows, and dehumidification.

• These examples apply to both residential and commercial buildings.

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• Human comfort is influenced by various variables such as air temperature, mean radiant temperature, air speed, humidity, metabolic rate, and clothing.

• The predicted mean vote (PMV) is a metric that combines these variables to determine comfort.

• Positive values of PMV indicate comfort.

• The Center for the Built Environment (CBE) website allows users to input variables and determine if the outcome falls within the comfort zone.

• The met is a measure of metabolism.

• The clo is a measure of clothing insulation.

Solution Box 7.5:

• The cooling load associated with dehumidifying infiltration air is calculated for a Houston house.

• The house has 2000 ft2 of floor space with 8-ft ceilings and an infiltration rate of 0.5 ach.

• The rate at which infiltration air needs to be dehumidified is calculated.

• The absolute humidity of the outside air needs to be dropped to the desired supply condition.

• The electricity cost for dehumidification is calculated based on the SEER of the air conditioner and the electricity rate.

• The total air conditioning demand for the house, including sensible heat transfer, west-facing windows, and dehumidification, is calculated.

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• Metabolic rate calculation for energy expenditure

  • Met rate = Metabolic rate (hr-lb) × Weight (lb) × 3.97 (kcal) × Met / Surface area (ft2)

  • Example met rates: sleeping (0.7 met), sitting quietly (1.0 met), standing (1.2 met), walking at 2 mph (2.0 met)

• Clo values for clothing insulation

  • Examples: "typical clothing" (0.5 clo summer; 1.0 clo winter)

• Importance of wearing lightweight clothing in the office in the summer

  • Center of the comfort zone suggests indoor temperature can be 10°F warmer with lightweight clothing

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• Passive solar heating as a way to utilize sunlight for heating houses

  • Passive solar approach: encouraging sunlight to pass through windows and other solar apertures

  • Simple, cheap, and reliable compared to active solar approach

• Design guidelines for passive solar

  1. Maximize envelope efficiency

  2. Orient the building along an east-west axis

  3. Provide south-facing glazing systems

  4. Design proper overhangs for summer protection

  5. Provide sufficient thermal mass to absorb excess solar energy

• Comparison of a conventional house and a sun-tempered house

  • Conventional house: higher UA-value and thermal index

  • Sun-tempered house: lower UA-value and thermal index

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• Rule of thumb: solar gain window area should be less than 7% of floor space area to avoid overheating

  • South-facing windows assumed to be thermally neutral in heat loss calculations

  • Example: 1500 ft2 house with 100 ft2 of south-facing windows reduces heat loss factor from 482 to 411 Btu/hr-°F, a 15% savings

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• Importance of thermal mass in passive solar house design

  • Increasing south-facing window area without sufficient thermal mass leads to excessive temperature swings

  • Example: doubling window area to 14% causes temperature to rise above 90°F, additional thermal mass needed

• Different materials for thermal mass storage

  • Concrete: volumetric capacitance of 28 Btu/°F-ft3

  • Water: volumetric capacitance of 62.4 Btu/°F-ft3

  • Phase change material made with hydrated calcium chloride: volumetric capacitance of about 8000 Btu/ft3

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• Three types of passive solar heating systems: direct gain, mass wall, and sunspace

  • Direct gain: south-facing glass, overhang for shading, and sufficient thermal mass

  • Mass wall: thermal mass directly behind glazing, often called Trombe wall

  • Sunspace: attached greenhouse with greater temperature range, heat directed into adjacent space

• Estimating solar performance using load/collector ratio method

  • Rough estimate of annual heat load for south-facing passive solar house

  • Solar aperture area used as a basis for calculation

Page 23: Solar Energy for Buildings: Approaching Zero Net Energy

• The key parameter that describes a passive solar house is called the load/collector ratio (LCR)

  • LCR is calculated using a heat loss term (numerator) and a solar gain term (denominator)

  • A low LCR indicates better solar performance

• The full LCR procedure involves finding the closest match between the house design and standard passive solar designs

  • Standard designs are defined in the passive solar design handbook

  • A huge table of numbers is used to determine the solar savings fraction (SSF) based on the LCR

• The energy that needs to be delivered by the heating system is calculated using the LCR and heating degree-days at a 65°F base temperature (HDD65)

  • Equation: Qdel (Btu/yr) = 24 (UA)ST HDD65 (1 – SSF)

• Simplified procedure and tables are provided for illustrative purposes

  • Table 7.2 allows selection of a generic passive solar design for a particular city

Page 24: Energy for Sustainability

• Table 7.2 provides LCR values for different passive solar designs and cities

  • LCR values are given for Trombe Wall (TWB1), Direct Gain (DBG1), and Sunspace (SSB1) designs

  • LCR values are provided for different solar savings fractions (SSF) and heating degree-days at a 65°F base temperature (HDD65)

• The LCR values in the table are specific to different cities across the United States

  • Cities include Birmingham, Phoenix, Tucson, Los Angeles, and many others

• The table also provides average temperatures (Tavg) and specific-heating energy data for 40 passive solar homes

  • Most homes require less than 2 Btu/ft2 per degree-day of auxiliary heating

Note: The transcript contains information from pages 23 and 24 of a document related to solar energy for buildings. It discusses the load/collector ratio (LCR) as a key parameter for passive solar houses, the calculation of solar savings fraction (SSF), and the energy requirements for heating systems. It also includes a table with LCR values for different passive solar designs and cities, along with specific-heating energy data for passive solar homes.

Page 25: Solar Energy for Buildings: Approaching Zero Net Energy

• LCR method for estimating annual heating demand

  • LCR2 and LCR5 values are used to calculate SSF (solar savings fraction)

  • Simplified LCR method provides a quick estimate of annual heating demand

• Month-by-month analysis called the solar load ratio (SLR) method

• Passive solar ideas have been proven to work

  • Study of 40 actual houses showed that most houses had furnace supplies of less than 2 Btu/ft2 per degree-day, compared to 6-8 Btu/ft2 -°F-day for new houses today

Page 26: Domestic Water Heating

• Conventional storage tank water heaters

  • Range in size from 40 gallons to 80 gallons

  • Fueled with natural gas, electricity, propane, or fuel oil

• Other approaches for hot water with greater energy efficiency

  • Demand (tankless or instantaneous) water heaters

  • Solar water heaters

  • Heat pump water heaters

• Efficiencies of water heaters rated by energy factor (EF)

• Minimum efficiency standards for water heaters in the United States

• Heat pump water heaters as a more efficient alternative to electric resistance heating

  • Extract heat from surrounding room air and deliver it to the tank

  • Can be ducted to the outside if located inside the home

Solution Box 7.7: Applying the Simplified LCR Method to a Direct Gain House

• Example calculation for estimating fuel bill with passive solar design features

• Calculation steps:

  • Subtract the (UA)-value of direct-gain windows from the total (UA)-value of the house

  • Calculate LCR using the (UA)-value and the area of the direct-gain windows

  • Use LCR2 and LCR5 values to calculate SSF

  • Use SSF and HDD65 (heating degree days at 65°F) to calculate annual heating demand

  • Account for furnace and duct losses to determine the final fuel bill

• Comparison of fuel bill with and without passive solar design features

Page 27

• Table 7.3 shows the energy factors for different types of water heaters

  • Energy factors for natural gas storage water heaters range from 0.675 to 0.0015 V for volumes of 20-55 gallons and 0.8012 to 0.00078 V for volumes of 56-100 gallons

  • Energy factors for fuel oil storage water heaters are 0.88 to 0.0019 V for volumes less than 50 gallons

  • Energy factors for electric storage water heaters range from 0.96 to 0.0003 V for volumes of 20-55 gallons and 2.057 to 0.00113 V for volumes greater than 55 gallons

  • Energy factors for natural gas instantaneous water heaters are 0.82 for volumes less than 2 gallons

  • Energy factors for electric instantaneous water heaters are 0.93 for volumes less than 2 gallons

• Solution Box 7.8 provides an example of estimating the operational cost of water heating

  • The example compares the annual cost of supplying 64 gallons of hot water per day heated from 58°F to 135°F using natural gas and electricity

  • For a 50-gallon tank meeting the minimum efficiency standards, the annual cost of using gas is $300 and the annual cost of using electricity is $558

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• Figure 7.31 shows the annual energy delivered per kW of PV-rated power for south-facing PVs at a tilt angle of 20°

• Table 7.4 provides operating costs for various types of water heaters

  • Operating costs for gas-fired water heaters range from $293 to $72 per year depending on the type and size

  • Operating costs for electric water heaters range from $556 to $168 per year depending on the type and size

  • Solar-thermal water heaters with a solar fraction of 0.7 have an operating cost of $72 per year for a 60-gallon tank

• Assumptions for the operating costs include a 64 gallons per day demand, 77°F temperature difference, natural gas costing $1.20/therm, and electricity costing $0.12/kWh

• Figure 7.30 provides an example of a heat pump water heater with an energy factor of 3.25

Page 29

• Heat pump water heaters can recover the additional capital cost in less than 3 years and have lower operational costs compared to natural gas water heaters

• Heat pump water heaters can use electricity from a rooftop PV system

• PVWatts, an online calculator from the National Renewable Energy Laboratory, can be used to determine the irradiance available at a site and the energy delivered per kW of PV-rated power

• The area needed for a PV array can be calculated using the rated power and module efficiency

• Solar thermal water heating systems take advantage of the concept of heating water with sunlight

• Solar water heaters have had a spotty history but have seen a resurgence in recent years

• The simplest approach to solar thermal water heaters is using black hoses on the roof to heat water

• When a hot water tap is turned on, cold water goes up to the panel and pushes solar-heated water into a conventional water heater

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• Solar water heating systems are inexpensive and reliable

  • No moving parts, pumps, controllers, or sensors

• Water heats up during the day and cools down at night

  • Better to take a shower at night than in the morning

• Systems may need to be drained during cold winters to avoid freeze problems

• Flat-plate collector is the heart of most solar water heating systems

  • Consists of a black absorber plate in an insulated box with a glass top

  • Water circulates from a storage tank to the collector

  • Pump or natural buoyancy causes circulation

  • Controller with sensors may be needed if pumped

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• Conventional flat-plate collectors lose efficiency as ambient temperatures drop

• Some collectors use evacuated cylindrical tubes with linear absorber plates

  • Vacuum eliminates convective losses and improves efficiency

• Heat pipes transfer heat from the absorber to the header

• Thermosiphoning systems use convective circulation without pumps

• Many choices of system types and freeze protection methods for solar water heating systems

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• Solar water heating systems face the challenge of achieving high solar fractions

• Storage is limited to the tank, which may store one day's worth of hot water

• Cloudy or rainy weather produces no solar output

• Photovoltaic-powered heat pump systems can achieve 100% coverage

• Photovoltaic systems are not subject to freeze problems and are cheaper

• Modeling energy requirements for net zero solar homes is straightforward using various energy efficiency measures

Page 33: Solar Energy for Buildings: Approaching Zero Net Energy

• Techniques described in this chapter can help size a PV system to meet loads on an annual basis.

  • Example: Heating, cooling, and hot water loads were calculated for a 2000-ft2 passive solar house in Denver.

• Plug and lighting loads make up a high fraction of the total loads.

  • Estimated using a rule-of-thumb of 2 kWh/yr per square foot of floor space.

• An electric vehicle driven 12,000 miles per year at 3.5 miles per kWh is included in the analysis.

• Figure 7.36 provides an overview of the overall project.

Page 34: Energy for Sustainability

• Aggressive efforts to improve insulation, windows, ducts, and heating/cooling systems can significantly reduce thermal energy demand in buildings.

• Passive solar heating can almost eliminate the need for heating in many locations.

• Careful building orientation, overhangs, natural ventilation, spectrally selective windows, and cool roofing materials can control cooling loads in challenging climate zones.

• Heat pumps can be used for space conditioning and water heater heating in net zero energy homes.

• A simple sizing technique is introduced to estimate the size of PV systems needed to meet building energy loads.

• The next chapter will focus on electricity demands for lighting and appliances in buildings.

• The concept of a whole building life-cycle assessment, including embodied energy, will be explored.

• Green building rating systems and the goal of zero-energy, zero-carbon buildings will be discussed.