Designing a Solar Home

Designing a Solar Home

Many homeowners today are concerned about pollution, greenhouse gas emissions and dwindling fossil fuels. A growing number of people would like to own sustainable and environmentally responsible homes. They would like to be able to promote sustainability and to leave resources for future generations.

Energy from the Sun

What is a solar home? The term has been widely adopted to describe a house that employs appropriate design principles and advanced technology to utilize the clean and abundant energy from the sun. Energy from the sun can provide heat and electricity for residential and other building projects.

Heat from the sun’s rays can be captured directly with passive solar building design. Passive solar design uses south facing windows with an appropriately sized overhang to collect the sun’s heat in the winter and keep the house shaded and cool in the summer. Thermal mass is often used to hold the heat overnight and moderate the temperature during the day. Thermal energy from the sun can also be used to heat a fluid in a solar thermal collector that then transfers the heat to be stored in a hot water tank where it can be used for domestic hot water and for space heating.

Light from the sun can also be converted to electricity by photovoltaic cells. Wind power from wind generators is also an indirect form of solar energy since wind results from differential heating of the earth’s surface by the sun

Designing a Solar Home
Standalone Experimental Solar and Wind powered house, designed as an RTM (Ready-to-Move) prototype for Suncatcher Solar Homes	For this lab, you will be visiting an experimental standalone solar and wind powered house designed and built by Angelika Ortlepp and Bill Campbell. A solar powered home presents a unique set of challenges for the designer. The home relies for its power on weather conditions, which are highly variable from site to site and even from year to year at any particular site. The technology is still fairly costly so designing a system with high reliability often comes at a cost that makes it no longer price competitive with grid tied options. Many engineers, scientists and homeowners have tackled these challenges to provide effective designs for their applications and geographic location. Solar power system design must take into account the solar radiation that is available at a particular site and the power needs of the consumer. If wind resources are good, a wind generator may also be an option. Passive solar home design is the most cost effective way to provide a substantial portion of a building’s heating requirements. The design method has been investigated extensively and guidelines are well established. Software is available for design and analysis of passive solar buildings.
1. Passive Solar Design Passive solar home design is not a new concept. For as long as people have built homes, they have used the sunshine entering through the windows as a source of light and heat. Much has been written about passive solar design in the last few decades and there are several available computer programs to assist in designing and analyzing passive solar design features. Basic Guidelines for Passive Solar Design: Passive solar design makes effective use of building shape and orientation, distribution of window glass and overhangs, and heat absorbing materials. Some general guidelines are: Building Layout and Orientation: The longest wall of the house should be oriented within 10 degrees of true south. Jogs, offsets and other projections should be minimized on the south wall, but porches and garages are useful on east, west and north walls for shading and insulation value. Window Glass and Overhangs: To maximize solar heat gain and minimize thermal losses, window glass should be distributed as (building code regulations permitting): South-facing Glass: 5% to 12% of the floor area of the house. East-facing Glass: less than 4% of the floor area. West-facing Glass: less than 2% of the floor area. North-facing Glass: less than 4% of the floor area. Windows on the east and west sides of the house do not receive much solar radiation during the winter months, so they are a source of heat loss rather than heat gain. In the summer, they are very difficult to shade so are an unwanted source of heat gain. However, windows are still desirable on these walls because they admit light to the rooms, provide more attractive house design and views in these directions, and operating windows assist with the natural cooling of the house in summer months. Overhangs should not cast shade on south-facing windows on the winter solstice (December 21) and should completely shade these windows on the summer solstice (June 21). Heat Absorbing Materials for Thermal Mass: Thermal mass such as concrete or tile floors and brick or stone facings are used to absorb solar heat during the day and reradiate it at night. Although the mass of the house itself (drywall, floors, cabinets, etc.) provides some thermal mass more may be recommended. If the south-facing glass is more than 7% of the floor area thermal mass should be added for the area of glass that is greater the 7%. In general, the maximum amount of floor mass that should be used is 1.5 times the area of south glass. If more thermal mass is needed it can be added to the walls. The recommended thickness for thermal mass is 2” to 4”. Thicknesses greater than 4” do not contribute significantly to heat storage. Energy savings of up to 80% are possible with the use of energy efficient, high solar gain windows and appropriate insulation for the climate conditions. The selection of windows is a tradeoff between achieving high solar gain and a high R value (insulation factor) to prevent heat losses during the night. Window coverings can reduce nighttime heat losses by up to 30%. 2. Solar and Wind Power Systems A stand-alone residential renewable power system consists of five basic components: A charging system, consisting of a solar panel array or a wind generator or both. An energy storage system, generally a bank of deep cycle lead-acid batteries. A charge control system, to prevent overcharging of the batteries. An inverter to convert low voltage DC to 110 V AC to power normal household loads. A backup generator operating on gasoline, diesel, natural gas or propane. The effectiveness of the total system depends on the availability of solar radiation and the wind speed distribution at the proposed building site and the load requirements of the home. The energy storage system should provide the power needs of the house for a specific number of days when no charging source is available (days of autonomy). The charging system must be able to run the building’s loads and recharge the batteries, based on available solar radiation and wind resources.
	Solar Power - Photovoltaics The process of converting solar energy to electrical energy is called photovoltaic power generation. The photovoltaic cell (or solar cell) was invented in the early 1950s, with the increase in semiconductor technology. Photovoltaic panels are made from silicon - either a single crystal, amorphous crystals, or a thin film. Considerable research is being done to find other materials, such as organic materials, that would be suitable. When sunlight strikes the surface of the semiconductor, it transfers energy to some of the electrons so that they are no longer bound to the nucleus and are free to move through the material. By connecting wires to the panel you can use this current of free electrons to do work on an outside circuit. Solar panels are specified by the open circuit voltage and short circuit current, and the Operating Point – also called the Maximum Power Point. This point can be determined experimentally by recording the current and voltage of the panel for various values of resistance, calculating the resulting power, and graphing it versus the voltage. The peak of the graph is the Maximum Power Point, or operating point, of the panel at which it operates the loads most efficiently. Photovoltaic cells have been criticized as taking more power to produce than they will generate in their lifetime. However, a study by K Knapp and T. Jester shows that photovoltaic panels recoup their production energy in two to four years, while their expected lifetime is in excess of twenty-five years.
Wind Power Generation Wind generators varying in power output from 100 W to several kW are common components in residential renewable power systems. The energy that can be produced by a wind generator depends primarily on the average wind speed and the wind speed distribution at the site, and on the swept area of the wind generator rotor blades. The power output of a wind generator varies directly with the blade swept area, so double the swept area will yield double the power. However, the wind speed has a cubic relationship to the power output. Wind Energy Output Wind Energy varies with the cube of the Wind Speed: Wind Speed doubles: 8 m/s – 16 m/s Output goes up by factor of 8 (2 x 2 x 2 = 8): 314 W/m2 – 2509 W/m2 Wind generators have a cut-in wind speed below which no power is produced and a maximum speed above which the generator would sustain damage, so it is either shut down or turned partially out of the wind for protection. Batteries The most common type of battery used for residential applications is the flooded lead-acid deep-cycle battery. The battery consists of a positive plate of lead dioxide, which is the active material, and a high surface area lead negative plate. They are immersed in an electrolyte of sulphuric acid solution. Batteries designed for solar power systems have thicker plates than automotive batteries so that they can be operated at a deeper Depth Of Discharge (DOD), typically 50%. Battery capacity is rated in “AH” – Amp hours. One amp hour is the ability to run something at one amp for one hour. If you multiply the AH by the system voltage, this will give you the kWh capacity of the battery. The capacity is not the same for all conditions, depending on the temperature, the discharge rate and the end voltage to which it is discharged. The batteries are usually rated at the 20 hour rate and the 100 hour rate. The slower discharge rate yields a higher capacity. In solar applications, the battery discharge rate is usually quite slow, so the 100 hour rate more closely approximates the actual capacity of the battery. Batteries should be fully charged fairly frequently and should not be discharged for extended periods or a condition called sulphation will occur and a higher voltage equalization charge will have to be applied to the battery to reverse the sulphation of the plates. Banks of batteries are generally sized to provide for the household power needs for approximately three days with no charging, if the charging system is purely solar, or for five days if the system is primarily a wind charging system. “Make hay while the sun shines” really applies here. If you use the power directly while the sun is shining, the power is not stored in the battery first so you eliminate battery losses. Charge Controllers Charge controllers are essential to protect the batteries from overcharging. They block reverse current that would discharge the batteries into the solar panels at night, but their main function is to prevent battery overcharge. If more charge is applied to a battery that is already fully charged it will separate the hydrogen and oxygen and “boil” off the gas. This can degrade the battery, cause overheating, and can stress the loads. Some controllers regulate the charge to the battery by simply switching the current totally on or totally off – ON/OFF Control. Others reduce the flow of current gradually. This is called pulse width modulation (PWM) and holds the voltage more constant. A more sophisticated method that has been introduced in the last few years is Maximum Power Point Tracking (MPPT). This device “tracks” the maximum power point of the panel. Inverters An inverter changes low voltage DC power (stored in the battery and produced by your solar or wind source) into standard (AC) alternating current, house power (120 or 240 VAC, 50 or 60 cycles). Modified Sine Wave Simple inexpensive inverters produce a modified sine wave output which is a very simplified version of what the power utility supplies. This is adequate for most small cabin and RV systems, but are not suitable for larger applications like year-round residential systems. Utility Grade Sine Wave For residential systems, a sine wave inverter more closely approximates utility power. Motors and electronics generally function better with sine wave inverters, which also deliver power more efficiently. Thus a lower power sine wave inverter will do the same job as a slightly higher power modified sine wave inverter. Pure Sine Wave The electric power produced by modern sine wave inverters is cleaner than the power delivered to wall sockets by an electric utility. And inverter power is uninterruptible--there are never any blackouts or brownouts. Modern sine wave inverters used for residential applications typically have efficiencies of 90 – 95 per cent. Some produce a very pure sine wave and have a high degree of programmability to allow the user to determine the optimum parameters and set points for the system. Designing the Power System 1. Sizing the System The size of the solar power system depends on how much power is needed to operate the various systems and appliances in the home. Loads can be continuous (running 24 hours a day) or intermittent. Many large loads, such as toasters, vacuum cleaners and microwaves are seldom used or are only used for very short time periods. On the other hand, small loads can contribute significantly to the overall load if they are a continuous draw. Small loads that are not obvious to the homeowner, because devices and appliances appear to be shut off but are still drawing power for quick power up, are generally referred to as “phantom loads” and are easily overlooked when calculating load requirements. These loads can be mostly eliminated by installing switch controlled outlets or a switched power bar that turns off the appliances when not in use. Designing a solar power system that is cost effective means making some choices about the household loads that are most important to you. While a solar power system is certainly capable of operating any size of load, cost considerations require a compromise and a consciousness of energy usage on a day-to-day basis. For a homeowner, this means making a choice of household loads that will supply the essentials such as water and mechanical systems and then selecting the appliances and devices that are believed to be most important to a fulfilling lifestyle. A load analysis determines the amount of energy in kWh used by the chosen selection of household loads so that the appropriate size of solar and /or wind charging system can be designed for the climate conditions of the area. The experimental house was designed to operate the necessary mechanical systems for the house, such as a jet pump to pump water from a sandpoint well and a submersible septic pump for the gray water overflow septic system, and also to operate the appliances and office and entertainment devices that were the basis of our lifestyle. 2. Determining Your Needs “Every watt not used is a watt that doesn’t have to be produced, processed, or stored.” - Richard Perez, Editor of Home Power Magazine Load Analysis for the Experimental House Load Rated Power (W) Hrs/day kWh/day Mechanical: Water pump 1200 0.3 0.36 Septic pump 800 0.05 0.04 Kitchen Appliances: Fridge 404 kWh/yr As rated 1.10 Coffee Maker 900 0.25 0.23 Toaster 900 0.2 0.18 Toaster Oven 1300 0.1 0.13 Electric Frying pan 1200 0.02 0.024 Slow Cooker 110 .1 0.011 Lighting and fans: 10 lights @ 15 Watts 150 2 0.3 Ceiling fans (2 @ 12W ea.) 24 4 0.1 Office and Entertainment: Television (27”) 100 2 0.1 VCR 30 0.5 0.015 DVD Player 30 1 0.03 Radio 2 15 0.03 Stereo 20 2 0.04 Aquarium lights 45 3 0.14 Aquarium filters 20 24 0.48 Laptop Computer 50 6 0.3 Printer 10 1 0.01 Modem and wireless router 40 4 0.16 Laundry: Washer (front loading) 227 kWh/yr 0.5 * rated 0.3 1 Dryer (110 V) – seldom used 398 kWh/yr 0.1 * rated 0.1 2 Iron 1100 .05 0.05 Small Power Tools 600 .1 .06 Car Block Heater 1200 .01 .01 Battery chargers 6 2 .01 (tools, cell phones, camera, etc.) Total AC Load: 4.31 @ 90 % inverter efficiency: 4.74 (1) The energy rating in KWh/year is based on 416 “Normal Cycle” operations per year and includes the energy required to heat the water[29]. This is more than one load per day, which is not necessary for two people. The usage estimate has been adjusted accordingly. (2) This energy rating in KWh/year is also based on 416 operations per year[29]. Our usage would be less based on fewer loads through the washer and we often hang the clothes to dry. Again, usage has been adjusted accordingly. Modern inverters have efficiencies of 90 to 95 per cent. Using the lowest rated efficiency of 90%, the anticipated average daily load for the experimental house would be about 4.8 kWh. 3. Energy Efficiency Measures The most important principle for designing a home with a solar power system is very aptly stated by Richard Perez, editor of Home Power magazine: “Every watt not used is a watt that doesn’t have to be produced, processed, or stored.”[18] This is a concept that is seldom considered by homeowners using grid power, but is essential for anyone who wants to live in an off-grid home. Energy efficiency is the best way to make the system more cost effective, but this does not mean doing without the customary appliances and conveniences. Many of the major appliances that we commonly use have become much more energy efficient over the last decade as both government and consumers became more aware of the importance of energy efficient appliances. One of the developments that has made solar power feasible is the introduction of the 1992 Energy Efficiency Act. The Energy Efficiency Regulations authorized by this act ensure that new appliances imported into Canada, or manufactured in Canada and shipped from one province or territory to another, comply with federal minimum energy performance standards (MEPS). A study was done by the Office of Energy Efficiency to assess the energy savings as a result of the MEPS between 1992 and 2001. Their findings showed an impressive energy saving. “Since the energy saved in any given year accrues over time, cumulative energy savings grew steadily between 1992 and 2001. They reached a total savings of 14.02 PJ in 2001, the equivalent of a year's energy for about 126 000 Canadian households.” The Energy Use Data Handbook, published by the government of Canada, lists the energy usage of various major appliances from 1990 to 2003. As shown below, the improvements are dramatic. Residential Appliance Unit Energy Consumption (UEC) UEC (1) for new electric appliances (kWh/year) 1990 2003 Total Growth Refrigerator 956 487 -49.1% Freezer 714 369 -48.3% Dishwasher (2) 101 52 -48.9% Clothes Washer (2) 97 57 -41.8% Clothes Dryer 1,103 914 -17.1% Range 772 718 -7.0% (1) Unit energy consumption is based on rated efficiency. (2) Excludes hot water requirements. Not only have improvements been made in the efficiency of appliances produced, but sales of major appliances also show that consumers are buying more energy efficient appliances. For example, figure 2.5 shows how improvements in energy efficiency have affected the energy consumption of refrigerators purchased in Canada. In 1990 there were no refrigerators sold with a consumption of less that 30 kWh/cu. ft./yr but in 2001 these accounted for 44.5% of all refrigerators sold. Energy Consumption of Refrigerators The drive for energy efficiency has also been proceeding in other areas. The development of the compact fluorescent light bulb was a major step forward, since incandescent lighting was notorious for high energy usage. Now, LED lighting is making strides in reliability, brightness and affordability at a much lower power rating. The same light output can now be achieved for about one tenth of the power needed for incandescent lighting. 5. Determining the Available Solar Radiation In northern latitudes there is a large difference between the length of the potential charging day on the summer solstice and the winter solstice. Designing a solar power system for such conditions usually involves a compromise that aims to meet the load requirements for three quarters of the year and uses approximately one hundred hours of generator backup to supplement the solar and wind charging for the one quarter of the year with the shortest days. This is a compromise designed to make the system more cost efficient, since designing a power system to meet winter needs would result in excess energy in the summer that would often be wasted. The load requirements and the average expected solar radiation at the proposed site are used to calculate the appropriate size of the solar array. Solar radiation data are available from NASA for the grid square from 51 to 52 degrees latitude and 106 to 107 degrees longitude. The proposed building site is situated near the northwest corner of this grid square. A solar cell receives the maximum amount of power if it is tilted at an angle perpendicular to the sun’s rays. To achieve this condition for all times of the day and days of the year would require a two-axis tracking device which would add considerable expense and complexity to the system. Most residential systems have the array at a fixed angle or at two tilt angles that are adjusted for the season. This method produces a reasonable compromise to maximize the solar gain and help shed snow in the winter. The table below shows the size of solar array required for a two tilt angle system. Array Size to Meet a 4.8 kWh/day Load Requirement with two tilt angles, at Average Solar Radiation and Minimum Solar Radiation Month Daily Ave. Radiation at tilt 51º (kWh/sq.m/day) Daily Ave. Radiation at tilt 66º (kWh/sq.m/day) Required Array Size (kW) Daily Min. Radiation at tilt 51º (kWh/sq.m/day) Daily Min. Radiation at tilt 66º (kWh/sq.m/day) Required Array Size (kW) January 2.92 3.1 2.06 2.66 2.82 2.26 February 4.22 4.33 1.47 4.05 4.15 1.54 March 5.11 4.97 1.25 4.68 4.54 1.41 April 5.19 4.74 1.23 4.96 4.53 1.41 May 5.23 4.56 1.22 4.67 4.08 1.56 June 4.84 4.2 1.32 4.37 3.82 1.67 July 4.38 3.85 1.46 3.86 3.41 1.87 August 4.26 3.81 1.50 3.77 3.38 1.89 September 3.99 3.77 1.60 3.31 3.11 2.05 October 3.5 3.5 1.82 2.73 2.71 2.36 November 3.02 3.16 2.02 2.29 2.38 2.68 December 2.58 2.76 2.31 2.41 2.57 2.48 Annual 4.1 3.89 1.56 3.64 3.45 1.85 The array size is calculated by dividing the daily load requirement by the daily average radiation and then multiplying the result by 1.33 to compensate for losses and inefficiencies in the system. Since the power system design includes both a solar panel array and a wind generator, the solar array only needs to supply about one third to one half of the load requirements. The system was designed with a 960 Watt solar array consisting of eight 115 Watt Evergreen solar panels. This product was selected because their innovative string ribbon manufacturing process provided excellent efficiency rating at a lower cost than other manufacturers because it eliminates the wastage of saw cuts. 6 Wind Power System The monthly load requirements for the experimental house were estimated to be 144 kWh. Part of this load will be supplied by a the 960 watt solar panel array, but a wind generator was desired to supplement the solar power, especially during the winter season where a solar array of over 2 kW would be needed to supply the load in November, December and January. The average monthly wind speed, based on Saskatoon airport data, is 10.4 mph and the average for the three winter months is 9.9 mph. A wind generator’s output at a given wind speed depends primarily on the swept area of the propeller blades. Manufacturers generally publish power curves to show the power output of their products at various wind speeds. Wind generators from several different manufacturers were compared on the basis of price and of the power output at the average wind speeds for the proposed building site[28]. The Whisper H80 (now the Whisper 200) was found to be the best value per watt. It is rated to produce 125 kWh / month at a wind speed of 10 mph and 160 kWh / month at a speed of 11 mph. The average wind speed during November, December and October is 9.9 mph, so this would be more than adequate to supplement the expected 50 kWh / month from the solar array during this period. The terrain at the building site is rolling prairie with some sections of bush so the power output was expected to be about 10 % less that at the airport with its flat terrain. The wind generator was designed to be mounted on a 40 foot tower to bring it above the turbulent layer that generally extends to about 30 feet. The Whisper H80 has the following specifications: Swept Area: 80 square feet Rated Power: 860 Watts at the rated wind speed of 22 mph Peak Power: 950 Watts at a wind speed of 24 mph Peak Amps: 33 A @ 29 V Cut-in Wind Speed: 7 mph Rated electricity at 12 mph average wind speed: 193 kWh/month Monthly Energy Output of the Whisper 200 (formerly H80) and Whisper 100 (formerly H40) Wind Generators 7. Sizing Your Battery Bank Most residential renewable power systems use a 24 volt battery bank and charging system because efficiencies are better than for the customary 12 volt systems, and because larger inverters are available at this system voltage. Other options include 36 volt and 48 volt system. The 24 volt system was chosen because it is the most common and therefore has the most complete selection of compatible components and accessories. Since large capacity deep cycle batteries are generally available in 2 volt cells, this system will require a bank of twelve cells. 8. Charge Controller A renewable energy system must include a charge controller that is rated for about 1.25 times the charging current that is specified by the manufacturers of the solar panel array and the wind generator. Specifications are based on test conditions at a particular altitude and temperature so actual current delivered by the units can be higher depending on the local climate conditions and the amount of diffuse radiation. Therefore the charge controller must be able to handle more than the rated current. The Whisper H80 wind generator includes a charge controller for the wind generator and a PV array. It is designed with a resistive dump load and a metering section that provides information on the charging, battery and DC load currents. Although more sophisticated MPPT controllers are available for the solar array, it was decided to use only the controller supplied with the wind generator for this experiment. 9. Inverter For residential applications that include a variety of loads, a sine wave inverter is the best choice because it delivers power more efficiently and eliminates noise that may cause problems with some sensitive loads. The power rating of the inverter should be sufficient to operate any loads that would run simultaneously. A look at the load analysis indicates that a 4000 Watt inverter is adequate for this design. The Xantrex SW4024 4000 watt sine wave inverter was selected for this design because it produces an acceptable quality sine wave and provides many features and programmable options at a very competitive price. Inverters with pure sine wave outputs are available, but at higher cost, and are not needed for most household loads. 10. Complete Power System The complete system design is shown in the schematic diagram ibelow. The EZ-Wire System Center, included with the Whisper H80 wind generator, is shown in block form in the diagram. The center provides bonding blocks, wind generator brake, solar array disconnect, dump load and the rectifiers and power supply circuit to provide the 24 VDC output for battery charging. The system monitor and charge regulator is included in the same interface. Schematic Diagram of the Stand Alone Solar and Wind Power System
	Interior of the house, showing the wood stove, kitchen area, and south facing solar gain windows.
Experiment - Solar System Design
1. Solar Cell IV Characteristics (weather permitting) Set the solar panel perpendicular to the sun's rays and connect a resistance box between the two terminals of the solar panel. Measure the voltage and current as you vary the resistance, starting with the smallest resistance value on the resistance box (just as you did for the Thevenin circuit measurements). Take most of your measurements at low resistance values. Plot the voltage vs the current. Calculate the power output of the solar cell for the various resistance values (Rload) that you measured. Plot the power vs the voltage, and find the point of maximum power (find this graphically). This is the "operating point" of the solar panel. From your two graphs you can determine what the operating point voltage and current are for the panel. Compare this to the information provided by the manufacturer (Iop = 330 mA, Vop = 15 V). 2. Design of a Basic Residential Solar System Using the Load Analysis Chart as an example, determine the load requirements for the size of system that you feel you could comfortably live with. Determine the size of solar array that would be required, using the example in Section 5: Determining the Available Solar Radiation. Assume that you will not be using a wind generator. Choose an appropriate battery bank from the example in Section 7 to give you 3 days autonomy (system will run for 3 days with no charging).