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March 19, 2019
The last article walked you through the general principles of passive solar design, which also inform the design and placement of on=site renewable energy systems. This article focusses on on-site renewable energy systems including solar thermal, photovoltaics, wind and microhydro.
Here’s the evaluation form, so you can rate yourself on your existing understanding of On-site Energy Systems. The rating is from 0 (I know nothing about this topic) to 4 (I’m an expert in this topic and can handle complex tasks on the daily). You don’t have to share this with anyone else, so rate yourself honestly!
If you find On-Site Energy Systems a challenge, read on. There are links to free resources in this article.
If you’ve got this part of the competency down, tune in next for the next part of High Performance Housing, Strategies and Approaches to High Performance Houses
Why use Renewable Energy?
Some of the environmental and economic benefits of including renewable energy for housing include:
Energy generation that produces no greenhouse gas emissions from fossil fuels
Energy generation that reduces some types of air pollution
Diversifying energy supply
Reducing dependence on imported fuels
Where renewable energy is paired with high performance housing, some of the benefits include:
Increased resiliency for homeowners and communities
Minimized space conditioning and water heating costs
Minimal household budget for comfort
Net Zero Energy/Zero Net Energy Houses and Buildings
Net Zero or Zero Net Energy (NZE or ZNE) houses and buildings produce as much energy as they consume over the course of a year. The first step is to reduce the energy loads for space conditioning, water heating and occupant-based loads like lighting and electronics. Then the size of the on-site renewable energy system that will feed back into the grid can be determined. In a typical NZE, the renewable energy system of choice is a roof-mounted photovoltaic (PV) system. Other systems that might be used to generate electricity are small wind turbines or micro-hydro turbines.
Not all renewable energy systems need to be sized to Net Zero capacity.
Active Solar Systems
PV generates electricity from the sun’s light. This is known as an active solar system. Another common active solar system is a solar thermal system - solar thermal relies on energy transfer between fluids (liquid or air) to provide heat, not electricity.
Like passive solar systems, active systems rely on capturing solar gain. The collectors (windows in a passive solar house, PV or thermal panels for active solar) need to be exposed to the sun’s path. In the northern hemisphere, windows on the north side of the house, and rooftop collectors will not provide significant solar gain. The sun’s path runs from East (morning) across the southern sky dome and sets in the West. There are two seasonal variations in the sun’s path that is more pronounced the further north you get. These are caused by the tilt of the earth’s axis away from the sun.
In the winter, the sun is lower in the sky than in the summer.
In the winter, the sun’s path is further south than in the summer.
Knowing this means we can determine the proper placement of the panels on the roof as well as any seasonal shading that might occur.
Photovoltaic systems convert light (photo) into electricity (voltaic). The name is typically shortened to PV. The basic element of these types of systems is the PV cell. The PV cell is made up of a semi-conducting material, like silicone, that allow photons, or particles of light, to knock electrons free from atoms, generating a flow of electricity. A solar panel, or collector, is made up of several PV cells, and produces anywhere from 25 to 200 Watts per panel. A series of PV panels is called an array. An array that can carry the load of a typical house might be in the range of 6 to 10 kilowatts of capacity. The PV array feeds the electricity to a converter and energy storage system if it is off-grid, or to a converter and onto the electrical grid if it is grid-connected.
Types of collectors
Collectors can be categorized by the materials that are used, their efficiency, and the type of junction used. In residential applications, there are two primary types of collectors. PV panels typically have a 25 year output warranty, with a lifespan of 30 years or more.
The high purity of the silicone used in these panels gives the highest efficiency rates - up to 20 percent, but more typically 13 to 18 percent. Mono-Si panels have high power output, occupy less space, and last the longest out of all the types of panels currently available. They can withstand higher temperatures as well. Mono-SI panels are recognizable by their uniform dark look and rounded edges.
Poly-SI panels are made by melting raw silicon. The efficiency of this type of panel is around 15 percent. The lower efficiency means more panels are required to meet a target system capacity. This type of panel can be recognized by it’s speckled blue look, and square shaped cells.
Interpreting Energy Performance Data
It’s important to recognize the difference between power (kW) and energy (kWh). In most of Canada south of 60°N, a kilowatt (1000 watts) of PV, mounted with good exposure facing south, could generate between 1000 and 1400 kilowatt hours (kWh) of electricity per year. This would take up 80 - 90 ft2 of roof area. Knowing the energy load of the house will allow you to do a quick calculation on the amount of energy you can expect a PV system to offset.
If a house is located in Atlantic Canada, and uses 10,000 kWh per year, divide that by 1000 kWh energy provided per kilowatt of power. The house needs a 10 kW capacity system, and would take up 800 to 900 square feet of unobstructed, south-facing roof space.
If the same house is located in an area with better insulation, like Southern Alberta, divide 10,000 kWh per year by 1400 kWh energy provide per kilowatt of power. Now the house needs a 7.2 kW capacity system, needing 570 to 645 square feet of unobstructed, south-facing roof space.
System size is based on the objective of the project: is the house to be net zero? Is the house to be net zero for the electrical load only (where the heating system is non-electric, for example)? Is the system only going to offset a certain amount of electricity because of limited roof area or budget size?
Balance of System
Balance of system (BOS) encompasses all components of a system other than the panels: wiring, switches, a mounting system, one or many solar inverters. In a grid connected system, the BOS is dependent on the utility and the connection requirements. Possible components include renewable energy credit revenue-grade meter, safety equipment, and other instrumentation. If the system is stand-alone (off-grid) or grid-interactive, BOS will also include a battery bank and battery charger.
Inverters are required to convert the direct current (DC) generated by the PV array into alternating current (AC). Inverters are used to condition electricity so that it matches the requirements of the load. Grid-tied systems need conditioning equipment that matches the system voltage, phase, frequency, and sine wave profile to that of the grid. A PV system is usually run by one of two types of inverters.
A standard inverter (also known as a string-inverter or central inverter) is a standalone box that is typically installed close to your fuse box and electricity meter. An inverter wired in a series circuit with 6-10 individual solar panels in what is known as a "string".
There can be one, two, or more, string inverters on a single residential solar installation. Putting panels into strings with multiple inverters means the system can work at a higher overall efficiency than a single inverter. This is especially true of arrays that experience some shading, as a standard/string inverter will cap the electricity production of each panel by the lowest producing panel on your roof. The voltage is reduced to the voltage of the lowest voltage panel in the string.
Micro-inverters, which are about the size of an internet router, are installed underneath each panel. Unlike a system with a few string inverters wired in series to several solar panels, each micro-inverter functions in a parallel circuit. Micro-inverters take full advantage of the production of each individual panel, converting the power generated by each panel to the grid voltage. If a solar system is installed on different roof faces, or if there are shading issues from trees or a chimney, the solar panels will be producing different amounts of electricity at different times of the day. Micro-inverters ensure you harvest all of the energy, while with a standard inverter you will lose some of this production.
Only need one, less expensive
Function well facing south, with minimal shading
Simple system wide performance monitoring
System expansion must be planned carefully
Production capped to lowest performing panel
System needs to be focussed to South for optimal gain
Higher production yield,
Multiple angles for panel placement (E/S/W faces of roof)
Better performance with shaded conditions
System expansion relatively easy
Inverter is most common part of PV system to fail - multiple inverters require higher level monitoring system to see performance
Integration into Building Design
PV products are available that can be integrated into the actual building assembly. Building integrated PV (BIPV) roofing materials are available. BIPV is currently better suited to commercial buildings as skylight, atrium and facade replacements. As building envelope material and power generator, BIPV systems can provide savings in electricity costs at a fraction of the installed cost of a separate system. Like other dual-purpose materials, the higher cost of BIPV can be calculated as a premium on the cost of the conventional material it is replacing.
Here is a tutorial on PV from the US Energy Information Administration.
An article from 2012 from Energy BC that covers the basics.
A great tutorial on the fundamentals from Solar Nova Scotia.
Solar thermal systems rely on collectors, essentially heat exchangers, that convert solar energy into heat energy using the same greenhouse effect that a passive solar house does. The collector captures solar energy and uses that energy to heat water that is typically used for domestic hot water (bathing, washing and laundry). This type of system can also supply hot water for hydronic space heating systems, or to heat outdoor swimming pools and hot tubs.
Like PV arrays, solar thermal collectors need to be oriented to the sun in terms of direction (south), and angle (roof slope). Solar thermal systems can be oriented to optimize performance for summer or winter, mounted at an angle that is perpendicular to one or the other solstice, or the collectors can be mounted at an angle that is perpendicular to the equinox.
Types of Collectors
There are two primary types of collectors for solar hot water systems: flat plate and evacuated tube.
Flat plate collectors can heat the fluid inside using either direct or indirect sunlight from a wide range of different angles. Unlike solar electric, or photovoltaic cells, solar thermal systems are not reliant on the light from the sun. They also function in diffused light, which is dominant on cloudy days as it is the surrounding heat that is being absorbed. How hot the circulating water gets will depend mostly on the time of the year, how clear the skies are and how slowly the water flows through the collectors pipes.
A flat plate collector consists of a large heat absorbing plate made of copper or aluminium (both good conductors of heat). The plate is painted or chemically etched black to absorb as much solar radiation as possible. Several parallel copper pipes are attached lengthwise to the absorber plate to optimize surface contact and heat transfer. In some models, blackened copper or aluminum fins attached to the pipes replace the solid plate. The pipes are filled with a heat transfer fluid, either water or a glycol mixture. The pipes and absorber plate are enclosed in a wooden or metal box with a sealed ‘lid’ of glass or plastic. Like the glazing in a house or greenhouse, the glazing material allows short-wave solar energy to pass through it. The incoming radiation is then received by the blackened absorber. As the absorber warms up, it transfers heat to the fluid within the collector but it also loses heat to its surroundings. To minimize this loss of heat, the bottom and sides of the box are insulated.
Flat plate collectors are usually 4 x 8 ft (32 square feet/3 square metres) and can weigh more than 200 pounds (approx)100 kg. One square foot (1000 cm2) heats about two gallons (10 litres) of water per day to well over 70°C. A single panel will heat about 60 gallons (300 litres) of water (about the size of a standard hot water storage tank).
Rule of thumb sizing is to allow 10 to 16 ft2 of flat plate collector area per person. So for a family of four persons this translate into 40 to 60 square feet of collector plate area, or two standard flat plate collectors.
Flat plate collectors are cost effective due to their simple design, low cost, and relatively easy installation. Flat plate collectors operate at maximum efficiency when the sun is directly overhead, when the sun’s rays hit the glazing at a perpendicular angle. At other times, the sun’s rays are striking the collector at varying angles, reducing their efficiency.
An evacuated tube collector (ETC) uses sealed glass tubes that each contain a smaller copper heat pipe filled with a heat transfer fluid, and an absorber plate to collect the sun’s heat. The glass tubes are sealed and the air is evacuated out of them, creating a vacuum. The vacuum reduces heat losses via convection and conduction. A series of tubes connects to a manifold or ‘header pipe’. When the sun’s rays strike the copper tube it brings the fluid inside to a boil. The resulting vapor then rises to the top of the tube where the heat is transferred to the manifold or header pipe and then to a storage tank. Once the heat has been exchanged in the manifold, the vapor returns to the liquid stage and runs back to the bottom of the tube, repeating the cycle.
ETC can operate at higher temperatures, at a higher efficiency for longer periods of each day because a section of the round tubes is perpendicular to the sun’s rays throughout the primary solar gain hours. In addition, the vacuum inside the tubes means the the collector works at a higher efficiency deeper into cold weather, and can produce usable heat even on overcast days.
ETCs can get very hot during summer months, and can require bypass valves and a way to ‘dump’ excess heat to avoid overheating and cracking of the glass tubes. On the other hand, in winter months, the vacuum keeps the outer glass tube from heating up. As a result, snow does not melt off the ETC array easily, like it does off a flat plate collector. Clearing snow and ice from glass tubes without causing damage to the tubes can be a problem, depending on the installation angle. They are lighter than flat plate collectors and so could be a better choice for roofs in areas with high snow loads, or where structural capacity might be of concern.
Evacuated tube collectors are grouped together into a functioning unit of 20 or 30 tubes connected to a manifold. This unit is called a ‘bank’. Evacuated tube collector sizes depend on the number of tubes used. A 20-tube collector (about 6×7 feet in dimension) would provide enough hot water for one to three people. A four-person household would likely require a 30 tube bank.
Like flat plate collectors, ETCs can be used in either an active open-loop (without heat exchanger) or an active closed-loop (with heat exchanger) system but a pump is required to circulate the heat transfer fluid from collector to storage in order to stop it from overheating.
There are two types of ETC: single wall tube, double wall tube. Single-wall evacuated tube collectors have the absorber plate and the heat pipe installed within the vacuum of the glass tube (glass-metal). Glass-metal tubes protect the absorber and heat pipe from corrosion. In double-wall systems (glass-glass), a double layer of glass is fused together at one or both ends with a vacuum between the layers. The absorber and heat pipe are installed inside this double layer of glass, and are not in a vacuum. Moisture may enter the non-evacuated area of the tube and cause absorber corrosion. The two layers of glass in a double wall tube reduce the light that reaches the absorber compared to a single wall tube.
Storage Tank Size
The rule of thumb for storage tank size is based on an average of 20 gallons of hot water per person per day.
# of People
Storage Tank Volume (in gallons)
1 to 3
3 to 4
4 to 6
In order to heat your water successfully and use it during both the day and the night, you will need to have both a solar collector to capture the heat and transfer it to the water and also a hot water tank to store this hot water for use as needed.
There are two primary ways to transfer the heat from collectors to the storage tank and then to household or other end use: direct and indirect. Regardless of the type of system, there are a few ways to ensure they run as effectively and efficiently as possible:
keep the distance between the collectors and the storage tank as short as possible
insulate all pipes
run pipes through warm areas of the house
A direct system, also called an open-loop system, typically uses flat plate collectors and a storage tank. The water from the storage tank that circulates through the collectors and back to the tank, or out to the end use. Open loop systems use the potable water from the house, and so cannot use glycol or other antifreeze chemicals. There are two forms of direct, open loop systems: passive/drainback, and active.
A drainback or passive system moves the water through the collectors by natural thermosyphon action. In this type of system, there are no pumps or control mechanisms to transfer the heat created to the storage tank. Instead, the natural force of gravity is used to help circulate the water around the system. In this type of system, a horizontally mounted storage tank is located above the collector. As the solar-heated water enters the tank, any cool water is forced out of the tank and flows via gravity to the bottom of the collectors (cold water is more dense than hot water). The cycle of rising and falling, known as thermosyphon action, continues until the solar energy is no longer strong enough to make a big enough temperature difference in the water to drive the thermosyphon.
This type of system is simple and low-maintenance, and uses no energy, but the water flow is relatively slow, which can lead to high heat losses from the pipes. There are some design restrictions to this type of system due to the fact that the tank must be located above the solar collectors and the pipes must have a continuous rise.
An active open loop system uses a pump to circulate the water around the system. The cooler water is pumped directly from the home to a central water storage or immersion tank and passes through the solar collector for heating. The hot water leaves the collector and returns back to the tank flowing in a continuous loop. From there, the water is pumped back into the house as hot usable water.
A low voltage DC pump, powered by a small PV cell runs the system. Direct systems are usually used in warmer climates with few cold days or is drained in winter to stop the water in the pipes from freezing.
In an indirect, or closed loop system, the solar collector is on a separate loop of pipe from the potable water in the storage tank. A heat transfer fluid, typically glycol, circulates through the panel and this closed loop of pipe. The heat is transferred from the closed loop to the water in the storage tank via a heat exchanger. The heat exchanger can be internal or external of the tank. The heat transfer fluid never comes into direct contact with the water being heated. The heat exchanger can either be a copper coil inside the lower part of the storage tank or a flat plate exchanger outside the storage tank.
There is some heat loss through the heat exchanger, but the advantage to a closed loop system is their ability to be used in cold climates.
Indirect systems typically use pump to circulate the heat transfer fluid.
An existing domestic water heating system can easily be converted to an indirect system, as the hot water storage tank can be placed anywhere in the home because it does not need to be higher than the collectors as in a passive or thermosyphon system. A drainback system, which circulates either water or an antifreeze mixture, using a temperature sensor combined with a pump to drain the fluid out of the system when it is too cold or when the sun stops heating the water.
One of the main advantages to this closed loop indirect heating system is that the antifreeze solution gives all year round operation in areas where the temperature falls below the freezing point as well as protecting the system from corrosion of the collectors by untreated tap water containing gases and dissolved salts.
The ability of a solar thermal system to operate is reduced seasonally the further north you go. The seasonal change in available solar gain is one reason for this. Colder temperatures can affect how well the collectors produce heat as well.
Solar water heaters don't generate as much hot water in the winter. On an annual basis, a typical solar thermal system can provide 60 to 80 percent of a household’s DHW. At latitude 45°N, from April through September, the solar thermal system will provide nearly all of the hot water required by a typical household. In winter, the percentage of hot water heated by the sun drops to as low as 10-20 percent, due to short days and weak sun. Solar hot water systems are typically connected to a backup conventional water heater to ensure that hot water needs are met through the winter months.
NRCan has a listing of solar thermal systems that meet the CSA Standard F-379 "Solar Domestic Hot Water Systems" The ratings in the directory have been determined using the standard solar day for a 300 litres/day hot water load. The directory shows the estimated annual performance rating for each system model in Gigajoules (GJ) per year (1 GJ is approximately 278 kW). The directory outlines the important features of each model:
Number of Collectors
Heat Exchanger Type
Solar Air Heating
Like solar water heaters, solar air heaters use a collector, typically on a south-facing wall, to provide solar gain into the building or house. These active systems are intended to supplement an existing heating system. Solar air heaters are direct-transfer systems that don’t store heat, or supply heat at night or on cloudy days. They are a good option for rooms or live/work spaces that see mainly daytime use.
A typical solar air collector is constructed on the same lines as a flat plate collector for water heat. They are similar in size and shape to a flat plate collector, 3 or 4 feet wide by 6 to 8 feet tall, typically. A dark metal absorber plate is encased in an airtight, insulated frame with glazing on the surface that faces south. To optimize heat collection, the absorber plate has rough surfaces or channels to increase the movement of air as it circulates through the collector. The flat box is mounted on the wall surface, with penetrations through the wall to accommodate the air exchange inlet and outlet.
Solar air heaters use thermosyphon action to provide heat to a room through a convection loop. Using the principle that warm air rises and cool air sinks, cooler air is pulled into the bottom of the heater from an inlet set at close-to-floor level in the room. The air circulates through the collector, picks up heat and blows the warmed air back into the upper part of the room through a port at the top of the heater.
Solar air heaters can rely on passive thermosyphon action or can have a blower to push more air and destratify the space that they are intended to heat.
Manufactured units are listed with capacities from 4000 to 20,000 Btus.
An article and a video on a home-made solar air heater from Green Futures.
A link to a blog article about solar air heaters.
Wind power is generated by turbines (not windmills, those grind grain). Turbines convert the wind’s kinetic energy into mechanical power that runs a generator.
Wind energy is often associated with large-scale wind farms, with each turbine having the capacity to produce hundreds or thousands of kilowatts, some with single blades measuring longer than 80 meters (260 feet). Small-scale wind energy, microgeneration, is what homeowners and property owners use to offset some or all of the onsite electrical consumption.
Turbines used in residential applications range in size from 400 Watts to 100 kW. Depending on the average wind speed in the area, a wind turbine rated in the range of 5 to 15 kW could meet most or all of the electrical needs of a whole house. For perspective on size, small scale turbines under 20 kW rated power typically have a rotor diameter of less than 10 meters (30 feet). The typical lifespan for a turbine is about 120,000 hours, or 20 to 25 years.
Like PV systems, small scale turbines can be off-grid, or they can be connected to the electricity grid through a power provider to help meet a Net Zero Energy target (energy used by the house/site is matched by energy produced by the house/site over the period of a year).
Types of Turbines
There are two common types of turbines, horizontal axis and vertical axis. Regardless of which direction the axis is, all turbines include three primary parts: the rotor, the gearbox and the generator.
The rotor is the part that spins around with the blades on it. Rotors are designed to capture the maximum surface area of wind to spin ergonomically. Blades are made from lightweight, durable materials.
The gearbox (or adjustable speed drive, or continuously variable transmission), situated between the rotor and the generator, amplifies the energy output of the rotor.
The generator is where the DC electricity is produced.
Horizontal Axis (HAWT)
Horizontal axis turbines are the most commonly seen type, with the rotational axis horizontal or parallel to the ground. Three or more blades rotating around a central cone, looking like a propeller for a boat. The blades create what is known as the ‘swept’ area, which determines how much wind energy the rotor can capture. The advantage of horizontal wind is that it is able to produce more electricity from a given amount of wind. The disadvantage of horizontal axis however is that it is generally heavier and it does not produce well in turbulent winds. A HAWT includes a nacelle (housing) to protect the generator, and a tailvane to direct the rotor in the direction of the wind.
Vertical Axis (VAWT)
Vertical axis turbines have the rotational axis vertical or perpendicular to the ground. These come in two typical categories, Darieus or giromill ‘eggbeater’ types, and Savonius or ‘two half-oil drum’ types. Vertical axis turbines are powered by wind coming from all 360 degrees, and even some turbines are powered when the wind blows from top to bottom. Because of this versatility, vertical axis wind turbines are thought to be ideal for installations where wind conditions are not consistent, or due to public ordinances the turbine cannot be placed high enough to benefit from steady wind.
Not surprisingly, wind turbines need wind to produce energy.
To perform at peak output, a turbine needs a constant wind that is out of range of most obstructions and objects that cause turbulence. This smooth, laminar wind typically occurs at 80 feet or more above ground. Due to the need to be in this smooth wind zone, turbines are installed on towers. How tall the tower is, depends on the site: trees and local structures, local topography all influence wind patterns.
The first requirement for an installation is a determination of the wind resource on a given site. Typically, a tower is erected and an anomemeter is installed at the proposed height of the turbine to measure the average wind speeds. Ideally, this is a three to five year test period, to get a good picture of the wind patterns. For small-scale installations, this step is often overlooked, and many installations are made with towers that fall short of the smooth wind zone.
When a tower and turbine are installed in a poor location and/or at a height that leave the turbine out of the smooth wind zone, the turbine performance drops off dramatically.
In general, annual average wind speeds of 5 meters per second (11 mph) are required for small-scale grid-tied applications.
Turbine capacity, or rated power, is based on how much power the turbine can generate under ideal conditions in the ideal smooth wind zone. The ratings for wind turbines are based on standard conditions of 59° F (15° C) at sea level.
The rotor-swept area determines how much wind energy the turbine can turn into electricity. The larger the rotor, the more energy it can capture.
The best predictors for turbine energy production are the diameter of the wind turbine (which gives you the swept area) and average wind speed for the turbine hub height.
All turbines have reports showing their performance, these include a graph of the power curve, which indicates how large the electrical power output will be for the turbine at different wind speeds. Power curves show the ‘cut-in’ and ‘cut-out’ wind speeds, giving the range of wind speeds where the turbine actually generates electricity.
A short video describing wind turbine power curves.
With the wind speed and turbine capacity in hand, the energy output can be determined. Turbine capacity is the rated power (kilowatts, kW), energy (kilowatt-hours, kWh) is the quantity of power consumed. An estimate of the annual energy output (kWh/year), shows if a particular wind turbine and tower will produce adequate electricity over the course of the year to meet some or all of the energy needs of the house.
A wind turbine manufacturer, a dealer/installer, or a site assessor will calculate energy output based on the power curve of a specified turbine, the average annual wind speed at your site, the proposed height of the tower, micro-siting characteristics of your site and, if available, the frequency distribution of the wind (an estimate of the number of hours that the wind will blow at each speed during an average year). As air density can affect wind speeds, the calculation could also include an adjustment for the elevation of the site.
The AWEA Small Wind Turbine Performance and Safety Standard indicates that the Rated Annual Energy of a wind turbine is the calculated total energy that would be produced during a 1-year period with an average wind speed of 5 meters/second (m/s, or 11.2 mph).
Balance of System
In grid-connected systems, the only additional equipment required is a power conditioning unit (inverter) that makes the turbine output electrically compatible with the utility grid. Some grid-tied systems may also have battery storage, in this case a controller may also be needed. Stand-alone systems (systems not connected to the utility grid) require batteries to store excess power generated for use when the wind is calm. They also need a charge controller to keep the batteries from overcharging.
Wind and Net Zero Energy Houses
Wind is not used often in Net Zero Energy projects for single houses, for some basic and practical reasons: safety, land availability and reliability, which all translate into increased costs for site development. The tower can require a safety zone - typically small-scale installations are on sites of over 1 Acre. Towers can can require a variation for height under municipal by-laws. Zoning bylaws and permitting processes can also affect wind installations. Issues that come up are aesthetics and community interests, including sound levels, visual impacts, possible wildlife impacts, as well as TV or radio interference and safety concerns around ice shedding or broken equipment. These factors make wind a less viable choice for urban and suburban sites.
As turbines have moving parts, they require more maintenance and repair/replacement of parts than static equipment such as PV panels. This also is a downside for most homeowners.
For detailed technical information about small scale wind systems, this article from Solacity Inc. is terrific.
Here is another excellent resource on small scale wind generation from WINDExchange and the US Department of Energy.
Microhydro refers to a small water-based energy generation system - up to 100 kW capacity - that uses the natural flow of water to provide electricity to homeowners and/or small businesses, including farmers and ranchers. These types of systems can complement PV systems in areas where water flow is high in the winter, during times when solar energy is at it’s lowest availability. Microhydro systems can be installed in creeks, streams, or rivers.
While systems range from 5 kW to 100kW in size, a 10 kW microhydro system provides enough power for a house. These types of systems use a turbine, a pump, or a waterwheel to take the energy of flowing water, transform it into rotational energy, and convert that into electricity.
Head and Flow
The capacity of a microhydro system is calculated in terms of both ‘head’ - the vertical distance the water falls measured in feet or meters, and ‘flow - the actual quantity of water falling, measured in gallons per minute (or cubic feet per minute, or litres per second). In general, the higher the head, and the higher the flow, the more power is available from the site.
‘Head’ is shorthand for hydraulic head, which is a pressure measurement of water falling in a pipe. There ar two terms that need to be considered when determining head: gross (or static) head and net head. Gross head is the actual vertical distance, while net head is the gross head minus the losses due to friction and turbulence in the pipe.
Head and flow are used in the initial calculation to give the potential power available in kilowatts (kW). Further calculations account for turbine efficiency, pipe friction and conversion losses to find the actual amount of power available.
While head and flow are key to sizing a system, other considerations include economics, permits, and water rights.
High head refers to site that takes advantage of a significant vertical height difference between a water intake and an outlet. to generate electricity, for example a small dammed pool at the top of a waterfall. Impulse turbines, primarily Pelton wheels, are often used in this type of installation, especially where head is 50 meters or more.
Low head refers to a site with minimal vertical difference between water intake and outlet - less than 10 feet (3 meters). Propeller turbines are used in low-head installations. Conventional pumps can also be used. A site with a head of less than 2 feet (0.6 meters) is likely not usable.
Types of Turbines
Pelton Wheel: This type of turbine uses jet force to create energy, funnelling water into a pressurized pipe with a nozzle at one end. The water sprays out of the nozzle, hitting curved buckets attached to a wheel, which rotates the wheel. Pelton wheels are highly efficient, and ork best in low flow, high head installations.
Propeller Turbines: this type of turbine uses three to six fixed blades at different angles.
Conventional Pumps: when the action of a pump is reversed, it acts like a turbine. Pumps are low cost and readily available. A microhydro installation using a pump requires constant head and flow.
Balance of System
Besides the turbine, there are several parts to a microhydro system. Before the water turns the turbine, it is filtered at the water intake, and then flows into a large pipe called the penstock. The water to flows down the penstock, with the pressure increasing in proportion to the increasing drop, and pushes the water at the turbine. The turbine spins the ‘runner’, and the energy is transferred to the generator. An electronic governor controls the entire system, monitoring and regulating the generator RPM.
Here is an excellent article on microhydro systems for residential use.