Solar Hot Water

Solar Hot Water has become a very effective means of providing hot water for a range of scales of projects. Solar water heaters can be used to provide hot water for washing and other domestic uses as well as space heating. The same set of solar collectors can be used to provide hot water for both space heating and space heating needs, although space heating will generally require a much greater collector area and storage capacity. Additional controls and heat exchangers are also needed. Due to these extra costs, and because sunshine is relatively scarce when heating loads are highest (for example at night and during the winter) solar energy is more often used to heat domestic water than it is for space heating. Solar hot water systems use the sun's energy to heat water for a variety of industrial and business uses. Sunlight passing through glass or plastic glazing strikes a light absorbing material. The material converts the sunlight into heat, which is prevented from escaping by the glazing.

The most common types of solar collectors used in solar water heaters are flat plate and evacuated tube collectors. A flat plate collector consists of a shallow rectangular box with a transparent glass or plastic window covering a flat black plate. The black plate is attached to a series of tubes through which water or some other transfer fluid passes. An evacuated tube collector consists of several individual glass tubes, each containing a black metal pipe. The transfer fluid flows through these pipes. The space between the pipe and the glass tube is evacuated, in other words, the air is removed.

Closed Loop - Glycol System

glycol systemClosed loop systems use a heat-transfer fluid to collect heat and a heat exchanger to transfer the heat to household water. Active closed loop systems use electric pumps, valves, and controllers to circulate the heat-transfer fluid, usually a glycol-water antifreeze mixture, through the collectors. This glycol-water antifreeze mixture makes closed-loop glycol systems effective in areas subject to freezing weather. For this reason, closed loop systems are preferred for year round use in cold climates where freezing is common.

 

A Basic Closed Loop Solar Hot Water System
Credit: DOE/NREL

Closed Loop - Drain back System
Drain back systems use water as the heat-transfer fluid within the collector loop. The water is forced through the collectors by a pump and then is drained by gravity to the storage tank and heat exchanger. These systems have no valves to fail and when the pumps are off, the collectors are empty, thereby assuring freeze-protection and auto shut-off if the water in the storage tank becomes too hot.

Open loop, seasonal, batch
Open loop systems heat and circulate household (potable) water directly in collectors prior to distribution in the household. One type of open loop system is a batch heater that is simply a black tank filled with water and placed inside a south-facing, insulated, glazed box, where it absorbs solar energy. The tank may incorporate a selective surface that that absorbs sun well but inhibits radiant loss. In climates where freezing occurs, batch heaters must either be protected from freezing or drained for the winter. Batch heaters are inexpensive and have few components, therefore they require less maintenance and experience fewer failures. These systems are good economical choices for seasonal applications such as summer camps.

HOW MUCH HOT WATER??

When designing for carbon neutrality, reduction is the bottom line when looking at activities that result in energy usage. Much of the reduction for normal hot water use will need to come from the installation of efficient fixtures such as low flow shower heads and faucets. The other part of reduction will need to come from an examination of lifestyle patterns.

Hot Water Useage

The total hot water requirement will be developed from the building program. What is the function of the building? How many occupants? What sort of fixtures are being used? Type of water use? Frequency? Architecturally you may be fighting with the availability of good south facing "real estate" with your desire to use photovoltaic. The solar potential of the building and site will need to be accommodated perhaps by the use of large storage tanks. These will need to be insulated much better than a standard hot water tank that is gas or electric fired. You will also need to super insulate all of the hot water supply lines to reduce temperature drops along the system. Locating the fixtures as close as possible to the supply tank can help to alleviate temperature losses in the lines.

Pipe Insulation at the Aldo Leopold Legacy Center
Pipe Insulation at the Aldo Leopold Legacy Center

This Carbon Neutral building uses full insulation on all of the piping leading from their solar hot water system, to the storage tank and back to the fixtures. The mechanical room is located in close proximity to the fixtures on the floor above.

FLAT PLATE COLLECTORS:

Flat plate collectors are the most commonly used. In a typical flat plate installation you should provide approximately one square meter of collector for every 45-50 litres of hot water to be stored.

Flat Plate Collector

System Losses from a Typical Flat Plate Collector

Flat plate collector systems are normally roof mounted. The angle of inclination usually follows the rules for photovoltaic arrays as both will benefit when the angle of inclination of the sun to the surface of the array is closest to 90 degrees which will minimize reflectance.

VACUUM TUBE COLLECTORS:

The vacuum tube collector uses a series of glass tubes that act like thermos bottles. The glass allows the light through, which heats up the fluid inside the inner tube. The vacuum between the layers of glass prevents that heat from escaping back to the atmosphere on cold days.

On warm, sunny days, the performance of the vacuum collector is equal to that of the flat collector. But it will increasingly outperform the flat collector as the outside temperature decreases or light levels are reduced.

Comparison between the Effectiveness of Vacuum Tubes and Flat Plate Collectors
Vacuum Tube Section and Comparative Performance to Flat Plate Collectors

Because vacuum tube collector systems outperform flat plate collectors on east and west orientations, as their ability to collect radiation is less dependent on the incoming sun angle, these sytems might be recommended for use on projects where there is inadequate south facing potential for an installation, or where preference must be given to the true southern exposure for the benefit of the photovoltaic array.

Evacuated Tube Collectors at the White Rock Operations Center
Evacuated Tube Installation at the White Rock Operations Center, Surry, B.C. LEED® Gold

Here they have used a vertical installation of tubes as they can take advantage of solar radiation coming in from all angles and do not necessarily have to be sloped to catch the sun's rays as is the case for photovoltaic arrays.

Solar System at the Aldo Leopold Legacy Center
Aldo Leopold Legacy Center

Evacuated tubes towards the ridge of the roof with photovoltaic below. Note the substantial insulation wrap on the pipes leading into and out of the evacuated tube array.

CALCULATING YOUR REQUIREMENTS:

For domestic applications, this link will provide you with a method for calculating your requirements. For larger commercial installations, more in depth analysis will be required.

Photovoltaic are increasingly becoming one of the more commonly accepted means of providing site sourced renewable energy to the project.

Photovoltaic science is the science of turning energy produced from the sun into electricity. Edmond Becquerel discovered the concept known as the photovoltaic effect in 1839. However, the first positive/negative (p/n) junction solar cell was not created until 1954 at Bell Labs.

PV Production Worldwide 2007

PV Production Worldwide

WHAT IS THE SOLAR POTENTIAL OF YOUR SITE?

Although Carbon Neutral design strategies include the use of photovoltaic systems to provide electricity for systems and equipment, you need to find out the solar potential of your site. The links below can be used to determine relative irradiation values for sites in the United States and Canada.

Natural Resources Canada: Interactive Map for PV and Radiation
link to map

How Do PV Cells Work?
Photovoltaic are solid-state semiconductor devices that convert light directly into electricity. They are usually made of silicon with traces of other elements and are first cousins to transistors, LEDs and other electronic devices. Production of PV has increased dramatically since the initial sustainable design conference in 1987. The efficiency has increased and the costs have decreased.

Solar cells are converters. They take the energy from sunlight and convert that energy into another form of energy, electricity. Solar cells convert sunlight to electricity without any moving parts, noise, pollution, radiation, or maintenance. The conversion of sunlight into electricity is made possible with the special properties of semi-conducting materials.

Sunlight Converted: At the atomic level, light is made of a stream of pure energy particles, called "photons." This pure energy flows from the sun and shines on the solar cell. The photons actually penetrate into the silicon and randomly strike silicon atoms. When a photon strikes a silicon atom, it ionizes the atom, giving all its energy to an outer electron and allowing the outer electron to break free of the atom. The photon disappears from the universe and all its energy is now in the form of electron movement energy. It is the movement of electrons with energy that we call "electric current.”

Sunlight to Electricity: A typical solar cell consists of a glass cover to seal the cell, an anti-reflective layer to maximize incoming sunlight, a front and back contact or electrode, and the semiconductor layers where the electrons begin and complete their voyages. The electric current stimulated by sunlight is collected on the front electrode and travels through a circuit back to the solar cell via the back electrode.

Solar cells are created from a semi-conducting material, usually silicon, that is treated or doped with a controlled amount of impurities such as Phosphorous (N type dopant) and Boron (P type dopant) to form a PN junction. Sunlight energy striking the face of the cell is absorbed by the semiconductor and excites electrons within the cell creating electron-hole pairs (negative and positive charges). These pairs are disassociated by the electric field generated by the PN junction: the electrons (-ve charges) drift towards the N region; the holes (+ve charges) drift towards the P region. The +ve and -ve charges are then collected at the top and bottom cell contacts and create a flow of electricity.

The surface of the solar cells is coated with an anti-reflective layer to provide higher solar absorption which gives them their typical blue or black color.

Solar cells alone cannot produce usable power. They need to be interconnected with other system components that ultimately serve a specific electrical demand, or ‘load’. PV systems can either be stand-alone, or grid-connected. The main difference between these two basic types of systems is that in the latter case, the PV system produces power in parallel with the electrical utility, and can feed power back into the utility grid if the onsite load does not use all of the PV system’s output. When the sun is shining, the direct current electricity (DC) from the PV modules is converted to alternating current (AC) by the power of an electronic inverter, and then fed directly into the building power distribution system where it supplies electric power.

Each of the modules must be wired and connected to the next, to eventually transfer the electrical charge to the inverter. In most cases this is done via thin, flat wires that run through the cells. In the spheral solar application, the silicon balls are embedded into a metal mesh sheet, and this acts to carry the electrical charge. Inverters come in various sizes/types and result in some power loss via the process.

An individual solar cell can vary in size from 1cm to 15cm and produce between 1 and 2 watts. Main types on the market are crystalline and thin film. Cells are combined into modules, and modules into arrays. Arrays are ganged on a surface to provide the amount of power required.

Types of Silicon Cells:
Mono crystalline cells: are made from very pure mono-crystalline silicon. This type of silicon has a single and continuous crystal lattice structure with almost no defects. High efficiency (15%). Energy intensive manufacturing process. Expensive.

Poly- or multi-crystalline cells: are produced using numerous grains of mono-crystalline silicon and have a more irregular surface. In the manufacturing process the silicon is cast into ingots which are rectangular/square in shape. These are cut into very thin wafers and assembled into complete cells. They can also grow this on a substrate. Less efficient (12%). Less expensive.

Mono-crystalline cells tend to be flat black or deep blue in color. Polycrystalline cells have a mottled (like galvanized steel), cobalt blue appearance.

PV types

Thin film cells can be amorphous silicon, copper-indium-diselenide (CIS) and cadmium-telluride (CdTe) cells. They are omposed of silicon atoms arranged in a thin amorphous matrix rather than a crystalline structure. Amorphous silicon absorbs light more effectively than crystalline and the product is much thinner. Cheaper to produce, but with efficiencies around 6%. These modules have a charcoal grey or bronze color and look like low-E coating or fretting when used on vision glass. Other colors are available but, the cost will be higher.

PV Relative Efficiencies by Type
Relative Efficiencies of PV by Type

The Temperature Factor:
Photovoltaic produce heat as a by-product of the process by which sunlight is changed to electricity. They must be installed so that they are vented, as overheating will decrease their efficiency.

Photovoltaic actually work better in cold weather situations. This makes the Northern United States and Canada good climates for their use.

Contrary to most peoples' intuition, photovoltaic actually generate more power at lower temperatures with other factors being equal. This is because photovoltaic are electronic devices and generate electricity from light, not heat. Like most electronic devices, photovoltaic operate more efficiently at cooler temperature. In temperate climates, photovoltaic will generate less energy in the winter than in the summer, but this is due to the shorter days, lower sun angles and greater cloud cover, not the cooler temperatures.

Rain and Snow:
Rain will not adversely affect a PV array system since during periods of rainfall the solar irradiance is already low. Roof mounted PV arrays can become covered with snow in the winter. If the array is covered, it will not work. In snowy climates, sloped arrays are preferred to flat installations as theoretically the sun will penetrate the snow, heat the dark PV layer, melt the base of the snow and it will slide off of the panel. It is important to prevent such snow from piling up at the base of the array, or sliding uncontrolled onto passersby below the installation. Sometimes it is necessary to shovel the array. Care must be taken not to damage it. These sorts of logistics must be carefully worked into the design of the project, its installation and instructions for use and maintenance to the owners.

Dirt and Pollution:
Any factor that reduces light transmittance to the PV surface will reduce the output of the system. If dirt is allowed to accumulate (more likely in urban areas), the output can be reduced by 2% to 6%. The higher value occurs if the slope of the array is less than 30 degrees. The occasional heavy rainstorm is usually sufficient to clean the array. If PV is installed on a wall surface, rain can keep it clean if the array is exposed to such. Otherwise, the surface can be cleaned in the same way as window systems would be. If installed in a dirty environment, the building must be designed to allow reasonably easy access to the arrays for cleaning.

CMHC Healthy House
CMHC Healthy House, Toronto, Ontario, Canada

The solar panels that form the large south facing overhang on this experimental housing is beyond the reach of any ordinary window cleaning equipment. The owners must hire a "cherry picker" to come and clean the arrays to remove environmental pollution.

Shading:

AVOID SHADING THE PANEL. The shaded area will not reduce the output proportionally to the area shaded -- loss is much higher. Within a chain of modules the output will be that of the weakest (shaded) module. If shade cannot be avoided at certain times, be sure to gang the affected modules together on the same circuit, leaving the sunny modules to fully function.

This goes for seasonal shading due to trees or vines, and even the shade from deciduous trees in the winter when they are bare. Watch for plant growth over time that can shade the panels. Locate the PV array away from the lengthening shadows of adjacent buildings or trees that will shade the panel in the early morning and late afternoon when the electrical generation is likely already being compromised by the lower sun angles and increased reflectance of the sun off of the panels.

Orientation:

It is essential to provide unobstructed access to sunlight to optimize efficiency.

Due south is ideal but deviations up to 45 degrees only result in a 10% loss of power. As a rule, BIPV installations are best when oriented south and tilted at an angle of 15 degrees higher than the site latitude; ie. The further north you go, the more vertical the panel as the sun angles are low in the sky and the system performs better when the rays strike at a right angle (less reflectance).

Orientation of PV at 52 degrees latitude
Effectiveness of PV at 52 degrees North Latitude

Optimum orientation must be worked for each latitude and can be derived from the climate data for the site, in particular the solar angles. PV Watts is an online performance calculator for grid connected systems.

PV VS. BIPV:

BIPV stands for “building integrated photovoltaic” systems.

These use PV, except attempt for a more “architectural integration” of the PV into the roof, wall, glazing and shading systems.

Integration aims to reverse the trend to think of PV as an “add-on” (and usually pretty ugly) system, and ensures that it works as part of the building envelope system.

This works with sustainable notions of having building elements “do” more than one thing. Roofs can easily accommodate another use -- by adding electrical production. The same with curtain walls, skylights, etc.

Lillis PV Installation
Lillis School of Business at the University of Oregon
Uses BIPV in its south facing front entrance wall to simultaneously capture the sun's energy and provide interior shading to the atrium.

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