I’ll review the last show on solar hot water systems. A system that is designed to heat the hot water only has a different angle to the sun than one that is used to heat the house as well. Summer hot water use is only slightly less than the winters, so year round collection is needed and a lower angle is appropriate. The backup for hot water can be small enough that an electric element can supply enough heat to supplement the solar when needed, without using much electricity. Electric resistance backup for home heating would require a larger heating element and would run longer than for the hot water.
The sun angle at noon in winter is as low as 30 degrees above the horizon and on June 21st it is about 75 degrees. An average angle half way between these two would give about an equal amount of heat winter and summer.
Tipping the panel somewhat toward the winter sun at about 55 degrees is a good idea. If the system is designed for heating the house and the hot water, a lot more energy is needed in winter.
Tipping the panel up to 70 degrees will shift the seasonal collection toward the winter and protect from overheating in summer. If you’re house is not well insulated, or you think you’ll use more hot water or like the house at 75 degrees in winter, add more panels.
Combined systems with back up can be complex. Size, sensor location, controls, choice of materials, insulation and sun angles all contribute to the effectiveness of the system. As I mentioned before about 50 sq ft of solar collector can supply enough hot water for an energy conscious family in this climate. A rule of thumb for a combined system is 10% of the floor area, so a 1,500 sq ft home would use about 150 sq ft of collector. If the house is tighter than most and well insulated somewhat less collector area is OK.
A temperature sensor is used to turn on the circulating pump, when the temperature in the panel is high enough to heat the water in the tank. After the water is raised to its maximum temperature, another sensor turns off the hot water pump and turns on the radiant floor pump.
The weather and the heating needs vary and designing a system that matches them consistently is a challenge. A backup system is used to fill in the spaces and sometimes turns on when it’s not needed.
Let’s say the water tank dips below the minimum temperature at 5 in the morning and you don’t intend to use hot water until dinner time. The sensor is not smart enough to know this and turns on the backup, when the sun could have heated the water before you needed it. This is another reason to have a larger and better insulated storage tank. A highly insulated house cools more slowly through cloudy periods than an average house and also requires less backup heat.
A little known fact about a passive solar house is that when the backup heat is used it has to heat the mass storage as well. This slows the heating and uses more energy than in a house with no mass storage. The mass can be isolated from the living space in the form of well insulated water tank. I’ll talk more about this later.
The angle of the solar heating panels to the sun is not adjustable like many solar electric systems. The purpose of adjusting this angle on solar electric panels is to keep the panel close to perpendicular all year round to maximize collection.
On a net metering system, collecting as much in the summer as in the winter is a good thing as it just feeds into the grid and spins the meter backwards. On solar heating systems there is no storage for the summer heat, for use in winter. For this reason the collection needs to be balanced seasonally.
A flat stationary panel can do this if it is tilted up to 70 degrees, at our latitude to protect from summer over-heating. This gives up collection power as it exposes the panel to a smaller solar window, thus requiring more panels. Also more energy bounces off as the angle moves away from perpendicular.
If the angle is set to 60 degrees, more energy is collected year round, but there is too much in summer. This extra heat needs to be dissipated or the pressure relief valve will open and spew antifreeze all over and the system then has to be recharged with more antifreeze. To avoid this, an extra zone can be added so that when the hot water tank is fully heated there is a place for the extra heat. This adds cost, complexity and a sense of wastefulness.
Making a solar thermal panel adjustable is difficult. They are heavy and the plumbing is not easy to rotate like the wire on an electric panel.
If the thermal panel is installed at an optimal angle of 60 degrees and a reflector is mounted above the panel, it can double the solar window in winter and cast a partial shadow in summer, protecting it from overheating. This more closely matches the solar collection with the seasonal heating needs and costs less build.
A reflector placed at the bottom of the panel can also increase the collection area of the system.
If a reflector is used on both top and bottom at the same time, a problem arises. The high summer sun bounces off the bottom reflector and hits the top one. At certain sun angles a bright light reflects out horizontally.
In most locations this would not be acceptable. This mirror-like material is 94% efficient. It would be like the low west sun shining deep into your neighbor’s house. This can be avoided by making the bottom reflector adjustable. This adjustability can also increase the production in spring and fall by focusing the higher sun onto the panel at an angle closer to perpendicular.
A bottom reflector can be used instead of a top one to provide a significant increase in solar gain. If mounted on a flat roof a 3’ high panel and a bottom reflector may be completely invisible from the street.
This design does not produce a shadow in summer to protect from overheating but with a panel angle of 70 degrees it will only collect enough heat for the domestic hot water in summer. With a bottom reflector added it will significantly increase performance above that of the panel alone and besides being less visible it also has less wind loading.
A drawback of the lower reflector is the accumulation of dirt and snow, which is not an issue on the top one. A small amount of dirt does not affect the performance very much, but as more dirt accumulates the reflective performance diminishes. Snow often melts off quickly but sometimes can remain for a week blocking the added power of the reflector. Rain is effective in cleaning this bottom reflector but varies depending on location and season.
The Reflectors I installed on my house in December of 2008 have been cleaned only once and I don’t think I’ll do it again. In the large concentrated solar electric power plants they use a special truck that washes the reflective surface to get every one percent of power possible.
Manually adjusting the reflector once or twice a year as is often done on solar electric systems produces more annual energy per square foot of collector. This option also gives added control as any one year may differ from another.
As the incident angle of the sun with the collecting surface increases, that is closer to perpendicular two things change. One is the projected area that the sun sees of the panel is larger. As the panel is tilted away from the sun this solar window gets smaller. At 30 degrees the solar window is half of the collection area as it is at 90 degrees. Thirty degrees is also an important number in terms of the amount of energy absorbed.
At an angle of 75 degrees to the panel most of the sun’s energy penetrates into the panel and is absorbed. This is true for the rays directly striking the panel from the sun and also the reflected rays from the mirror. At 45 degrees there is still considerable heat gain, but much below this it drops off quickly and by 30 degrees it’s very little.
Concentrating the sun’s energy can be dangerous. Most of us have seen what can happen when the sun passes through a magnifying glass and strikes a piece of paper. A curved surface is needed to concentrate the energy.
Since this system uses flat reflectors there is actually no concentration or the associated fire hazard. The added collection area and seasonal balance are the benefits of this system.
Combining the solar hot water with the solar heating system is more cost effective than hot water only in the long run, but is significantly more expensive. A large part of the added expense is the backup for the heating system. A well designed system can produce 90% or more of the homes heating needs. Eliminating the backup and wearing a sweater on occasion would be more earth friendly and save some money.
Next week will be about building a zero energy house with little added expense compared to a standard house
Quote of the week
“To get something you’ve never had, you have to do something you’ve never done before”