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Solar Water Heating Fundamentals
Solar thermal collectors
This is a durable aluminum framed box with a tempered glass aperture area. It has the appearance of a typical solar panel. Inside the box is either harp or serpentine style copper tubing welded to a copper and chrome absorber plate. The dimensions vary from 3"-6" deep, 3'-4' wide and 6'-10' high. They are durable, have high pressure ratings and no components that can fail.
• Unglazed (pools)
• evacuated tube
• Integrated collector storage
ICS units have a tank that is attached to the collector and goes on the roof. Hot water moves into the tank from the panel by a natural convective process. DHW is usually drawn directly through the tank/ panel assembly. If there is any risk of freezing the system must drain down completely.
• other technology
Solar air heaters
Parabolic trough and parabolic dish
Building integrated solar is a loose term. It includes using solar panels as a deck cover, using panels as a building material in the construction of the façade or even embedding collector tubing into a roof or wall surface. Perhaps more artistic than experimental, there are known to be many feasible and productive methods the question is which one best suits the design conditions. Since each project is unique and site built, high engineering costs and complicated installation may challenge the investment capability.
Choosing an installer
Becoming an installer
Place the panels where they are not shaded, if possible. Performance will diminish in proportion to the shaded area. Also consider foliage growth and future building plans. It is sometimes hard to visualize how shading will change during the seasons when the sun is in a different position. Tools such as the Solar Pathfinder reflect the site surroundings on a lens which simulates the curve of the earth and shows where the shadows will fall at each season.
• Attachment methods
Remember it will be harder to resurface the roof after panels are attached, so it is better if the roof is new.
If the joists are in good condition and the roof surface is new, the panel racks can be bolted through the center of the roof joists making sure to pre drill for a burly bolt and locating the exact position of the joist. Some owners want the penetrations to be covered by a warranty and if this is not done by the original roofer the warranty may become void for the rest of the roof. The installer is usually capable of sealing the roof so if the owner agrees, bolting to the roof is easiest. If the joists are exposed in the attic but are weak or staggered, the rack can be bolted, not to the joist, but by using a rod and bar to hug the joists from the underside.
Weighing down the rack works if the roof can handle the continuous load which must equal the maximum expected wind load. It will also present an added difficulty if roof resurfacing is needed.
The benefits of embedding a platform for the panel footing into the roofing material are that the roofer has complete control, the roof can be resurfaced, the roof structure does not need to be interfaced and the panels can be at any orientation in relation to a flat roof. This can be done on flat or shingled roofs by an experienced roofer.
• Racking and panel connections
Avoid using metals which are galvanicly dissimilar, most notably; steel should not be in contact with copper or aluminum since accelerated corrosion will occur. Where aluminum racking is mounted on steel put an EPDM or butyl rubber layer between the two. Triple check your racking hardware order because if you come up short on nuts or bolts you won't be able to substitute them with galvanized or zinc plated steel bolts but you may replace them with stainless steel which is compatible with most other metals, stainless steel is OK.
The top and bottom copper manifolds of the panel must be joined with a level of precision unseen in regular plumbing so it is super critical that the panels are absolutely level with each other with no dips or curves where racks are coupled together, particularly in panels connected by "unions". Some brands allow the headers to be joined by soldering which is more forgiving during installation and is more durable but more difficult to repair later on. Union type connectors usually have gaskets which are very sensitive to damage during installation and are not designed to "squish" or allow any deviation in the position of the flanges. Some with brass compression fittings may need tightening after start up. The space between the connector and the panel is very slim and the copper may be delicate so it is necessary to be prepared with 2 or 3 low profile wrenches.
The best way to avoid problems with union type panel connectors is to adjust the panels so the header is very straight and tighten the unions BEFORE committing the panel clips.
PVC trap with flashing collar
A 4"-8" PVC pipe is inserted through the roof with a hook on the top so water will not get inside. A prefabricated flashing collar is used to seal the PVC to the roof membrane. There is plenty of room inside the PVC pipe for the solar pipe insulation and sprayfoam can be used to seal the remaining spaces. Try to use sweeping bends or flexible tubing where the solar pipe passes through as it creates a small up trap in the pipe which could trap air.
Essentially a copper cap two pipe sizes than the solar pipe can be soldered directly to it thus covering the penetration and allowing insulation to butt right up to the cap. Some thermal losses will occur where the soldered cap, touching the pipe, is exposed to the outside air but the profile is close to the roof so an essential pitch can be maintained.
A square of tar is cut out around the penetrations. Sometimes the pipes might pass directly through the new bed of tar. But the pipes are hot so a shield should be around the pipe with a rain cover, such as a coolie cap. The area is then filled on with new tar.
At the foundation wall
Usually the base of the building will be poured concrete, concrete block, stone or brick. Using a hammer drill or core drill make an opening large enough for both pipes and insulation to pass through. Make sure you are not harming the structural integrity. The pipe must be encased in a strong shield that will bond with the cement and reinforce the structure, such as a piece of cast iron pipe. Cement corrodes copper so don't make contact with these materials. Fill the area around the opening with cement and use a rubber cap if further weatherproofing is needed around the solar pipes.
Sometimes the array is sighted away from the building it is supplying. This is often the case with pool heating. Trenches can be dug by hand or machine and the trench must be at least 2' deep to with stand impacts from above. Since the pipe is a low spot it is a wise addition to put an access box with drains somewhere along the channel. The pipe must still be insulated and the minerals in soil will corrode copper pipe. Fiberglass insulation will deteriorate quickly and hold moisture so use elastomeric insulation. Additionally, foam board can be placed around the insulated pipe to help protect it from the dirt.
Freeze protection and overheating control
Propylene glycol is non-toxic anti- freeze. Brands designated for solar have re-soluble inhibitors. The inhibitors prevent the glycol from becoming too acidic for the pipe. The glycol should be rated for at least 325 degrees. It will keep its' integrity for 5 years under normal conditions and less per occurrence of overheating conditions. When glycol degrades it becomes acidic enough to eat through copper especially where there is extra friction in the pipe such as at elbows. It also loses its freeze protection gradually, as the inherent chemical compounds eventually separate.
• Fluid expansion
For the most striking description of fluid expansion consider that 1 part water occupies 17,000 x its' original volume when it is steam. Steam formation temperature is retarded by pressure and most strategies make the goal to avoid steam either by limiting the temperature or increasing the pressure.
The fluid in its liquid state also expands dramatically according to temperature. Where steam is to be avoided, the expansion tank should have the capacity to hold 1/3 the liquid volume of the system. The expansion tank should be compatible with glycol. The tank has a Schrader valve on it and should be pre charged to within 5 lbs of the average system pressure. If the expansion tank cannot take up the expanded liquid, the system pressure will increase until the pressure relief valve opens.
• Pipe expansion
• Heat dumps
When the storage tank reaches its maximum safe temperature the controller will signal the circulation pump to shut off. See below for more detail. Now temperatures in the collector will continue to climb. If a heat dump strategy is employed; rather than the pump turning off, the relay will activate a diverter valve that will instead direct the fluid through a radiator located outside of the building. Thus the fluid is kept from boiling until hot water demand brings the tank temperature to a point where it can accept more heat input. Swimming pools make excellent heat dumps and also energy on pool heating. Simply divert to a pool heat exchanger. The extra heat can also be stored in a seasonal storage pit and used later in winter as a contribution or with a heat pump.
• Drain back systems
• High pressure and steam back systems
High pressure system
In this heat protection method, high pressures are used to control steam formation. All components must have at least a 150psi pressure rating and withstand temperatures up to 350 degrees. A well sized expansion tank is still needed to take up extra fluid volume. Pressure relief valves on this type of system do not open until temperatures reach 350 degrees.
Steam back system
These would be set at up to 50 psi for a boiling point of 300 degrees, usually the setting is lower. Here boiling is anticipated. Steam forms in the collectors and pushes the liquid down. The water hides safely in the cooler zone while the collectors continue to heat the steam portions of which then also condense. The pressure spikes when the liquid is about to turn to steam and then levels out as the liquid is forced to occupy the expansion tank which is the specialty object in a steam back system. Expansion tanks up to 50% of the volume are used and the pipe to the expansion tank is as short as possible with no check valves to block the flow of water to the expansion tank. Tank and glycol must have exceptional temperature tolerance.
The basis of tank sizing is that the tank should able to hold and utilize all of the extreme heat that may be collected and is there for the useful way to control overheating. Tanks sized for space heating are designed at a ratio that will allow the tank to be heated entirely by the solar array in winter and in the summer will need to have a heat dump plan. In multi-family building with high consumption the water may never reach its' point of use temperature but will still contribute significantly to the water heating load. See more about storage and use below.
• Plumbing components
Storage tanks and heat exchangers
The daily consumption needs to be determined when sizing the tank or any other hot water source. Tables can be found in plumbing code books and many other reference sources that show the rate of use at each type of fixture. A suggested rule of thumb is to assume the first two residents of a family will use 20 gallons/ day each and additional member will use 15 gallons. If it is intended that this hypothetical family will heat their use entirely with solar March- September, we now know that the storage tank will need to be at least 70 gallons. Now consider occupancy rates and weather conditions such as: what if they vacation for a week in the summer? What if it is rainy for 3 days and I'm only getting 1/3 of the seasonal irradiation? The possibility of a lapse in occupancy results in the necessity of an overheating protection method. This concern is not normally applicable in a multi- family building. Industrial facilities would want to be able to store the heat during weekends and holidays but residential systems do not usually have room for that much storage. If possible you might size the system for sub- prime production with a high collector to storage ratio, such as in a space heating system, and divert the storage to an additional tank when the input is great or there is not enough use.
Colder water is heavier than warmer water and will sink or stay at the tank bottom. Conversely, the water will be hotter at the top of the tank. The effect of stratification could easily create a temperature difference of 40+ degrees between top and bottom in a 100 gallon tank. Cold water needs to put in at the bottom of the tank so it does not cool the warmer water and hot water should be drawn from the hottest top of the tank. Usually the effects of stratification are utilized and turbulence in the tank is counter productive. When putting heat into a tank be sure to give solar the priority by injecting it lower that an auxiliary source. The heat will fill the tank from the top down naturally and so the auxiliary source will not be signaled. When there is no sun and the auxiliary source is relied upon, the top portion of the tank must hold the load of the demand and this is another reason why solar storage tanks are often larger than fuel fired tanks.
Each 1 Sq. ft. of collector area should have a minimum of one gallon of storage. The collector area is not always apparent as with evacuated tubes and the rules change with other collector types. It is preferable to base storage needs on the collector output rating, referenced from the SRCC (Solar Ratings and Certification Committee). The rating will show the daily performance of a collector under various weather conditions. New York latitude puts us in category C of the ratings tables for the summer. If, for example, your collector has a rating of 40.000 Btu on a clear day in the summer and the family you are serving uses 55 gallons each day and needs to raise that water 100 degrees to make it hot enough: 40,000/ 8.34 (lb.s/gall)=4,796 (gall/degree), 4,796/100= 47.96 gallons heated 100 degrees each clear day March- September. In large commercial systems where the use is much more than can be produced with the usable roof area the tank size determines how much usable heat can be transferred and not necessarily stored. Imagine if the tank temperature was the same at top and bottom on a sunny day because the heat was being consumed at the same rate it was produced. Even were that only 30% of the total load it would be very efficient for heat transfer and the system would always be productive.
• Heat exchangers
Heat exchangers are rated in Btu/ hour of heat transfer potential. The Btu production/ day can be divided by 6 prime solar hours each day but when finalizing the calculation add an additional 25% to the production. To begin the calculation: determine the collector output and the flow rate recommended for that panel (say 1 GPM per panel/40,000 Btu day). Convert the GPM into pounds per hour because hour answer will be in hours, not minutes. 60GPHx 8.34 lbs=500.4 lbs/hr. 500 pounds of liquid are moving through the system each hour. Convert 40,000 Btu per day into hours 40K/6 hr=6,600 Btu/hr. 6,600 Btus need to transfer into 500 lbs.= 13.2 degrees continually transferred from a 6.600 Btu rated heat exchanger. Add 25% for seasonal variation and to keep the returning water temperature optimized. You do not want to transfer all the heat because you can't raise it all the way back up from 0 when it makes the next pass through the panels. Having a slow flow rate in an attempt to raise the fluid more than 20 degrees will cause scale to form on the heat exchanger and reduce the operating hours. In this case the temperature in the panel will need to be at least 13.2 degrees hotter than the bottom of the tank to be effective. This is the primary Delta T that will signal the solar pump to turn on, when it knows there is enough heat in the collectors to transfer to the storage water.
• 60-1000+ gallon steel tanks with 1-12 exchange coils in the tank
Sub merged coil heat exchangers are not specified by Btu. They are sold as proportionate to the storage volume so are pre- engineered. Tanks up to 130 gallons are available for sale with one or two submerged heat exchangers built in. Larger tanks for domestic water can be custom built on or off site with heat exchangers welded in to specification. The tank must be insulated with 2"-4" of foam and should have a sacrificial anode.
• 40-2000+ gallon steel tanks requiring external heat exchanger
Brazed plate heat exchangers are compact external units piped to and from the heat source and also to and from the storage vessel. They can be purchased from a variety of sources and selected based on their Btu rating.
Tube in shell
For large volumes and applications where flow is critical but an external heat exchange unit is required. The choice for pool heating or applications with very large pipe diameters. Tube in shell or tube in tube is more like a submerged coil but is external to the tank.
• Collapsible EPDM tanks
• Other storage methods
Excess heat produced during summer can be stored in sand, earth, salt or water for use during winter. Since the chamber is not pressurized the temperature can be allowed to climb as high as the piping and insulation will allow. It is feasible to heat a building all winter and yet reduce the necessary collector area.
District heating serves separate residences or multi-family units by piping stored hot water from a large thermal mass such as an underground pool located away from the building. Large, ground mounted arrays can supply hot water and heating remotely. This is commonly practiced in European countries.
The salts being used to store thermal energy are compounds like Sodium and Potassium Nitrate that melt at over 750 degrees and don't vaporize until they reach thousands of degrees allowing them to hold mass amounts of latent heat. The salts then flash water to steam which powers electric turbines. The ability to store heat at such high temperatures means it is usable overnight and through unfavorable weather conditions making it a stable and plausible source for grid tied electricity production.
Back up heat source
• Integrated into solar tank
• Supply hot water heater
The classic prefeed system supplies the cold inlet of a hot water heater with preheated solar water usually from a solar storage tank with a heat exchanger to boost storage capacity and regulate the solar control strategy. Here the HWH will respond to the present temperature all or some of which will have been boosted by the solar input.
• Supply tankless
• Supply boiler
The same strategy and burner activation requirements apply to the boiler as to the tankless HWH. Modulating boilers are designed to fire according to hot mixed inlet temperatures which is a great efficiency advantage. If there is a recirculation line supplying the boiler the solar should be diverted back to its' own storage tank until its' temperature is hotter and otherwise must be designed to not interfere with the recirc loop. Solar storage is required and a pre heated domestic tank may be necessary especially if there is recirculation.
• Boiler, tankless, or hot water heater supplying solar tank
• Heat pump
• Direct injection
Certain thermostatic mixing valves will accept pre-heated water to be injected on the cold inlet port.
The valve will not introduce a hot water supply to the hot port except to boost the temperature of "cold" water. Of course, a domestic mixing valve will be installed down stream to temper the supply to the fixtures. In this way none of the solar heat is unusable or lost to the operating parameters of the auxiliary heat source.
Integrating with domestic hot water piping
• Mixing valves
• Check valves
A check valve on the solar loop should be installed to prevent convective flow in the pipes from temperature, even when the pump is off. Swing checks offer low head losses while a spring check offers a positive seal though creates flow losses. Even when flow is arrested at the turn off setting, temperatures can still migrate within a pipe run. If temperatures persist in escaping from the tank, additional check valves might need to be installed.
Check valves are located to direct flow in only one direction and disallow unwanted mixing of sections of connected pipe.
Back flow prevention
Hot, warm or circulated water cannot be introduced into municipal water supplies. Pressure differentials will result where backflow is prevented because of draw and temperature dynamics so a vacuum breaker is needed to stop backflow if the drawing side still has a pressure higher than the main.
• Plumbing permits
A plumbing permit is required for the alteration of domestic water pipes where they are rerouted and connected to the new tank or appliance.
• Label the pipes
• The importance of low temperature heat emitters
• Sizing the solar array
First the load of the heating system is calculated based on the room volume and heat loss in the heated area. Usually the heat load will be 3 times higher than the domestic hot water load. The panels will be collecting less energy for fewer hours and will collect best during winter at a 60 degree tilt. Refer to the collectors SRCC rating to see the variation in seasonal output. You will notice evacuated tubes have less of an efficiency drop when collecting for winter, but they don't shed the snow so chose the location carefully. The output will be around 1/2 of the summer output. An array designed to meet all space heating loads will be up to 9 times larger and a strategy to store or disperse extra heat during the summer is necessary.
• Combi tanks
• Forced air
A popular means of heat distribution, forced air can be easily supplemented by inserting a fluid filled heat emitter inside the ductwork.
For space heat only on south facing walls or roofs products are sold or can be fabricated that are designed to capture and hold heat from the sun. A small fan powered by a solar PV panel will move the warm air into the room. Only a small hole is needed to penetrate while the unit is mounted and sealed to the outside if the structure. These units are sized up to 6000 btus.
• Passive Solar
This refers to a building designed to incorporate solar heat gains and heat retention by placement of windows and walls and good insulating practices.
Piping, insulation and controls
• Balanced flow piping
• Routing the pipe
• Pipe elevation
It is also recommended to avoid low spots where debris can settle. This debris could be bits of solder, copper shavings, pipe dope or burned glycol. Debris is inevitable and should be allowed to flush from the system. Low places can cause difficulty when flushing the system or repairing the system and can never be completely dried. If you must design with a low spot, please, put a durable drain on it.
• Types of pipe
Copper pipe comes in 10', 20' or coils up to 100' lengths. There are four grades of copper pipe with different wall thicknesses, medium to high to high pressure applications where the PH might be low should use "L" copper. "M" copper could degrade quickly under stagnation conditions and, since the glycol is slightly acidic, will have a shorter life span. Copper pipe can be soldered or brazed and is easy to cut and repair. PEX, cross linked polyethylene, is used in heating applications and can handle temps up to 225, is impervious to acid, comes in long rolls and is flexible. However the temperatures of a solar thermal loop may exceed the tolerance if the system is not designed with a positive heat dump, no other pressure stagnation protection method will work. Flexible, Stainless Steel line sets are a popular choice and often sold by the collector manufacturer. The temperature tolerance and chemical resistance is excellent. Drawbacks include moderate pressure ratings, ridges that disrupt the flow, difficulty repairing and the attachment method requiring specialty gaskets. Iron pipe can be used in closed loop heating and closed loop solar applications. It is cheap and has very high tolerances but requires more skills, time and equipment to install.
• Sizing pipe
Formulas for sizing pipe are based on the flow rate required and the frictional resistance of the pipe. These are the limiting factors and your pump will be selected based on the flow rate and friction once the pipe size is selected. The amount of fluid traveling is usually rated in gallons/ minute (GPM) and the speed at which it is travelling is usually rated in ft/second (FPS). Greater GPMS means greater FPS and when the pipe size increases more GPMs can travel at less FPS. More than 8 feet/ second will greatly accelerate wear on the pipe and the pipe can only sustain a certain amount of fluid flow before the frictional resistance will cancel out flow entirely. Take as an example 100' of 1/2" cooper; at 5 gpms the head loss due to friction is 65' of head (Hazen- Williams formula), each elbow adds another .5' of head, and the collector will add another .5-1..5' of head. In other words, even a giant pump will not make it possible to supply 5 collectors on a 4 story building. For this hypothetical installation we want to minimize friction losses so we can reduce the size of the pump and not put excessive wear on components. We have 5 collectors in a string so we need to run the flow fast enough to gain about 15 degrees as the fluid moves through the array, a bigger heat gain is less efficient since hot water collects less heat when passing through. Product specifications usually recommend the best per panel flow rate and you want to increase with each added panel so let us aim for a flow rate of 5 gpms. We have a 50' pipe run (100' of pipe), 20, 90 degree elbows, 3' of head loss at the collectors. The formula tells that 5 gpms can barely make it through 3/4" pipe at under 8 fps so we will need to use 1" pipe and calculate the losses to determine the size pump to use. The 1" pipe loses 2.2' of head in friction, .05' for each elbow = 1' of head and we need to overcome 5' at the panels. We need 1" pipe to deliver enough flow and we need a pump with a 8.2 head at 5 gpms. That is how to size the pipe, pump and set the flow rate.
• Minimizing friction loss
As can be seen in the example above, friction loss sets our operating parameters. By comparing 1/2" to 1" pipe we discover the benefit of using larger diameter pipe but the elbows and fittings are an equally substantial part of the equation and though larger pipe offers less resistance, larger fittings offer more resistance. Minimizing elbows is key. Long radius elbows have almost 1/2 the resistance of regular ones and 45 elbows are about 1/2 as much as 90s and tees create a lot of friction and should never be arranged to bullhead flow causing flow imbalances. Avoid bumps and sharp edges within the piping as this could again double the friction. It is important to ream the pipes. Use full port valves and fitting with smooth, amply sized ports.
Types of insulation
Good exterior insulation is expensive and important. Most elastomeric brands say "UV resistant" on them but everyone knows this is not the case. It will turn to brittle powder and shrink long before your warranty is up unless you put pipe coverings on it. It also should not be wet because you will lose precious heat. PVC or aluminum jacketing is cheap compared to the insulation and worth the effort in effort and energy saved
Inside where there is not excessive moisture, 1" wall fiberglass pipe insulation can be used. It has an adhesive seal that commonly becomes unstuck so it is good to put some extra insulation tape on the seams. A roll of special tape to go with the insulation will be available from your retailer. You can miter the elbows or get PVC elbow covers for cheap. A bread knife works great for cutting the insulation. It is often difficult to get 1" wall insulation and accessories from plumbing suppliers so it saves time and money to get it from an insulation distributor. Pipe insulation sizing is based on the OD of steel pipe so it doesn't correspond to copper pipe sizes. Be sure to specify you are applying it to copper pipe, when you order.
"Elastomeric", "rubber" or "closed cell foam" insulation all refer to the dense, flexible type often seen on refrigeration piping. Unlike fiberglass it will not degenerate if it becomes wet and it has a higher R value, therefor it is best for use outside. It is easy to cut with scissors. It can be bought with or without seams and can be mitered and taped or glued at corners. The tape is a roll of similar material with an adhesive backing. Use 1" wall insulation. Remember, you may need to go directly to the distributor and specify the size is for copper pipe.
• Avoiding heat loss from conduction
Have you ever had to handle metal when it is cold outside? It is not as bad if you are wearing gloves. The gloves with the fingers cut off are almost like not wearing any gloves if you are touching metal. For this very reason all pipe supports need to be on the outside of the insulation or losses will be substantial, not 100's but 1000's of Btus. Just get hangers that are as big as the outside of the insulation. Where the weight of the pipe is being supported a saddle made from a PVC pipe or piece of flu pipe can be used to keep the insulation from getting crushed in the hanger. Insulate everything.
The insulation clad pipes are a cause of aesthetic concern for most owners. Color choices are very limited so lending a smooth look to the available materials will be very important. Shop around at you insulation supplier to see what your options are and be prepared to spend time on applying the insulation. Building owners need to understand that pipe insulation makes up a sizable portion of the job.
Control strategy and monitoring
Size the pump according to pipe and collectors it supplies. The pump is the primary flow controller in the system. Preassembled pump stations need to be checked for compatibility with the size of the array and the piping. To match a pump to the system, first find the systems friction head as covered in the section on sizing pipe, then look up the pump curves for the brand you are interested in. Pumps are now available that have adjustable speed control making them more versatile for changes in pipe design and seasonal needs. Some smart pumps now have integral temperature sensors that will change the flow based on heat gain at the collectors and they can interface with the solar controller. Drain back systems need a second pump that is activated on start up to assist in lifting the water up into the collectors until gravity assisted circulation begins. Pool systems use the pool pump to circulate the water through the pool panels en route to the pool, the pump size should be verified to be sufficient and usually is.
• The function of the controller
A basic solar controller would need only two temperature sensor inputs, one for the bottom of the tank and one for the collector temperature. It would also send 120 v power to the solar pump when the collectors were hotter than the tank bottom, to activate the solar loop. Then there is a need to limit the temperature of the tank, for safety, so we also need a tank top temp sensor. The tank top sensor can stop the power to the solar pump. If the tank top is not hot enough for use then we need call in the auxiliary heat source to supply the demand so a 24 v signal can also be activated when the tank top is too cold or a 24 v signal could be activated also if the tank top is too hot that activates a diverter to a heat dump. The solar controller usually has a transformer to provide up to 10 different 24 v signals for various control strategies, each one independently programmable to activate a valve or device within certain parameters. Most controllers have at least two 120 v power outputs that are programmed to activate pumps within the set parameters. More sophisticated controllers can vary the speed of the pumps or take flow and temperature data to calculate btu production or have an output to connect to a internet based monitoring system. Some controllers even contain wireless capability. The controller can be used to activate almost any function in a heating system. Drain back and pool systems have unique operating requirements and need a controller specific to them.
• Flow meters, pressure and temperature gauges
Mechanical pressure and temperature gauges are a common and important item that can be purchased from a plumbing supply store. Some thread directly into the pipe or specialized ones have immersion wells. Temperature sensors that connect to a controller operate on resistance, OHMS, which changes according to heat. Pressure sensors that connect to a monitor or controller come with specialized wells for the sensor and can alert the user to a leak. Flow meters are usually included in a pre assembled pump station and are important especially if there is no digital flow monitoring. Mechanical flow meters can be purchased best as a solar specialty item from meter makers. Flow meters that take digital readings are available and needed to monitor btu production. It is desirable to have temperature gauges on the supply and return of the solar piping as this is a visual indication of production and proper flow rates.
Temperature sensors are located wherever the controller needs the temperature data. A sensor goes at the collector outlet, where the hottest fluid is exiting on its' way to storage. If there is not a well for the sensor it can be clamped to the pipe with SS. Don't forget to run the temperature wire along with your pipe, on the outside of the insulation. 18 gauge wire can be used to run the signal. Most solar controllers accept 10K thermistor type that takes a reading of resistance in OHMs, the OHMs go down as the temperature goes up. Another type of sensor is an analog sensor or RTD and here the OHMs go up as the temperature goes up. These types of sensors are not compatible with the same controllers. Operation can be verified with an electrical multi meter.
Relays open or close a 24 v signal after it has been stepped down from 120 v by a transformer. Each automatic valve or burner switch and some mechanical sensors need 24 volts and controllers will have relay out puts for optional functions.
• Monitoring production
Though electricity is commonly metered, metering heat production is only now coming to demand due to an increase in awareness about the cost of heat and need to conserve it. Monitoring your solar thermal functions can include programs to monitor BTU production if a digital flow meter is installed in the piping. If a pressure sensor is also installed it will alert to failure and could pay for itself in one call, by preventing permanent damage to the system. Claims based on peak performance often sell a system and it may help to verify the promised performance to maintain integrity. There is a great need for real production data to be made available so the amazing results can be seen and unnecessary disappointment can be avoided. The utilities need the data to calculate their load reduction based on solar and those savings, in turn, fund incentive programs. Among the many ways that the cost of monitoring will pay for itself is by ensuring a performance based relationship with the installer.
Motivation and financial benefits
How much energy does it really make?
There are more solar thermal systems installed, world wide, than solar electric systems. Solar thermal panels collect 4 times as much of the suns' energy/ sq. ft. than do electric panels. In the example above see how 3 thermal collectors convert to the equivalent production of electricity. 274 therms = 8,235 KWH. Three 32 sq. ft. thermal panels = 48, 8 sq. ft., 75 watt PV panels at ¼ the required roof area. In the US, low public awareness and the low cost of subsidized fuels have, so far, kept the power of solar thermal a secret. Even if created sustainably, it does not make sense to waste electricity on heating when the resource is available. The only cost effective way to make heat sustainably is with solar thermal panels. The only reason we can waste fuel on making heat is because of the artificially low cost of heating fuel in the US and this is bound not endure.
• Making energy sustainably
Since the mayor implemented plan 2030, the Greater Greener Building Plan, emissions have been dropping steadily and are now 13% lower than in 2005. The elimination of #6 heating oil had a huge impact on emissions and public health. Several new local laws are making it easier to use more roof space for solar. Limitations on energy consumption are now being written into the local codes and these reductions will allow a better portion of energy to be generated on site. All hands are on deck to fund and ease the installation process.
• Reducing bills
Some quick calculations regarding fuel heat: Nat gas appx. $1.20/100,000 btu, Propane appx. $3.00/92,000 btu, Oil appx. $3.50/140,000 btu. A 3 panel system makes about 27,375,000 btu/year and costs in the range of $10,000.
If you have a heating system older than 10 years, an expertly done system overhaul could reduce the bill by as much as 10% and cost in the range of $4000.
If your hot water heating is a gas unit older than 10 years, you should change it anyway for about a 5% energy savings at a cost of around $1,600. If you got a solar upgrade this would be included.
If you have an oil/steam system that makes hot water also you must take action. Don't use the steam boiler to make DHW in the summer the options are many. Ideally you would convert to a high efficiency natural gas heater and convert your radiators run on hot water. This alone could cost $12,000 and save up to 50%. Add solar to any upgrade option and save an additional 65%.
Installing solar water heat entitles the residential home owner to an automatic 30% federal personal income tax reduction which carries over successively until the full amount is credited. State income tax incentives can be obtained as well for water heat or space heat, up to $5000.
Commercial customers can find incentives through utility companies and federal, state and municipal programs. Utility programs and NYSERDA programs as well as local weatherization programs have cash rebates or tax credits for equipment upgrades, pipe insulation and oil to gas conversions for residential customers. All incentives can be researched starting with www.DSIREUSA.org.
Buildings with new heating systems and a low utility consumption report are worth more than the cost of installing the system. Soon, as more people continue to install solar, tax assessors will automatically increase the value of the building and there is already a law in place that prohibits the tax bracket from increasing because of solar.
Calculate your payback
If fuel operating costs are $1500/year, an $8000 installed solar pool heater would cost $0 to operate under the same parameters.
• Air heaters
An 8'x4' solar air panel produces 6,000 btus /hour when sunny and costs $3500 installed. If offsetting natural gas for 100 days/year saves more than $70/year and gets a 25% tax credit.
• Domestic hot water
When considering the cost of a solar heating installation, subtract from the equation the cost of installing a conventional appliance instead. Solar thermal systems are very durable and will operate for 30+ years at an extremely low operating cost compared with only slightly lower up-front costs to install other conventional appliances.
If exactly 100% of the hot water demand is made during the summer, the savings can be calculated according to the fuel bill. Production in the winter can be expected to be 50% of this. Each resident uses between 15-20 gallons of water each day, 10,000 btu. Each person, $0.12 day x 365=$43.50. Minimum possible system size is 30,000 btus/ day(20,000 annual average) or $100/ year. Costs $6000 installed- 55% incentives=$2700/$100. The bigger the system and demand the faster the payback, for example, a 3 panel system costing $12,000 installed saves at least $300/year if consumed.-55%= $5400/$300. 6 panels, $15,000-55%=$6750/$600, etc…It is important to consider a fraction of the cost that offsets the purchase of non-renewable equipment, for example, a hot water heater sized equivalent to a 3 panel system would probably cost $3000 installed, where as it is included with the solar water heating system.
• Space heating and water
In order to produce 100% of winter, water space heating needs, the array will need to be about 3 times the size of one for DHW only. Space heating not only requires higher temperatures but operates during non-prime months. The savings are, however, proportionate to the added production and the price per panel of installation decreases a lot with each additional panel. A back-up heat source is necessary for cold days and overheating must be avoided in the summer. Even single family homes commonly spend $600 or more in space heating needs alone with natural gas plus another $200 (4 people), at least, for hot water, so if the space heat system installed costs $22,000-55%=$9900/$800.
• Whole building efficiency