If you’re considering installing a new solar hot water system on your home’s rooftop as an alternative to traditional heating options, then undoubtedly, some of the burning questions on your mind are about the differences between a passive and active solar thermal systems, costs, and pros and cons of each option.
To get right to the bottom line, here are the average costs of having a completely new solar hot water system installed on a typical residential rooftop.
Passive Solar Water Heat: $2,500 to $5,500 with an average of $3,500 before the 26% federal solar investment tax credits that are good through the end of 2022. These solar tax credits from the federal government will be reduced to 22% in year 2023.
Active Solar Water Heat: $7,500 to $10,000+ with an average of $8,700 before the 26% federal solar investment tax credits.
If these numbers seem to cover a wide range, it’s because of the wide variety of options available when purchasing and installing a solar water heating system. Here are some descriptions and examples of the different types of systems and how we arrived at the above costs.
The heart of either a passive or an active solar water heating system is the solar collector. This is the part that transfers the sun’s energy into hot water. There are three common types of collectors, and the type used will greatly affect the overall cost of having a solar water heating system installed.
The three types are: integral collector/storage systems (ICS), flat plate collector panels, and evacuated tube collectors. We will describe how each works before explaining the difference between a passive system and an active system.
Integrated Collector and Storage System
The integral collector/storage system (ICSS) has several large diameter tubes or a black tank inside a sealed, insulated box. It will have a clear cover – usually made of one or more layers of glass – to let the sunshine directly onto the surface of the tubes and/or tank.
As the dark surfaces absorb the solar energy, the water inside them is heated. It is considered a collector/storage unit because it holds the heated water in its internal tank before it goes into the house.
This is the simplest type of solar collector, and many people have constructed their own. It is also the least expensive type of solar water heat collector and the least efficient too. It is sometimes called a batch collector or batch water heater.
Flat Plate Collector Panel
The flat plate collector panel is basically a shallow aluminum-framed sealed box that has an absorber plate with a special dark coating behind a glass cover. Attached to this absorber plate are copper pipes through which the water flows.
Anytime the sun shines, the absorber plate generates heat which is transferred to the pipes inside the collector.
The water inside the collector is heated from bottom to top in a serpentine fashion through the pipes. It then either moves by thermal activity into a tank mounted higher than the collector (hot water rises) or directly back into the house. When hot water is called for, fresh cold water moves into the bottom of the collector as hot water moves out of the top.
This type of collector could be a DIY project if a person is very handy but is most often a purchased product. They are more expensive than ICS collectors, but less expensive than evacuated tube collectors.
Evacuated Tube Collectors
Evacuated tube collectors are available in two configurations: heat pipe and direct flow.
The heat pipe evacuated tube collector consists of parallel rows of transparent, double-walled, vacuum-sealed tubes which have a special metallic coating in them to absorb the sun’s heat. This heat is then transferred to an aluminum fin assembly which heats a sealed copper tube inside each glass collector tube.
An alcohol solution inside the copper tubes vaporizes and causes a condenser bulb at its top end to get very hot. These condenser bulbs extend inside a header manifold full of water at the top of the unit.
The heat is transferred from the hot condenser bulb into the water in the manifold. This hot water then either migrates into a tank above the collector or is sent directly into the house, depending on the configuration of the whole system.
The direct flow evacuated tube collector is sometimes referred to as a “U” pipe collector. It is different from the heat pipe collector in that it has two copper pipes running through the center of each double-walled, vacuum-sealed glass tube.
One pipe is for the incoming water while the other is the return pipe containing heated water. The pipes are connected at the bottom of the tube in a U-fashion, which gives this type its name.
There is a heat-absorbing plate inside the tube that provides a physical separation between the incoming and return pipes from each other. Both the heat absorber plate and the hollow pipes are made from copper with a special coating to maximize the efficiency of the collector.
The water that enters the tube travels the length of the tube twice (once down and again back up) and exits warmer than at the entrance.
The evacuated tube collector is not something that can normally be made at home, though some engineer-types might purchase the vacuum-sealed glass tubes and go from there. — They are the most expensive and offer the highest efficiency of the three types of solar water heat collectors. One reason for this is that the round surface of the tubes is always at the optimum angle to the sun throughout the day.
Passive vs Active Solar Water Heaters
Now that we’ve discussed the anatomy of most of these innovative water heaters, here are the differences in the key types that use this technology.
Passive Solar Hot Water Systems
via Energy Depot
The simplest and oldest passive solar water heater is a barrel or other container on an elevated stand, which is filled with water and warmed by the sun. All that was needed was a device at the bottom to let the solar heated water out when it was wanted.
Commercially manufactured passive solar water heating systems today operate in the same basic way as the barrel on a stand, but we don’t have to carry water to fill the barrel.
The passive solar water heating system uses the pressure of the city supply or the well that serves the house.
The passive system uses no mechanical means to circulate (or recirculate) the water. The power of the sun is what provides the heat for the water.
What they have in common: Passive systems do not contain pumps, controls, switches, sensors, or any moving parts. The simplicity of the passive solar water heating system is that it is only the water pressure that is normally found in a house that makes the water flow.
There are two main types of passive solar water heater systems: thermosyphon systems and integral collector-storage systems.
Thermosyphon Passive Systems
These systems produce a limited amount of fully heated water. More often, they are used with a conventional tank water heater. This results in saving either gas or electric energy because the incoming water to the tank heater has been preheated by the sun.
Flat-plate collectors and evacuated tube collectors are common equipment in thermosyphon systems.
Normally installed on a south-facing roof or wall in the northern hemisphere, a thermosyphon solar water heating system can also be mounted on a ground stand. This type of system will have a solar collector – often of the flat plate type – which uses the natural property of water to rise as it gets warmer.
The cold-water supply enters the bottom of the collector and moves upwards to the outlet as it is heated by the sun. It then either goes back into the house or to the inlet at the bottom of another flat plate collector for more solar heating.
Multiple collectors increase capacity: When several of these flat panels are connected, the outlet of the last panel – which will be the hottest – then goes back into the house, often into a conventional tank water heater.
If the solar collector is of the evacuated tube type, the principle is basically the same. Supply water comes into the system and is heated by the action of the vacuum-sealed tubes as described above. The thermal tendency of water to rise as it gets hotter is what makes these configurations “thermosyphon” types.
When a hot water tap is turned on in the house, the pressure of the incoming water pushes the heated water out of the solar heating system, through the indoor pipes to the faucet.
Either type of solar collector of the passive thermosyphon kind can be paired with a storage tank mounted higher on the roof than the collector.
Normally, the bottom of the storage tank is connected to the bottom of the collector and the top of the collector is connected to the top of the tank.
The thermal activity of the warm water will keep it rising above any cooler water in either collector or tank. The hottest water is always at the top of the tank ready to be sent into the house.
Integral Collector-Storage Passive Systems
As before described, the distinguishing factor of the integral collector-storage system is that it not only heats water with solar energy, but it also stores a significant amount of water. With the ICS collector, there are several large tubes inside which will hold about 10 gallons of heated water each – much more than the other collectors do by themselves.
Active Solar Hot Water Systems
via Energy Depot
Active systems differ from passive systems in that they rely on various kinds of equipment to circulate the water, regulate its temperature, or both.
There are many ways active solar water heating systems can be configured. They are often connected to a traditional indoor tank water heater, but they can also have bypass valves to operate without it in the system if so desired, supplying heated water directly to the plumbing.
Equipment used in active solar power water heating systems: They can utilize any of the different types of solar collectors, but the active system will also include one or more of the following components:
- A pump to circulate the water from a traditional indoor water heater tank back to the solar collector for heating or reheating.
- One or more sensors to measure the temperature of the water at various points in the system.
- A controller that responds to the sensors to regulate the circulating pump or pumps.
- A pressure relief valve to prevent damage to the pipes in case the water becomes so hot that it builds up excessive pressure.
- A mixing valve to mix very hot water with cold water before it is sent to the faucets in the house.
- Ball valves and/or check valves to allow or prevent the water from flowing in certain directions under certain conditions.
- Drain faucets to allow the water to “drain back” out of the outdoor collectors and tanks to prevent freezing in cold climates.
Direct or Indirect?
Active solar hot water systems are either direct or indirect. In a direct system, the water that will be used by the people in the building runs directly through the entire solar heating system. Just as in a traditional indoor water heater, fresh, cold water goes in and hot water comes out, being heated as it passes through the system.
With an indirect solar hot water system, it often isn’t water that is in the solar collector(s), but a different liquid – usually a water/glycol solution similar to antifreeze.
When this fluid is heated in the solar collector, it travels into a heat exchanger made of a series of spirals or loops inside a tank of water. This might be a separate storage tank or that of an indoor water heater specially designed for this purpose. As the solar-heated fluid flows through the heat exchanger, the water inside the tank is heated indirectly.
The glycol inside the solar hot water system travels in a closed loop through the system using completely different pipes than that for the water to be used in the building for drinking, bathing, and other domestic needs.
Actually, water could be used as the heat transfer fluid in this type of system, but by using glycol, the need for draining it during freezing weather is eliminated.
When a Passive Solar System is a Smart Investment and Why
- Passive systems are less expensive than active systems, usually less than half the price.
- They don’t require any special equipment to circulate or control the water.
- They work best in areas where temperatures rarely fall below freezing.
- Off-grid households will need no electricity to produce hot water.
- Work well in households with significant daytime and evening hot-water needs.
- Low maintenance and higher reliability since it has fewer moving parts.
- Cost efficient – adds nothing to the utility bill.
- Doesn’t work as well in colder climates – lower efficiency and may freeze.
- May overheat and bring scalding water inside the house, since there is no thermostat or sensors to control temperature (though this is rare).
- Heavy – systems utilizing a large storage tank require adequate roof frame strength.
- Almost always require a backup gas or electric heating system for cloudy days and times of increased demand.
When an Active Solar System is a Smart Investment and Why
- Can be more readily used to heat water in cold climates; the water can be stored in a tank that can be kept indoors to prevent freezing. No liquid outdoors except antifreeze.
- More efficient than passive systems.
- Water temperature can be easily controlled with specific components.
- Can move more water than passive systems – better for larger families.
- May qualify for federal and/or state tax credits.
- Much more expensive than passive systems.
- Much more labor involved to install.
- More components to fail and cause the system to overheat or shut down.
- Energy savings may be partially offset by use of backup heating sources to supply hot water.
Cost of Solar Water Heating System Components
The cost of the equipment and associated parts and supplies for solar hot water systems cover a very wide range. Active systems are much more complex than passive systems and require many more components. There are many different configurations, so the specific components must be chosen for each.
Below we have listed some examples of the cost of equipment associated with both types.
Solar Hot Water Collectors – to be used in either passive or active systems
Flat Plate Collectors:
Most are 20 to 22 square feet or about 40”x80”
Cost: $550 – $750
Evacuated Tube Collectors:
- 10 Tube Evacuated Tube Collector and 45° Frame: $500 – $650
- 15 Tube Evacuated Tube Collector and 45° Frame: $875 – $1,000
- 20 Tube: average cost – $1,100
- 25 Tube: average cost – $1,350
- 30 Tube: average cost – $1,550
Evacuated Tube Collectors with Stand and Storage Tank:
- 15 Tube Evacuated Tube Collector with Stand and 40-gal tank: $2,750 – $3,500
- 20 Tube Evacuated Tube Collector with Stand and 80-gal tank: $3,900 – $4,750
Integrated Collector Storage System:
ICS Passive Solar Water Heater with Roof Stand and fittings: $2,850 – $3,500
ICS Collector with 38-gal internal tank: $3,300 – $3,750
Passive Solar Hot Water System Component Costs (not including solar collector)
Solar Water Storage Tanks – Average Costs:
- 60 gallons: $2,000
- 80 gallons: $2,500
- 100 gallons with built-in heat exchanger: $4,000
- 120-gallon tank with built-in auxiliary heat: $5,000
- Solar Insulated Piping: $380 per 100 ft (3/4”) or $570 per 100 ft (1”)
Active Solar Hot Water System Component Costs (not including solar collector):
- Water Circulating Pump: $65-$125
- Flow Balancing Pump: $85-$150
- Solar Controller: $250-$500
- Solar Pump Station with Integrated Controller: $1,000 -$1,500
- Isolation and Diverter Valve: $75-$300
- Air Venting Valve: $125-$250
- Straight Pipe Fitting: $7-$15
- Solar Non-Insulated Piping: $25 per 50 ft (3/4”)
- Solar Insulated Piping: $380 per 100 ft (3/4”) or $570 per 100 ft (1”)
- Piping with Sensor wire: $400 per 100 ft (1/2”) or $600 per 100 ft (3/4”) or $1,450 per 100 ft (1”)
- Glycol Solution (for indirect heating of tank): $50 per gallon
Solar Water Storage Tanks Average Costs:
- 45-gal Indirect Heated storage tank: $1,500
- 50-gal Direct Electric Solar Storage Tank: $800 – $1,200
- 80-gal Direct Electric Solar Storage Tank: $1,000 – $1,500
- 108-gal Indirect Electric Solar Tank: $1,750
- 50 Gallon Pressurized Electric Backup Tank with Single Heat Exchanger: $1,000 – $1,200
- 130-Gal Stainless Steel Solar Hot Water Heater Tank Dual Copper Coil Heat Exchanger: $2,350 – $3,000
- 119 Gallon Indirect Double Wall Stainless Steel Water Heater: $5,000 – $6,400
Active Solar Hot Water Kits Average Costs:
- Flat Plate 2 Panel Standard Hybrid Solar Water Heater Kit: $750
- Flat Plate 4 Panel Standard Hybrid Solar Water Heater Kit: $1,000
What’s in the Kit? These kits include collector, pump, all fittings, adapters, and connectors to connect directly to an existing water heater and a 10W Solar PV panel to power the pump.
Evacuated Tube 65-gal Solar Water Heater Kit: $5,700 – $6,000
What’s in the Kit? This kit includes collector, pump station, controller, tank, and backup energy source.
Using the power of the sun can be as simple as setting up a barrel of water or as complex as a multi-panel rooftop system that is fully automatic and will guarantee that you never run out of hot water regardless of the climate in which you live.
As shown by the prices of the various components above, the cost of a complete solar hot water system can be as little as around $2,500 fully installed and can run well over $10,000 for a larger system, depending on the type of system you choose to purchase before the 26% federal solar investment tax credits that apply to the full cost of the system installed before December 31st, 2022 (Solar tax credits will be reduced to 22% in 2023).
On average, most homeowners pay between $7,500 and $9,500 for an active solar how water system before the current 26% federal solar tax credits.
Do your own research and be sure to determine if all the system components you need are available in your area before starting your purchases. Some are only marketed in certain parts of the world, and shipping can add significant costs to your bottom line.
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