In a typical residential house, the energy used to heat hot water takes 25 to 35% of the utility cost. This equals $250 to $500 per year. It is the second largest energy user, after heating/cooling of the home.
It is efficient to use solar thermal collectors for heating domestic hot water; average efficiency (amount of the sun's insolation converted to usable energy) of a solar thermal collector is up to 70%. Compare this to Solar Photovoltaic panels with an efficiency up to 14%.
Using solar hot water is the easiest way to lower the utility bill and reduce the carbon footprint. Generally 2 panels with a total area of 64 sq. ft. will be enough to reduce the hot water bill by 70%. The investment for a traditional solar hot water system will be earned back in 8 to 12 years. It's only 4 to 5 years for the Sunnovations system.
The investment required for solar hot water systems is much lower ($2k-$8k) than for solar PV($20k-$40k). If a solar PV installation is deployed, it is advisable to include a solar hot water system – as it provides a higher efficiency and requires less roof surface area. If 2 solar thermal collectors are installed (64 sq ft) one can save on installing 4 to 5 times as many PV panels (256 - 300 sq ft). This savings will easily pay for the solar hot water system's cost.
Why a novel solar hot water system concept is needed?
There are many different types of solar hot water systems: batch, thermosyphon, open loop, closed loop glycol and drain-back (see solar hot water basics for more information). For most of the United States, freeze-protected systems are needed. Until now, basically two systems could be used: Closed Loop Glycol (CLG) and Drain-back systems. The most commonly used system is CLG, the Drain-back system is a little more complex (and expensive) and also has some drawbacks.
Despite the benefits, the percentage of solar hot water systems in the United States is very low - especially compared to other European countries which even have less favorable sun conditions. The federal government offers a 30% tax credit and various states and utility companies offer additional subsidies.
There are 2 main reasons why solar hot water systems are not widely deployed:
1) Cost of systems and installation is fairly high
2) People generally don't like to think about operation & maintenance of the solar system
Sunnovation developed a system which take these concerns away.
Closed Loop Glycol Systems
In a Closed Loop Glycol System, glycol (antifreeze) is circulated in a closed loop through the collector(s) and a heat exchanger – which transfers heat to a solar storage tank. Glycol is used to protect the system from freezing.
The system requires the following components:
- 1 or 2 electro-pumps
- Electronic controller to activate pump(s) at correct collector and tank differential
- Sensors & sensor wiring
- Expansion tank to allow thermal expansion of glycol
- Check valve(s) to avoid (nightly) thermosyphon
- Air vent valve(s)
- Vacuum breaker valve(s)
- Double walled heat exchanger (due to risk of leakage of toxic glycol)
The Closed Loop Glycol system has following drawbacks:
- Lengthy, costly installation, requires significant plumbing and electrical installation time
- System depends on availability of external power (can't be used in remote locations and will fail during black-outs). This is external power (called parasitic power) needed to run the system; it reduces overall efficiency.
- A number of mechanical and electronic components are present and hence require regular maintenance to avoid failure. Known failures are:
o Pump/impeller breakdown
o Controller failure
o Sensor failure
o Wire breaks
o Check valve failure
o Plumbing failure (leaking of toxic but sweet glycol - risk for pets/kids)
o Expansion tank failure
o Pump noise
The worst aspect of CLG systems is the problem with stagnation & overheating. This is a situation where the pump is not running – either due to a system failure, a power outage or simply because the temperature differential between tank and collector doesn't result in pump activation. This can simply be because the tank is fully heated.
The result is an overheating of stagnant glycol in the collector(s). Glycol will break down if it exceeds a certain temperature and it will turn into a corrosive substance. This will even corrode copper tubing – which will lead to system leaks and hence proper system operation. Glycol may also lose its antifreeze properties – which can result in busted pipes and collectors.
Drain back Systems
Drain back systems use normal water in a closed loop; they are freeze protected by draining all fluid from the collectors to a drain back tank. It is a good system, but it has the following drawbacks:
- A stronger (more expensive) pump is required to be able to lift the water from the level of the drain back tank to the top of the collectors – at least several meters.
- This type of pump requires more power – this is energy that needs to be added to the system. This is called parasitic power – which is relatively high in drain-back systems. This can lower overall system efficiency by 20%.
- A small PV panel for powering the pump can usually not be used due to the high power requirements of the pump.
- Drain-back systems can be noisy, water can be heard tricking through the pipes.
- An additional drain-back tank is needed; this requires extra space and cost.
- Installation of a drain-back system is critical – as all pipes need to run down to allow water to flow out to avoid freezing. This usually requires that the collectors are mounted under tilt or slightly rotated – this is not aesthetically pleasing.
- Drain-back systems cause stress in the collectors. An empty collector heats up and then suddenly cold water is pumped through the collectors. This causes a temperature shock in the collector tubing, which suddenly shrinks. This can cause brazed joints to crack and result in leaks.
- The temperature sensor mounted on the empty collector may heat up quickly and cause the pump to kick in. But once fluid flows, there may not be enough sun to heat the water to minimum temperature differential and the system shuts off again. This cycle can continue all day long and cause significant extra stress to pump and collectors.
- Drain-back systems are more expensive (due to need for extra tank and more expensive pump) and require extra installation space (for the extra tank).
- After a drain-back system stops, a large quantity of hot water drains to the drain-back tank. This solar heated water will not be used, and hence it reduces the efficiency of the drain-back system.
Conclusion on existing systems
Both Closed Loop Glycol and Drain-back systems have drawbacks. These systems are also relatively expensive to install (it adds significant to cost price of the solar hot water system). If the systems are not annually checked and maintained (a common situation) the systems will eventually fail – often before the payback period ends.
Many solar hot water systems installed in 1980's have broken down and have been deactivated or removed prematurely.
What is needed for care-free solar hot water?
We know solar hot water makes economic sense, but both CLG and drain-back systems exhibit too many problems. What is needed is a system that is low in cost, is easy to install and operates for decades without having to care or worry about.
Design parameters:
- No mechanical components
- No electrical components
- No valves
- Easy installation - no or minimal plumbing
- No glycol
- High performance
- Care-free operation (free from maintenance, no stagnation or overheating)
- Significant lower cost – reduce payback time to 3-6 years.
- Aesthetic design
Sunnovations' system complies with all above parameters!
How does it work?
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