“To him whose elastic and vigorous thought keeps pace with the sun, the day is a perpetual morning.”
- Henry David Thoreau
Over our many years making solar hot water and solar heating systems, we’ve arrived at our own preferred solutions, coming from a particular perspective. This perspective has included a focus on simplicity, ease of manufacture and handling, the ultimate need for mass adoption of solar technology, and scalability. We’ve found our own methods and preferences, and always have kept the system efficiency (not the panel efficiency) paramount. We’ve also noticed the “devil is in the detail” applies to solar if it applies anywhere. This page is intended as an introduction to some of our work, and some of the concepts we find important, and often overlooked, in solar thermal systems.
Solar Basics
Solar energy basics
The sun shines 24/7 and provides plenty of free energy available for us to harness. When it shines on you, know that its warm rays could be heating your house and your hot water. Note that we are discussing solar hot water, or solar thermal systems. While solar photovoltaic (PV or solar electric) has improved over the years, it still cannot match the efficiency or savings of heating water with the sun. Solar heat and hot water is a better investment than solar electric and affords us the greatest opportunity to reduce both our household energy use as well as our national energy requirements. Solar energy use reduces greenhouse gas production and represents responsible stewardship of our natural resources and the Earth.
Nuclear energy
If heating water doesn’t seem as interesting or as sexy as getting electricity direct from the sun, consider this: All nuclear power plants that produce electricity use nuclear power reactors for none other than heating water, so its steam can turn a turbine and produce electricity. And they operate, in round numbers, at a system efficiency of 33% at best while polluting our biosphere, producing highly toxic waste.Nuclear energy with cosmic containment
The sun shines on a solar collector, which absorbs the rays, gets hot, and transfers the heat to water that is circulated back to a solar tank. Presto! Hot water for dishwashing, showering, and heating the house! No waste products, and a system efficiency that can range typically from 30% almost up to 100%. Photovoltaic systems can produce electricity at a solar power conversion rate up to 15% at best. This high grade power should never be used for heating or hot water, unless it is strictly excess.Larger arrays collect more heat
The rate at which you collect energy is directly related to the array size. A larger array will collect energy faster than a smaller array. The most effective array
Type of Thermal System
We are concerned mostly with hot water and space heating. Direct heating of potable water in non freezing climates can use direct heating methods such as batch heaters, but we will not consider them here.
Drainback and pressurized systems
One important distinction of a solar system is whether it’s a drainback or pressurized system. In a drainback system, gravity causes the collection fluid to drain back to the solar tank when the pump is not operating. The fluid can be just water or a mixture of water and antifreeze. In a pressurized system, all parts are under pressure at all times, and there is nowhere for the collection fluid to drain back to. In this case, the collection fluid is a mix of water and antifreeze to prevent freezing, which would occur on cold nights. However, antifreeze won’t prevent the collectors from overheating when the tank has already reached a maximum safe temperature limit.Choosing a system type
The decision is simple: always choose drainback unless the building configuration prevents it. We make solar tanks for both types of systems but highly recommend drainback. While both types of systems protect from freezing, only the drainback system protects from overheating. While small pressurized systems can use "heat dumps" we consider them a poor design solution. More importantly, overheat protection is essential in larger systems used for space heating. Drainback systems are simpler, safer, and more foolproof. Even in the case where solar panels are installed at ground level, a drainback system can often be installed with proper planning and "lowboy" tanks.
Solar Heat Storage
Tank size matters
This is a very important point, and is less obvious than the array size. The amount of energy you can collect and use is directly related to the size of the solar tank. If you double the tank size, you double the storage size and you collect energy more efficiently. Larger tanks operate more efficiently because they they can collect more energy at lower temperatures. Large tanks will store more heat, giving you more time to shower at night (and extend use over cloudy days). With space heating in particular, a large tank is very important to store enough heat to supply the system once the sun goes down.The array and tank must be sized to match each other
The array and tank must be sized together according to the need. A large array will heat up a small tank quickly. A small array will take longer to heat up a large storage tank to usable temperatures. Generally speaking, larger is better: the amount of energy collected is directly proportional to the size of the array, but also proportional to the tank size. The amount of energy stored for later use is directly related to the tank size. In a residence, storing hot water for nighttime and early morning use is essential, but a car wash or laundry needs a large array for quick recharging and little storage in comparison. While the panel and tank size matter, correct heat exchanger location and sizing is equally important.How to value a solar tank
The most important characteristics of a solar tank are its size, longevity, and insulation level. If it’s a conventional glass or stone lined steel tank, there is a good chance you’ll have to replace it within 8 years. Consider this: we’ve made unpressurized plastic solar tanks for hundreds of solar systems, and we’ve had no known tank failures in 28 years! The Earthstar tank has proven itself, whether built for a pressurized system or a drainback. Also, our ease of handling for shipping and installation means a quick job onsite and no concealed damage.Life of a solar tank
Historically, the largest source of failures in standard pressurized systems has been tank leaks. The higher water temperatures of a solar system are more corrosive than the lower water temperatures of electric and gas heaters and corrosion is the main cause of tank leaks. In a closed, pressurized system, a pressure relief valve must open to prevent boiling if the system overheats. The system then requires an expensive service call. A faulty relief valve can make the system inoperative or become a safety issue. A drainback system avoids these problems by allowing the water in the panels to drain back once the tank reaches its maximum temperature.The limitations of commodity solar tanks
Almost all system brands compromise the efficiency and affordability of solar by using hot water industry standard tanks that are designed and made for fossil fuels with rapid recovery capability. Standard sizes of 40, 65, 80, or 120 gallons are simply too small and offer too little design variation for solar storage! Only Earthstar offers a wide range and larger sizes, using unpressurized plastics, that have proved themselves for almost 30 years. Standard sizes are 50, 100, 150, 200, 300, 500 gallons and up, in vertical and horizontal configurations. Now that's design flexibility!The importance of the heat exchanger
The solar array must transfer heat to the domestic water through a heat exchanger. An improperly sized heat exchanger can greatly limit the capacity of the array to deliver heat. Undersized exchangers drive up the collector loop temperature, reducing collection efficiency. Tanks with built in heat exchangers allow no flexibility in sizing the exchanger, and the exchanger must be discarded when the tank fails. The Earthstar solution is to move the heat exchanger from the collection loop side to the load or hot water distribution side. This increases both efficiency and design flexibility, and the ability to add on panel area in the future without penalty.Tap water is better than antifreeze mix
Water stores and transfers more energy per gallon than antifreeze mix, takes less energy to pump, and costs nothing. At solar panel stagnation temperatures, antifreeze breaks down, becomes acidic and can corrode equipment to the point of failure. A pressurized system requires antifreeze, and in pressurized systems there is no way to remove the fluid from the panel when the panel overheats except by opening a relief valve and releasing scalding fluid or steam. A drainback system filled with tap water offers a simpler, more reliable system with the highest efficiencies at the least cost.
Benefits of an Earthstar Drainback System
• Intrinsic overheat and freeze protection
• No commissioning requiring pressure charging
• Uses tap water, no antifreeze required
• No system failure resulting from relief valve operation
• Unlimited storage tank capacity
• Reduced cost
• Fully expandable- adding more panels or tanks doesn’t require heat exchanger resizing
• No tank corrosion failures
• High efficiencies using flat plate panels
• No known failures of our drainback tanks.
© 2009 EarthStar Energy Systems, Inc.
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