COMMERCIAL SOLAR HOT WATER HEATING
HOT WATER HEATING IS COLD WATER HEATING
Almost half the energy we humans consume heating our hot water is applied to water temperatures lower than swimming pool temperatures. The water coming into your facility is cold. Your boilers heat it up to hot water temperature. Solar hot water heaters pre-heat the hot water tanks. If we can raise the incoming cold water temperature from 10C to 35C (55F to 97F) we've done half the work that we're asking the boiler to do. This is the low hanging fruit for solar energy because we can tackle this half of the job cost effectively with inexpensive solar collectors and inexpensive storage.
THE HISTORY OF SOLAR THERMAL
The solar thermal industry has been around longer that the natural gas water heating industry. Cavemen were warming their water with the sun long before anyone thought of burning fossil fuels. We've diligently developed high tech solar collecting panels that perform well at extreme high temperatures, insulated ASME rated steel tanks, double walled plate and frame heat exchangers, computer simulations to support this futuristic endeavor and we have learned from our mistakes of history, slowly but surely.
Evacuated tube solar panel concept- the most advanced solar thermal collection technology
Boxed and glazed or "flat plate" solar panel concept- old school solar thermal collection technology that is still competitive with evacuated tubes.
The solar collector technologies above represent billions of dollars in investment by forward thinking entrepreneurs, government, and advocates of a clean energy future but there is one little Achilles Heel. Its no secret that all too often, solar thermal technology has a tough time competing with fossil fuels where cost is only the market cost of taking them out of the earth and burning them.
SOLAR THERMAL TECHNOLOGY
Hot Sun's founder, Ken Wright, was a pragmatist from the beginning. He began in the solar industry in 1986 and by 1990 he had realized that he couldn't compete with low cost fossil fuels in Canada so he adapted. He moved to San Diego! He also did nothing but solar pool heating right up until the year 2010.
THE MINORU AQUATIC CENTER
Click here for a brochure on this unique solar thermal system.
In 2010, it all came full circle for Hot Sun. Finally someone in a position of influence saw the obvious. Swimming pool heating solar technology needed to be used to heat commercial hot water. Against low cost natural gas there was no way, even with the heaviest water usage imaginable, that any solar thermal technology but unglazed solar collectors could provide anything close to a payback period anyone would consider viable. The Minoru Aquatic Center in Richmond BC consists of two large indoor municipal pools and 16 shower heads. The City was smart about this. From 2008 to 2010 the Canadian federal government had a significant rebate program available for commercial solar thermal but that wasn't enough to get this city jumping on the solar bandwagon without due diligence. An engineer with an understanding of the laws of nature came to the conclusion that pool heating solar panels belong on this roof space. As a result, Hot Sun was able to be part of demonstrating commercial solar hot water heating at this facility, solar hot water that throws the towel in on the expensive technology of the future and presents what can be done today cost effectively against the lowest cost fossil fuel in the world in a place known for rain.
The basic performance curves for the various solar thermal collector technologies are below. You have to realize that half the load we're working on when heating hot water falls well to the left of 0.03 on the x-axis. Heating hot water is heating cold water and cold water can be heated with a simple black plastic heat exchanger with the sun and sometimes just the air. Even at night we can be gaining energy because the air is warmer than the water that we're heating! The Minoru Aquatic Center solar hot water heater is monitored so we've learn how to specify these systems and predict their performance with certainty. This could develop credibility and sustainability for an industry that desperately needs to wean itself off government grants but the solar thermal industry in Canada is dead and what killed it was the bad economics associated with the tried and proven untrue technologies of yesteryear. We're really starting over from scratch with our cost based approach.
THE BASIC SYSTEM DESIGN
200 square meters of Powerstrip solar collector are mounted to the south facing flat tile roof of the Centennial Pool (one of two indoor pools at the facility). The roof is at a 45 degree angle. Its 3 banks in parallel. We won't discuss the specific plumbing and flow design of the collector bank as this is nothing new. We've learned how to do this kind of thing over the years in solar pool heating. Solar pool heating is a well developed technology in terms of piping to and from unglazed solar panels and back to the pool. The piping is all cpvc but this wasn't necessary. We didn't have a pool of these systems to refer to when getting approvals from all the parties involved.
The above is the data collected by our swim pc operating on Oct 5, 2012. Click the graph for access to the monitored site. There you can look at any day we have data for.
Our preference has always been to use the "drainback" concept. All the piping and the solar panel headers slope downhill back to the tank. Water flows downhill. That's not a difficult plumbing challenge. When solar is available and the tank isn't already at the setpoint of 140F (60C) the solar pump activates and pumps water to the top of the solar panels where it falls back down to the tank. Its an open system. The pump shuts off, the water drains down by gravity. Its pretty reliable. If there is a power failure, the system drains.
A roto-moulded and spray foam insulated tank is the storage. A heat exchanger is used between the tank and the cold water on its way to the hot water tanks. Flow goes from tank to pump to heat exchanger to three way valve. The motorized three way valve either sends the flow to the tank or to the solar panels. Return from solar panels goes to the tank. The pump is controlled by a variable speed drive so that the flow can be maintained when subjecting the pump to the extra height of the solar panels. Pumping thru the heat exchanger before water goes to the solar panels takes more pumping power but it puts colder water into the solar panels and unglazed panels like colder water. Unglazed solar panels are used because the cold water we're heating is cold water and the load is high so the tank will never get too warm and even if it does in summer the bang for the buck is still much better than expensive glazed collectors.
The above is a flat roof system that would be winterized. A minor loss of power only in the weakest solar time of the year. It is a common misconception to think that we need to focus on collecting solar energy when it is least available and most difficult to collect efficiently (in winter).
The temperatures a boxed and glazed or evacuated tube collector can stagnate at are high enough to melt solder. Back in the days of 50/50 lead solder, connections between collectors would fail and we had to specify special lead free solder for solar panels. That kind of high temperature is well past the limit for glycol inhibitors. There are some high temperature glycol breakdown inhibitors now but they are expensive. No glycol inhibitor can handle the extreme stagnation temperatures that high temperature solar panels can stangnate at. Instead the idea with evacuated tubes is check your glycol and replace if necessary and if the system freezes up and is destroyed its past the one year warranty. With boxed and glazed collectors the standard Canadian technology is to use the drainback concept but to also use glycol as a back up safety measure. At Hot Sun we don't understand the need to insure against gravity. We're pretty confident gravity will always be there for us. Our best commercial systems dealer, Renew Energy, is also a major proponent of drainback solar with water instead of glycol. Renew has installed many systems this way and Renew is one of the few companies in canada still surviving in solar thermal. We don't like glycol. Its breaks down and becomes toxic if its overheated and when it fails it doesn't protect against freezing. There are a lot of dollars on some of these systems entirely dependant on glycol. For solar thermal to be viable in hot water applications it has to survive long enough to pay for itself.
Unglazed systems are self limiting in temperature so glycols can operate within their specified limits but we don't want to use 165 gallons of antifreeze in this system anyway. The heat transfer fluid we want to use is water. It can be changed out easily without expert help. If there is a leak its not a toxic spill emergency.
INSULATION AND PLUMBING
Solar hot water systems designed to work on the entire load must be insulated. There's no point using a more expensive high temperature efficient solar panel and then losing the heat through uninsulated piping. Due to the high temperatures especially in stagnation, piping must be copper meaning greater heat transfer to the air potential. Copper piping is very expensive. Proper insulation is very expensive. Cheap foam insulation simply does not hold up outdoors or to the high temperatures. Proper foam insulation and a full metal jacket is required and this isn't easy to do outdoors. It adds enormously to the costs.
Unglazed collectors are unglazed. No insulation because we aren't going for the high temperature end of the spectrum. We're happy with the low temperature half of the load, the half we can displace cost effectively. That means there is no need to insulate the piping either. The piping itself can be regular PVC on the feed side. We do like to employ cpvc sometimes on the return side at elevations equivalent to the collector. This makes sure nothing sags when the high heat of stagnation transfers to the upper piping. Cpvc is pricey but we don't need much and it goes together as fast as PVC. Note that all piping on a solar heater will undergo a large temperature swing. Its very important to allow the piping to move with temperature.
Look at the slip collars we always employ on any long run of piping on any solar heater. How can you do that if everything is insulated and clad with aluminum? All these little details add up to the difficulties the solar thermal industry has endured over the decades.
TANKSWhich do you think makes more sense? A low pressure vented roto-moulded polyethylene 5000 gallon tank worth about $3500 or a high pressure vessel or series of smaller tanks totalling the same 5000 gallons of storage worth $100,000? Its the same volume of storage. The low cost storage fits well with our water based draindown preference. Even if you use evacuated tubes it still makes sense to use water as the heat transfer fluid. Don't pressurize it, limit the tank temperature to 140F (60C) and save tens of thousands of dollars on storage costs. Cost matters no matter what when it comes to anything. The industry's answer to this up to now was to go light on the storage. Storage is really important.
Why are we seeing so many solar water heating designs that employ expensive ASME certified pressure vessels that are 20 times the cost per volume? Is it wrong to use tanks that are 20 times more expensive? Yes it is and only because of the cost. Cost matters. A relatively small amount of prewssurized storage is useful depending on the load profile. It can save on heat exchanger sizing enough to justify its cost. We're just beginning now to learn to optimize these designs. Up until now its been impossible to fine tune the design of these systems because the whole industry was based on a lie (the lie that solar thermal is a 3 or 5 payback).
Double walled heat exchangers are a code requirement in some areas. Ironically in the most solar prevalent regions like Southern California and Florida where probably 20-50% of all solar water heating is done, there are no double walled heat exchanger requirements. Shell and tube heat exchangers are less expensive and a lot thicker and less likely to rupture which is ironic. Each case is different. Unfortunately heat exchanger engineering is a bit mysterious. Heat exchanger salesmen will give you a recommendation for any given operating point you specify. The problem is that especially in a solar heating system, the duty varies all over the map. Computer simulations can not handle this. Abbreviated compromises like assuming the heat exchanger effectiveness is constant over the entire operating range are common in all computer simulations so you can't use them to size a heat exchanger. This heat exchanger is obviously the heat exchanger between the tank and the load. It makes sense to solar heat the storage water directly and let the solar water drain back to the tank by gravity when solar is off...obviously. Its obvious because exchanging the stored heat with the load happens over the entire day whereas exchanging the solar heat from the collectors with the tank can only happen over the shorter actual solar day so it makes sense to exchange the stored heat with the load not with the collectors. This also separates the low pressure and high pressure where we want. This design makes sense. The pieces all fit.
In our design concept laid out here, we have to pump up to the roof height and we want high flow for good collector efficiency. If we pump slow the average collector temperature is higher and therefore we operate less efficiently. Note this is the solar pump we're talking about. It pumps the water to the solar panels and back to the tank whenever the sun is strong enough and the tank isn't fully hot already at the setpoint of 140F (60C). Closed loop systems allow a lower powered pump as mentioned above so we should put some numbers to this at this point. If we run a 3HP pump full out on a 2000 sq ft system with a pumping head of 55 feet we use about 2000W of electricity. In full sun of 1000W/m2 that solar heater could deliver as much as 200,000W. Typically we only operate at half a full sun and electricity is more expensive than heating fuel so this ratio can be as bad as 20:1. Pumping electricity costs can be 5% of heat energy delivered costs but this is still a small price to pay for the advantages this more straightforward design affords. Every time we've evaluated this we've always come to the same conclusion. Glycol closed loop isn't worth the small energy saving. In the case of the more expensive collector types this calculation is different because the economics are so far out there anyway that the economics of a lower powered pump stack up comparatively. Note that we could create a closed loop without going to glycol. We just have to drain down when freeze conditions occur and refill with pressurized city water. The only issue is failsafing a closed loop in the event of a power outage. That is why closed loops usually mean glycol and we really don't like glycol. We have other tricks up our sleeve where we can reduce that electricity demand and we'll let you know once we've had a chance to demonstrate them. Nobody is giving us free money because our technology is already viable. It doesn't need a government grant. We have to finance our own R and D by finding progressive customers who will put their faith in us or we have to find engineering firms willing to step outside their comfort zones.
SOLAR PANELSUnglazed or low pressure evacuated tubes or both. We are an unglazed solar panel manufacturer with a vested interest in our own technology but we love evacuated tubes. We don't sell many of them but we see them fitting into the mix moving forward. Look at the curves above. Look at the fact evacuated tubes hold their 50% efficiency right up way beyond what the chart shows. You can operate them at 60C and run them thru a much smaller heat exchanger than we need for unglazed. The low pressure tubes can use water as the fluid and they can empty when off. We can limit the temperature same as we do with unglazed and with our sophisticated solar controllers and intelligent plumbing design we can make sure high stagnation heat doesn't migrate into the plastic tank. We do have to plumb with copper and insulating it is still a problem. Evacuated tubes are expensive and fragile to work with and we don't know of any made in the US or Canada but let's be fair. They belong in the mix and we should always consider them for our customers whenever the discussion isn't strictly economics. Sometimes the green environmental statement is more important than the green money issue. Its really important to understand that if you want to use evacuated tubes on any commercial scale project, you must still use unglazed panels for the first portion of the load. It makes no sense using high temperature conventional collectors to heat cold water. Pick the low hanging fruit first and if you want to invest more money displacing more of the load by aiming at the higher temperatures (if you have the roof or ground space) then add evacuated tubes or possibly boxed and glazed but do so with your eyes open to the fact that the marginal economics may not be palatable. Let's prove it. Let's find out. Let's independently monitor these systems. There are hundreds of them all over the countries and they are all touted as demonstrations. They are all done with environmental pride. Let's see what they are doing and let's learn what the realities are. Its hell trying to figure it out from a computer simulation.
Hot Sun Industries Inc., San Diego, California. Email:firstname.lastname@example.org