nkaskrod-admin – Solar, radiant, and all that is sunny

Is your drainback tank sight glass cracked? Here’s the solution

Here’s a service tip for those whose systems use an HTP drainback tank – specifically on the maintenance or replacement of the sight glass. The sight glass is the transparent tube that lets you know the water level of the drainback tank when both the pump is pumping and when the system’s drained down.

This is the pex repair on a system’s drainback tank, in which the glass has been replaced with a piece of ½-inch pex tubing that is not opaque. It’s difficult to see, but the water mark is at the half mark.

I received a call last week from a man who installed his solar system about two years ago. He’d noticed that a slight crack had formed in the sight glass, just below the water level. And as time progressed, the sight glass started to drip. He shut down his system and removed the sight glass, but was unable to repair it. So he ordered another one. Meanwhile, the system’s not working until the replacement arrives.

I suggested a repair – which can be either permanent or temporary – that has worked on my system: Replace the glass with a piece of ½-inch pex tubing that is not opaque.

The pex fits perfectly into the same connections as the factory sight glass. Pex has a temperature rating of over 400 degrees with no pressure, so it’s an excellent, industrial material for this application. I used Wirsbo pex for the 10-gallon drainback on my system. The length you’ll need depends on the size of the tank, but my tank required about 11 inches.

Earlier HTP drainback models seemed to have some flexing in the stainless steel tank and occasionally would break the sight glass. Also, sometime after years of exposure to heat, the sight glass simply needs to be replaced

Filling the pressure: Sizing an expansion tank correctly

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Before you get to the exciting part, you’ll need a few basic details about your system.

Here are some good questions about the correct sizing for an expansion tank from a for the conscientious installer — which I know includes all of us.

Question 1: Fill pressure vs. expansion tank size? (Or in other words, what should the loop pressure be and how much pressure should be in the expansion tank?)
Question 2: Expansion tank size vs. system fluid volume/ number of collectors? (Or, how to size the expansion tank to your solar system, including collectors, length of pipe, etc.)
Question 3: Fill pressure vs. PRVs? (How to size the pressure relief valve.)
Question 4: Fill pressure vs. head? (Does head or distance to the panel affect fill pressure?)

If you can visualize the concept of a correctly installed pressure glycol system, the answers will become more evident.

The goal is to keep the expansion tank from blowing off when the power fails on a super-hot day. To do this, the system needs to be designed along the lines of a drain-back system. (Sometimes it’s called a steam-back system.) This is the only sure way to design this to make it work correctly.

Let me explain, and your other questions will start to become clear once you understand how it works.

All plumbing coming out of the collector array MUST drop down several feet to form a thermal lock or trap for the steam in the collectors. The liquid/steam must not migrate at all. Water expands 1,130 times when it flashes. If in any way it can migrate and flow out of the collectors, it’s not possible to have a large enough expansion tank.

With this understanding, you can now start making the calculations you need. For example, a single 4’x10’ collector holds 1.3 gallons of water. So, if you have a string of eight 4’x10’ collectors, the water total is 10.4 gallons. This should have 1″ pipe (copper handbook). Never oversize the plumbing or you’ll lose control.

As the array heats and starts to flash, it will start to push the fluid back and DOWN the plumbing. Once it settles, the collectors now contain 10.4 gallons of steam volume, and about 10.4 gallons have been pushed down the plumbing into the expansion tank. Calculate expansion for the liquid in the plumbing (maybe 0 to 200°), and you now know what size the expansion tank needs to be. Or, I have an Excel spreadsheet I created years ago that will help you with the calculations. (Go to the “Sizing Tanks, Panels and WH” tab.)

Most systems have between 20 to 40 psi. Remember, every 100 feet of water has 44.4 psi. Add this to whatever you start with. Oh, and make sure you size the PRV with a good margin above set pressure.

The expansion tank should have a cold unloaded pressure set before you start to fill that is close to the psi your system will be set at. You cannot change or check the pressure on an expansion tank after it has been loaded because the reading won’t be accurate.

You might get some other suggestions for setting the pressure. I know Velux and Heliodyne have some interesting guidelines. Then too, I’m not a pro on pressure glycol systems. We do drain-back every chance we can. But, European collectors generally can’t do drain-back when they have a serpentine riser inside.

Good luck — or, do it correctly and you won’t need any luck!

Incoming water temp can determine size of collectors, storage

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As you get ready to design a solar thermal system for your home, there’s one factor you may not be considering that could make a bucket load of difference: What’s the beginning temperature of the water you’ll be heating?
In the United States’ northern climes, it’s likely 50°. In warmer, more southern climates, you’re likely beginning with 70° water.
And common sense tells us, the colder the water you start with, the more solar uumph you’ll need.
I’m asked all the time how to design a residential system. And, as I worked on the attached chart, I realized it pretty much tells you the basics of what you need to know – the size of the storage tank and the number of collectors.
This attached chart will give you an idea of what to expect. The chart allows you to determine if, for instance, you’ll be heating an 80-gallon tank in a system sized for up to three people, or a 119-gallon tank sized to serve three to five people. I chose these sizes because they are averages for family-sized systems. These numbers can be easily customized.
So, to read the chart: Say you’ve decided on an 80-gallon tank and you live in a colder area, then you’d chose line 1. And, reading across, it will tell you the number of solar collectors you’d need based on the sizes of the collectors you’d like.
Continuing this example, you decide that your home in Michigan has a large roof area and you’d like the largest collector — the 4’x10’ collector. Based on this chart you’d need two of those large collectors — or 80 square feet of collector real estate — to heat that 80-gallon storage tank.
As someone who lives in a northern climate, I shake my head sometimes knowing that we colder people all need a little extra uumph. But I know that we snow people – I live in northern Utah — will enjoy our hot water all the more.
Just as a side note, the input water temperature also has a bearing on how steep should you tip your collectors, believe it or not. If you live in 70 degree, you’ll living in a 30 to 40° latitude, so you’d use a tilt or angle of 30 to 40° for optimal performance. Likewise, those in the colder climes with colder water probably live in a 40 to50° latitude, therefore using a 40 to 50° tilt or angle.
To download an Excel file of the chart, click here.

Designing a medium-sized commercial solar thermal system for apartment complex

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Other posts have discussed light commercial systems for restaurants and other similar businesses. But what if you need a solar thermal system with a bit more uumph? Here’s the recipe for a solar hot water system that would be perfect for, say, apartment buildings or hotels. It provides 330 gallons of hot water per hour, with significant backup strength.

The example I’ll discuss below is for a two-building apartment complex in Los Angeles, in which the solar thermal is targeted to provide 50 percent of hot water demand.

This size of solar thermal system — six solar collectors backed up by a Solar Phoenix modulating water heater — also works great for large restaurants or smaller hotels. To illustrate this, take a look at this case study of a solar thermal system on Catawba College in Salisbury, NC. Cost analysis indicates that energy costs at the school were cut by 58 percent. Now that’s a success story.

Here are the details on the new, low-income housing project in LA that needs a cost-effective solar thermal system:

Building A has a hot water demand of about 500 gpd. It’s made up of 16 two- and three-bedroom units; a laundry room with four washers; a community kitchen; and a staff restroom.
Building B has a hot water demand of 800 gpd. It includes 25 one-bedroom units; three washing machines; and one mop sink.

The solar thermal system I designed for this apartment complex features a 199-BTU Solar Phoenix, a highly efficient water heater that stores nearly 120 gallons of solar-heated water and kicks in as backup with more water in needed than can be heated by the sun. It’s a drainback system to guard against overheating. Also, I’ve upped the solar water storage with the addition of a second 119-gallon tank. While a light commercial system may require three to four solar collectors, for this application, I’ve recommended six 4’x’8’ collectors.

Here’s the parts list:

1 ea. Solar Phoenix, PH199-119SNHX, stainless-steel, modulating, condensing water heater with solar input
1 ea. SSU-119 stainless-steel 119-gallon indirect tank
1 ea. SSU-20DB stainless-steel drainback tank for overheat protection
1 ea. Variable speed solar pump control with 4 sensors (8600-047)
1 ea. Solar-rated anti-scalding valve (8600-068)
6 ea. 4’x8’ flat plate solar collectors (FP-32SC)
6 ea. FP-RM mounts (or select mounts for roof application)
1 ea. Field-supplied pump that will supply 7 GPM and the lift from the drainback tank to the collectors
1 ea. Field-supplied tank-to-tank transfer pump 1/2″ (I recommend a Taco 006B)
1″ copper and insulation to the collectors

The control strategy uses a tank-to-tank transfer pump that will transfer the solar heat to the water heater, but will not allow the water heater to heat the storage tank. This is not a solar tank preheating a water heater; rather this is solar heating 240 gallons of storage and the water heater only firing to make up what the solar will not produce.

Data logging that allows solar production to be posted on the internet is available if the apartment building supervisors have an interest in seeing how well the solar system is performing.

In colder climates, I would recommend increasing the number of collectors to eight collectors or upgrading the size of the collectors to 4’x10’ panels.

This design for a light commercial thermal system is effective, simplified

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I’m asked, more and more, to provide a solar hot water system design for light commercial systems. There is, indeed, an effective and simple design template for smaller commercial systems that need to heat up to 330 gallons of water per hour or less. The actual system this template is based on is for a fire station; the solar is designed to cover 80% to 100% of the load in a southern climate.

However, this category of light commercial could also include restaurants and cafes, hair salons and spas, grocery stores, laundromats and car washes. Some of these businesses may require even larger systems, which we’ll discuss in an upcoming blog post.

Why am I recommending this template? Because it’s the most efficient in its design, and it offers greatest simplicity in installation. It has freeze and overheat protection. Also, it requires little or no maintenance.

The virtue of this system is that solar directly heats the water to be used. Too often I’m sent designs that preheat water for some conventional water heater that can’t be heated with the solar directly.

This template includes three 4’x8′ flat-plate solar collectors. The number of panels may be adjusted according to the climate of the location. On that fire station I mentioned, because it’s in Texas, a hotter climate, fewer panels are required. If I were to spec out a similar system for a northern location, I’d possibly use one more panel or use larger panels.

This system also allows data logging with web monitoring so that its performance can be checked at any time online.

Following is the components list for this system:

1 ea. Solar Phoenix, 199,000 Btu and 119 gallons (PH199-119S) stainless-steel modulating condensing water heater with solar input. Will produce continuously 335 GPH at 100° rise. If this is more than needed, use the PH130-119S
1 ea. SSU-10DB stainless steel drainback tank for overheat protection (10 gallons)
1 ea. Variable speed solar pump control with 4 sensors (8600-047)
1 ea. Solar rated anti-scalding valve (8600-068)
3 ea. 4’x8′ flat plate collectors (FP-32SC)
3 ea. FP-RM mounts (or select mounts for roof application)
1 ea. Field-supplied pump that will supply 4 GPM and the lift from the DB tank to the panels
1″ copper and insulation to the panels

The photos below show a system using this design at Corry Station Bldg 3782 (a GSB support building) at Florida’s Eglin Air Base. The contractor is McDonald Construction of Fort Walton Beach, Fla., and they really know what they are doing (850-862-2151). The engineer was Jimmie Johnson of Johnson-Peaden Engineers, also in Fort Walton Beach. Manufacturers’ rep is Coleman-Russell.

Above shows the system at Eglin Air Base, which uses a Solar Phoenix in a simple elegant design. It includes four 4’x8′ collectors.

Template below for a drainback solar hot water system for light commercial, with three flat-plate collectors and a Solar Phoenix

Drainback vs. pressure: A question for the ages

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Pressure or drainback?

That’s probably the question I get asked more than any other: Should I choose the more popular pressure-glycol system, or go with a drainback system?

Here’s the answer: It depends.

Don’t you hate that kind of answer? Well, here’s a more specific answer: If the slope of the collectors and plumbing is such that all the fluid will drain out, then go with the easier and less expensive drainback design.

That doesn’t mean I favor one over the other; it really does depend on the building’s design.

Let’s backtrack for a moment. Here are definitions of each type of system.

Solar pressure system: Closed loop arrangement that generally has 20 psi or more at the panels. Besides the panels and the tank, basic components include check valves, expansion tank, air purge and air traps, a pressure relief valve, pump, solar control with sensors and nontoxic antifreeze.

Drainback system: Closed loop arrangement that generally has 10 psi. Besides the panels and the tank, basic components include drainback tank, a pressure relief valve, pump and a solar control with sensors. May or may not contain antifreeze.

The virtues of each system type:

Solar pressure system: Pressure systems generally contain glycol to provide freeze protection.
Drainback system: A properly designed closed-loop drainback system provides freeze protection without glycol as well as overheat protection due to the absence of fluid in the panels when either condition exists.

Let’s start with the more popular pressure glycol-filled system:

Pressure systems take time and patience to fill, charge and purge all the air out. If some air remains, controlling the flash point may be impossible.

A pressure system must be in perfect balance to handle the expansion of fluid when the pump isn’t pumping — such as a power failure or when the tank is satisfied. You must have a properly sized expansion tank that is kept at a consistent parallel pressure with the glycol loop pressure. Will this pressure stay exactly the same forever? Nope, every year you need to check and recharge the system.

And what happens if the closed-loop system pressure drops a few pounds below where you set the expansion tank pressure? The system will eventually blow antifreeze all over — via the pressure relief valve. The reason is that water-to-steam expansion is immense — 1,172% to be exact. When the pump shuts off during, say, a hot summer day, the fluid in the panels will go to boiling point and flash to steam if it is not in perfect balance.

Let’s say that we have a collector that holds one gallon of water. If the system pressure to expansion tank is maintained in concert and the pump shuts off under full sun, the panels will build heat slowly and the fluid will start to phase-change. That one gallon of water will start to phase-change and expand; it will push fluid out of the collector and down the pipes where it will stay condensed (as fluid). Eventually, you will have one gallon volume of steam in the collector and the gallon of water you pushed out will be in the expansion tank. If there is air in the system or the system pressure is not in harmony with the expansion tank, violent boiling will transfer down the plumbing, expanding several gallons of water by the aforementioned 1172%, out of control.

When the system is in balance, only a small amount of fluid will expand that immense amount, likely filling the collector’s one-gallon capacity.

Because of this, larger systems should use a constant pressure auto-fill device.

The closed-loop drainback system:

A drainback system is a fluid-filled closed loop with an air bubble and a small tank to isolate and capture the air when the pump comes on. When the pump is switched off, the heavier water drops from the panels and pushes the air bubble back up into the panels and exterior plumbing. If the tank is satisfied or the power fails, the pump will be off, and no water will be in the panels to flash to steam. No fluid to expand, no need for expansion tank. It’s the same for freezing — when the temperature at the panels drop below freezing, there is no water in them to freeze. Fewer safety measures need to be considered.

However, a pressure glycol system will need to be considered if the slope for panels and plumbing cannot be achieved to drain all the panels’ fluid into the drainback tank.

Advantages of using some pressure

I would recommend you add a small amount of pressure in a closed loop drainback system, for two reasons. First, it will confirm you have integrity in your closed loop. If you have a completely sealed system, the air will go inert and turn mostly to harmless nitrogen.

Second, if you fill and close up the system when it’s warm, then at night when the panels get cold, the closed loop will go into negative pressure and start boiling the stored fluid in the drainback tank when it is at room temperature.

Pump and pump sizing rules change considerably with a drainback system. Pump size will be determined by the lift required to deliver from the drainback tank to the top of the panels. After the pump has overcome the lift to the panels, it’s important to have the variable solar pump speed control slow the pump down.

A variable-speed pump control should be used on both pressure and drainback systems to keep solar heat transfer to a maximum based on Delta T. Over-pumping a solar loop may void the panels’ warranty. Consider raising the drainback tank as high as possible in the building (within conditioned space) to allow a smaller pump. The pump should be sized to deliver the GPM needed for the solar and the lift from the drainback tank to the panels.

Sizing home system based on temp of incoming water, with helpful chart

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In recent years, I published a chart that helps size a solar hot water system for your home, but after hearing from some people I realized I needed to make it more simple.

So, to back up and provide a bit more information:

This chart shows what system is right for you, using actual sizing logic instead of guessing. (Believe me, I see that more than you’d think.) Instead of a rule of thumb, it considers a southern or northern location in the U.S. to determine what the incoming water temperature might be. The amount of energy you’ll need is based on raising the water temperature from where it starts as it comes into your home. With a little math, we can easily determine how much solar will be needed to fill up the tank, based on the incoming temperature.

The chart lists four choices of collectors with differing measurements. For example, if the load requires two 10-foot collectors and your roof height is only 7-feet tall, you still have several choices to make them fit nicely on your south-facing roof. (Unless you live in Brazil, then please face them north.)

Chart for sizing solar based on temp of incoming water. Click on chart image for full-sized PDF.

Commercial solar: With tankless water heater, you don’t pay to heat it ’til you need it

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Here’s a new concept for commercial solar: a 250,000-Btu tankless water heater. It comes down to this simple philosophy: Don’t pay to heat ’til you need it. As you study this concept illustration, you’ll realize how simple this is. And, when you compare this setup to a conventional design, the cost will be half. back-up.

The Hydra Smart water allows for a gas or electric backup. In locations that have 300+ days of sunshine (all the southern U.S., for instance), I’d recommend this tankless with an electric backup. You can actually get real close to having 100% of the domestic water heating covered all by solar.

With the addition of the new Cocoon Tanks — large, super-insulated and designed to fit through any door — you can store several days of hot water. The simple drainback design lends perfect protection from freezing and overheating. Most importantly, the backup heater will rarely get used.

If you’re looking for a small domestic hot water system for your home or a client’s commercial job (laundromat, car wash or restaurant, for example) these new products and designs will dramatically cut the cost in half and make solar the big hit it should be.

Drainback with tankless is less costly, more efficient

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Today, as part of our ongoing series of basic solar designs, we look at a drainback system using an efficient solar storage tank and a tankless water heater as backup.

This design is fairly uncomplicated and is, perhaps, the least costly system for residential applications. For all its simplicity, it delivers freeze and overheat protection with water. It also features low heat loss.

The tank is the solar storage and the drainback tank combined. The hot water storage tank I’m using in the drawing is made of expanded polystyrene (EPS), which has an insulation factor of about R5 per inch. Dimensions of the tank are 60 inches in height and 30 inches outside diameter, with 4-inch walls. Usable Btu storage is 81 gallons, which when filled with solar energy up to 160 degrees equals about 67,000 Btus of energy.

The system has very few components — basically the storage tank, pump, controller and collectors. The tank contains a 50-foot 1″ HX coil. Potable water is drawn through the coil, picking up heat from the contents of the tank. The backup water heating is located after the coil exits the tank. A tankless gas or electric water heater works well as a backup, or this could be connected to any existing water heater and used as a preheat design.

This system is an excellent choice for the HTP Hydra Smart tankless water heater. That is the tankless water heater I’ve used in the design drawing. For more information on this advanced, modulating tankless, click here.

Like other drainback designs, all plumbing must be sloped from the panels for complete drainage.

Here’s more general information on how tankless water heaters work.

Condensing boilers and baseboards: A great matchup

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Do you need a more efficient (and expensive) modulating condensing boiler if you have baseboards?

That was the question posed in a discussion group I follow, and many contractors said, “No, a cheaper, conventional boiler is just fine.” The baseboards require high temps of 180° at all times, they said, and condensing boilers save fuel by heating at low temperatures. Plus, baseboards respond too slowly to the outside-reset feature of condensing boilers.

I respectfully disagree. Condensing boilers can result in significant energy savings when used with baseboards.

First off, there is a good part of the heating season where you don’t need those high temps – such as the “shoulder” months of October and March, for instance.

Secondly, systems using conventional boilers tend to be over-sized, and huff and puff more heat up the chimney than is used.

Consider this, a 30° reduction in water temperature will pencil out to a 10% fuel savings. In the shoulder months, you may run your baseboards at 130°, while in colder seasons, you may run the baseboards at 160°. So, running your baseboards at 130° would save you 18% over the high temps of a conventional boiler. When you consider that a condensing modulating boiler runs at 95% efficiency, while a conventional boiler is at 80% or so efficient, you’re adding that 18% savings on top of your already higher efficiency rating.

Thirdly, you absolutely need a good reset control. Contractors put reset controls on old-style set-temperature boilers all the time, but their effectiveness is limited. Condensing modulating boilers do it now. Some do a crummy job with poor control, and some are very sensitive and dead on for comfort at the lowest operating temperature possible (and energy savings).

I watch my new HTP Elite Plus with gas adaptive technology all the time. I can see exactly how my house is performing. I retrofitted with underfloor plates in the 1/3rd basement that I have and low-temp (Heating Edge) baseboards in the rest of the home. Not to get too carried away with my details, but I found that my basement stayed warm just from the staple-up for the upper floor.

As an experiment, later in the next heating season I installed low-temp baseboards in the basement. My condensing boiler adjusted the temp down about 8° overall compared with the previous year’s operating conditions. I have my program set from 110° to 140° with an 11° swing. It follows outside temp and sets the water temp to about 123° on average.

When a T stat comes on after being off for a while, the boiler control measures the Delta T in the zone in accord with the outside temperature. If it is below 16° outside and the zone fluid temp has a Delta T difference of 11°, the boiler will ramp to max setting of 140° until the Delta T narrows, which doesn’t take long. It does that so that the zone doesn’t have an under-swing because of the off/on setting of the T stat.

So, to recap, baseboards love modulating, condensing boilers – and we love the accompanying energy savings.

Flat plate vs. evacuated tube: Which is the winner?

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By Tom Scheel

Red vs. Blue. Tastes Great vs. Less Filling. Kirk vs. Picard. Epic battles for which there really are no right answers. Enter into that pantheon the great debate between flat plate vs. evacuated tube solar thermal collectors. The answer, much to the chagrin of the true believers on either side is: it depends. Let’s first briefly look at the two types of technology.

Flat plate solar thermal collector:

The flat plate collector has been around forever (at least since the 1890s). An insulated box, with glass on one side and copper tubing running through absorber plates is the basic design for flat plate collectors.

Evacuated tube solar thermal collector:

Instead of evacuated tube, think thermos bottle. Each tube is actually its own mini-collector. A clear thermos bottle allows sunlight in, an absorber plate absorbs more heat, and the vacuum holds the heat in. Most designs have a heat pipe – a small diameter, sealed copper pipe inside the thermos bottle. Inside the heat pipe water, antifreeze, alcohol or some other medium, under a mild vacuum, flashes to steam around 80F. So the liquid heats up, turns to steam, rises in the heat pipe to the heat exchanger (usually a larger section of the heat pipe dry fitted to the water-or-glycol carrying header), gives up its heat, turns to liquid, flows to the bottom and repeats the cycle.

Comparison of generic evacuated tube and flat panel collectors

	Evacuated tube*	Flat plate*
Dimensions	9’X7’X6”	4’X10’ X4”
Weight	250 pounds	150 pounds
Effective absorber area	~30 square feet	~38 square feet
Piping	Top only	Top and bottom
Max weight per heaviest piece	105	150
BTU output	Depends	Depends

*off-the-shelf, readily available models. The point is not a particular brand/model, but roughly comparable panels between the two styles.

Each type has a role to play, depending on where the project is, and what the heating needs are. If you are installing a solar water heater in Phoenix, the flat plate solution is the obvious choice. If you are installing a space heating system in Burlington,VT, evacuated tubes are the obvious choice. Almost anything in between requires some analysis.

To start the analysis, let’s look at price, performance and ease-of-installation.

Price

The advantage goes to flat plate collectors, but not by as much as is typically assumed. For example, general prices for the two panels above are: flat plate: about $1,100; evacuated tube about $1,400. How and if discounts are available from list prices is a whole other subject in the game of solar.

Performance

Regardless of price, how much energy can we get from a panel? Here we will use data from the SRCC OG-100 tables. Data for any rated panel can be found here: http://www.solar-rating.org/ratings/og100directories/OG100dirfull.pdf. Because panel performance varies greatly, you should check the specific performance of the panel you are considering.
The table (click to enlarge) is based on three different sunlight conditions: Full sun, partly cloudy and cloudy. At those three conditions four different temperatures were tested (the temperature is the difference between the outdoor temperature and the water you are heating at the inlet to the collector). Pool 1 is an estimate of pool heating requirements in a warm climate; Pool 2 is an estimate of pool heating requirements in a cool climate; the same applies to DHW 1 and DHW 2. Black plastic pool panels are recommended for seasonal pool heating.

Somewhere in row C the performance per panel begins to favor evacuated tube collectors. Note that this compares a ~55 square foot evacuated tube panel with a ~40 square foot flat panel. If you are minimizing the solar footprint on the roof, flat plates probably win (but you may not get the performance you want in the winter). This graph presents the same information in a slightly different way.

As you can see from the chart, somewhere between 70 and 80 degrees Fahrenheit delta T between your incoming water at the solar panel and the ambient air, the evacuated tube is more efficient. An example would be heating 120°F water when it is 40°F outside. The 80°F differential is right at the efficiency crossover. This is a typical case for overnight storage in a solar hot water heater. You want to heat the water to 150°F or higher so you can use that heat energy for night-time and early morning hot water demand.

However, if your ambient temperature is 50°F or 60°F then the flat plate is more efficient almost the whole time. The ambient temperature we are considering is the average from 9am to 3pm (the solar window).

Now let’s consider space heating – as a stand-in for winter, we will assume partly cloudy conditions (~800 watts/square meter)

Now the crossover point is about 60°F. If the outdoor temperature during the solar window is 40°F then heating water over 100°F benefits (at least slightly) from evacuated tube collectors.

Ease-of-installation

If you are a one man crew, the evacuated tubes are the hands down winner. Other than that, the flat plate wins. Flat plates are big, heavy and bulky. Full roof harness safety gear for a pitched roof and some sort of mechanical lift are required to get them on the roof safely. A 3 to 4 man crew can muscle them up by hand, but even with a full crew this is a challenging step in the process.

Both styles require some frame to attach to, and typically require a tilt-angle frame to lift them away from parallel to the roof. The panel or panel header/frame is set in place once the tilt-angle frames are built and installed. It will take a few people to get the flat panel positioned, whereas one person can maneuver the manifold and frame of an evacuated tube collector. While the flat panel is ready for piping, the evacuated tube panel still needs all its tubes put in place (the correct order is complete piping, and then put in tubes). It takes roughly 60 seconds per tube. If you are putting up a single 30-tube evacuated tube collector that is an additional half hour of time. If you are putting up a 10 collector array, that is an additional 5 hours.

Miscellaneous considerations

* The evacuated tube collector is more efficient at chasing a dwindling resource. So the gross BTUs the extra efficiency gets you are not as large – but they may come at the critical time (typically winter conditions).

* Windy conditions affect flat panels much more than they do evacuated tubes (due to the surface area of the un-insulated glass). If your project site is typically windy during the day, you might want to give evacuated tube collectors extra consideration.

* Evacuated tube collectors only require flow through the top (header). This can be very helpful when installing drainback designs (the most efficient freeze-proof design).

* Flat plate panels melt the snow due to their high losses through the un-insulated glass. Evacuated tube collectors are so well insulated they do not melt the snow. If the south facing roof at your location does not routinely melt the snow during a typical winter, flat plates are probably a better choice. The evacuated tubes seem to work well under light-to-medium frost. And they will collect energy when half covered with snow (about half as much as when fully exposed).

Summary

The chart at left (click to enlarge) highlights the advantages of each type of collector. As the issue can be sliced many ways – cost per square foot, cost per BTU, performance per square foot of collector, of absorber, of net aperture, etc., the debate can be never ending.

Tom Scheel is the owner of Radiance Heatingand Plumbing in Flagstaff, AZ. His website is http://www.radianceheating.com/. This article was originally published in Contractor magazine. Originally posted on askrod.com on March 12, 2012.

Drainback solar system at airport gains every drop of energy from sun

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Out with the old (over complicated, expensive solar). In with the new (better and more efficient solar at lower cost).

Case in point: the Minneapolis- St. Paul international Airport’s cutting-edge solar thermal system with a new innovative design approach.

Two decades ago, conventional solar called for installing a water heater or, for larger systems, a boiler that heated a tank. Later, solar morphed into separate, individual components that drew heated water into the water heater storage only if someone washed their hands. This design suffers from too much heat loss, and the solar can’t heat the water heaters to prevent them from firing. Preheating is now an outdated and inefficient design.

The new design we’re sharing today integrates the drainback tank and storage tank, reducing costs and components. At the Minneapolis-St.Paul airport, the engineering firm of Michaud, Cooley & Erickson did this with great finesse and accuracy. All 152 collectors charge the storage tank. Because the storage tank is a huge drainback tank, no energy is left to waste in a separate individual drainback vessel. That cost is eliminated too.

Conventional designs use a heat exchanger to transfer the solar BTUs into the storage devise. Any time you launder BTUs through a heat exchanger, you’ll pay an efficiency penalty. Store the pure energy until it is needed, and then use a heat exchanger to deliver it to the needed source.