Solar Evacuated Tube Domestic Hot Water System

Two days ago the pros (plumbers, electrician and tech support) finished the install of the re-designed evacuated tube system at Deep Portage and completed the initial programming of the controller. This is a 300 tube system, 10 manifolds of 30 tubes each, and is designed to provide domestic hot water (with LP hot water heater backup and supplement) to an environmental learning center that serves around 10,000 visitors per year in a 56,000 sq ft facility. The initial design and install had serious deficiencies based on the operating experience which resulted in shutting the system down last year. I did the new design based on experience with the deficient system and studying the literature, and I also advised the plumbers and electrician as they did their work.

I will post pictures shortly and provide performance data once the system is fully operational. There still is some controller programming to fine tune, but initial indications are that the system performance deficiencies have been corrected and the system should work as well as possible subject to some minor constraints which were unavoidable under the circumstances of the facility.

I've been slow in responding due to time spent monitoring the evacuated tube system to identify and document any issues as they arise and take immediate action if needed to avoid any adverse consequences while an appropriate control scheme or other remedy is implemented. The manufacture/supplier of the tubes and engineering/tech support, the manufacturer's representative, the plumber and the electrician all have been outstanding throughout the installation and continuing commissioning process of the system.

One major factor with an evacuated tube system is that once the tube manifolds and system are filled and pressurized with the heat carrying medium, water or glycol, it is very important to make sure as best as possible that no air lock will raise its ugly head once the system is put into operation -- easier said than done.

Second, I think good practice includes a means to manually turn on or off all critical pumps and motorized valves once tubes are installed and the system is first being tested. This allows an ability to quickly respond to any developing situation. On initial start-up any air lock, vapor lock, pump not operating, or controller issue can result in immediate consequences. The system needs to be immediately capable of accepting and handling the very hot water that is quickly produced. Temperatures in the manifolds can rise from 70F to over 300F in a matter of a few minutes if the hot fluid is not moved into the system as it is being produced.

A few pictures, and more info to follow.

Vent locations were pretty good. The main venting problem was in-line thermal valves installed a few inches from the hot water discharge side of the manifolds (under the insulation and not visible) which greatly restrict flow at temps under 155F. We pressurized the system before installing the tubes, as they only make thermal contact with the manifold, no fluid contact. It just took a long time to work the air out the system. And since this was a re-do (original contractor not in the picture), the "little" detail involving the thermal valves came to light only after frustrating efforts attempting to achieve a flow rate that would expel air.

These were five major issues the redesign is intended to solve. The first issue: system delivery of up to 180F+ hot water directly into the domestic hot water supply and inability of the mixing valve to rapidly react to deliver water not exceeding 130F to domestic usage.

The initial design directly used domestic water rather than glycol in the evacuated tubes, and it supplied hot water from the tubes directly into the plumbing from the hot water storage tank (1000 gal) to the domestic supply. If demand for hot water, this resulted in surges of very hot water into the domestic supply. If no demand for hot water, the very hot solar water flowed into top of storage.

The redesign employs a glycol/heat exchanger design. Hot water from the HX now is supplied to the storage tank at a mid tank injection point on the opposite end of the tank from the hot water supply to the domestic system. This is intended to temper the hot water as it moves through the tank and obtain a more even supply of hot water to the domestic system, which also should improve the ability of the mixing valve to further temper hot water to the 130F maximum supply temperature.

At this early point it is not known how well the objective will be met. Some experimenting likely will be needed in setting maximum top of tank water temperature; also possibly some further adjusting of the mixing valve.

The water input range of the mixing valve is 110-140F. The controller has a sensor at the top of the tank and has the ability to limit top of tank temperature to 140F (or any set temperature above or below that), and we will be testing the mixing valve to determined its regulating ability at temps above 140F. The desire would be to raise storage as much as possible above 140F during the daytime to increase hot water supply from solar to last through the night and into the morning hours until solar again could provide hot water. The LP hot water heater is always available as needed, but the goal obviously is to reduce LP usage to the minimum.

The second of the five major issues was storage over-heating during periods of low usage of hot water and failure to implement an overheat scheme to prevent blow-off of boiling water and steam. The evacuated tubes shut down and stagnate at 250F and above, but that still leaves very hot water (now glycol solution) moving to the system through the hx. This issue is obvious and I don't know why it wasn't dealt with on the initial install.

The solution is the installation of a heat dissipater (hot water fan unit heater) on the solar hot water line to the system plumbed to a motorized diverter valve which will divert unneeded solar hot water through the dissipater to cool down solar hot water so as not to over-heat storage. The heat dissipater circuit is activated when hot water in the storage tank reaches a maximum set point and is deactivated when hot water in the storage tank falls to a setting currently 6F below the maximum.

In periods of low use the hot water at the top of the tank could reach the maximum due to stratification while the tank still has capacity to store additional hot water. The solution at this point is a circulator to mix the tank to a more uniform temperature to achieve maximum total tank hot water supply.

renewablejohn said:

Why?? With PV now as cheap as chips and immersion controllers and immersion heaters designed for solar PV I thought solar thermal was dead. Its far easier getting high temperatures with multiple immersions then trying to achieve the same with mixer valves.

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The premise is good and solar PV for hot water heating remains under consideration, but three factors dictated against this path now. First, this is an addition to an existing install with an LP hot water heater and a 1000 gallon storage tank. Plumbing changes to add immersion heater ports to the tank were an obstacle due to tank modification concerns, and plumbing changes to the tank for solar hot water were not needed. At such time as the hot water tank and/or the LP hot water heater may need to be replaced, the option of solar PV hot water heating becomes more practical to consider.

Second, DP has reached its limit of solar PV under MN law and still obtain full net metering advantages. Further solar PV would materially reduce the net metering utility purchase price which adversely changes the economics of solar PV vs LP. Future price increase of LP may shift the economics towards solar PV regardless of the net metering disadvantage. Even with solar PV, a backup hot water heating system is needed, utility electric or LP, when solar PV is not available.

Third, at the time of the initial install in 2011 a major consideration was the learning opportunity with an evacuated tube solar hot water system. DP is an environmental leaning center, and adding the solar hot water system was and remains a learning opportunity.

...but can you pencil out the case for building it in the first place??

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Good question, but in 2011 a major consideration was the learning opportunity and not solely cost. DP has a wind turbine, solar PV and with this system solar evacuated tube hot water, as well as woody biomass hot water space heating with a Wood Gun E500, Garn WHS3200, and a Froling FHG-L 50

The third issue was under-utilizing the 1000 gallon hot water storage tank as a heat store to achieve the full benefit of hot water available from the evacuated tube system. Not only did the original design inject hot water injected directly into the top of the storage tank, but also supply to the solar arrays was taken from the tank midpoint, not the bottom. While the tank mixes quite well with a mixing circulator operating and low usage periods, any advantage from cooler water at the bottom of the tank was lost with the original design.

The new design not only injects hot water into the tank midpoint but also takes supply water for the arrays from the bottom of the tank to achieve as much delta-T and and to obtain as much btu storage capacity as possible. Cold makeup water also is supplied to the bottom of the tank. During periods of high use, mixing effect will be the least, the coldest water will be available to the arrays, and heat store capacity will be maximized.

The fourth issue was failure of the freeze protection scheme to prevent system freeze up during the winter. The glaring design failure here I think was use of domestic water in the arrays and not a glycol mix and heat exchanger. The original design also employed a freeze protection expansion valve, heat tape on the exterior exposed supply and return plumbing, and a programmed exercise cycle to flow water through the arrays. All of these together failed to prevent freeze-up, and the only remedy was to drain the system and shut it down during the winter.

The solution uses a glycol mix rated to -18F, which is satisfactory for most of the MN winter, but temps well below that are not unusual. The heat tape will continue to be used along with an exercise cycle. The desire is to use the system during the winter because although December usually is very cloudy, beginning in Jan and continuing sunny days are very common (along with cold into March). Working out an effective freeze protection plan will be a work in progress and should be achievable. Whether there will be sufficient hot water btu gain to offset freeze protection losses during January/February particularly will be an open question.

Finally two sunny days with very low building usage and an opportunity to test the effectiveness of the system to handle potential overheat conditions. Immediate observations indicate that the storage tank is responding well, near even heating top to bottom and maximizing hot water storage, use of the dissipater to reduce system output btu's, and running the solar and domestic circulators into the night to cool the tank down and have it ready to take the next days' sun. These last two days likely would have been disastrous with the old design.

I'm waiting for a firmware update and a revised programming scheme taking advantage of the update to automate what is now being done manually. Things look positive.

Lesson #1 now well learned: an evacuated tube hot water system must have an effective means to dissipate the btu's in heated water that is not needed for use. Sufficient storage is the first critical element, additional heat dissipation is the second critical element. A design which relies on the ability of the tubes to stagnate and shut down as the means to reduce or eliminate unneeded output is seriously deficient.

Interesting idea. The problem of excess hot water occurs only on days of low visitor usage (very little domestic hot water used) and long hours of sun on clear days, almost only during the summer. Another option is to use a heat exchanger to transfer heat to the space heating storage tanks, 7200 gallons total, if that heat could be used. Unfortunately, excess solar domestic hot water normally is produced at times when space heating hot water is not needed. Another idea is a powered awning to shade the tube arrays. That also could be used to protect the tubes from hail damage, although hail is a rare occurrence and the tubes are quite robust.

What's working right now and may be the simplest and least expensive is to keep the solar/domestic circs "on" during the night to circulate hot storage water through the hx to the tube manifolds to cool the tank down at night, the result being sufficient btu storage capacity during the daytime to accept the full output from the tubes. Both circs "on" consume about 300 watts. The controller can manage this to shut the circs down when tank temperature reaches a minimum temperature setpoint, possibly about 135F.

The July 4th weekend provided 4 days of sunny weather and minimal need for domestic hot water. The simple solution resulted in no over heating from too much hot water. The same scenario last year would have had staff in panic mode.

This coming fall, winter and spring will be of interest to determine how much and how often, if at all, excess hot water is available. If available, rather than blowing it off I'm thinking about the possibility of adding heat dissipation into the hot water recirculation system to put usable heat in a place where it is needed. Summer excess heat still is problematic: a hot tub is a great idea as a perk for staff, the kiln idea is also good except that the wood location is not close to the point where the plumbing and excess hot water heat is available.

A very tiny issue has held up the final programming of the Resol controller. Maybe newer models are better, but this one requires not more than a 2GB SD card with formatting that I have not found to be commonly available. Probably an easy fix for a computer techie, but not for me. Still waiting for the proper SD card so that I can load the parameters to finish automating the controller operation.

Finally obtained a proper SD card that works with the Resol, and tech support provided a pre-programmed file (which still need to be fine tuned) to upload to the controller. After a long hiatus, I uploaded the program file and set about yesterday to verify the programming, doing some test changes in the programming to understand the Resol programming procedure, and made substantial progress in moving towards a fully automated operation of the system. The programming is not difficult in the absolute sense, but is unfamiliar and not especially intuitive.

Goal: Obtain Maximum Usable Hot Water Without Day-time Overheating of the Hot Water Storage Tank.
Subject to verification later today, the heat dissipater appears now to be fully automated and operating correctly. Some testing is needed to determine the appropriate "on" and "off" temperature to prevent day-time overheat. This needs to be coordinated with an automated night-time cool down cycle of the hot water storage tank.

Example 1: Sufficient hot water in the storage tank needs to be available in the early morning (before solar supply is available) for kitchen use, showers, and other domestic needs. Ideally, the hot water storage tank pre-mixed supply should be no lower than about 135F (?) at the start of a day. Assume the storage tank ended the prior day at 150F after a day of solar loading. The controller should run a cool-down cycle during the night to bring down the tank temperature to the desired start temperature of 135F. The cool-down cycle operates the system during the night to dispel heat from the tank through the tubes into the cool night air until the tank temperature drops to the tank start temperature (135F ?) and then shuts down the night operation.

Example 2: During a day of intense solar with a tank start temperature of 135F, tank temperature rises to a high temperature set-point (150F ?) and will continue to rise due to solar supply exceeding demand. The heat dissipater needs to cycle "on" to dissipate incoming excess solar heat to prevent the tank from rising above a maximum temperature (170F ?) and continue to operate until the tank is cooled down to the high temperature set-point. Further cool-down to the 135F next day start temperature can occur through the night time cool-down cycle.

Further use experience will determine the appropriate minimum daytime tank start temperature and the heat dissipater set point temperature to prevent the tank from rising to the maximum temperature.

Still to be addressed is the frost/freeze protection scheme.

Use of propane for domestic hot water has taken a big nosedive. While some tweaking still is necessary, the system finally appears to do what it is supposed to do from 300 evacuated tubes without overheating to the point of emergency shutdown and without delivering scalding water to the domestic supply. Night cool down as needed to bring the tank temp down to about 135F has been effective. This leaves lots of capacity for tank heating during the day, even with very low hot water usage. This capacity, combined with the heat dissipating unit heater as needed, is estimated to eliminate any tank overheating issue.