Solar Hot Water Heating System [1]

7/28/2019 Solar Hot Water Heating System [1]

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4 4 A S H R A E J o u r n a l a s h r a e . o r g S e p t e m b e r 2 0 0 9

By Gaylen Atkinson, Member ASHRAE; and Tom Colvin P.E., Member ASHRAE

To offset the environmental impact from the Societys annual meetings,

ASHRAE launched its first sustainable footprint project at the 2008 Annual

Meeting in Salt Lake City. ASHRAEs Utah chapter designed and installed a solar

domestic hot-water (DHW) heating system at a home for teens. These projects

are an ongoing way for the Society to give back to the community where its

meeting is held, and can serve as a way for the chapter to learn more about

solar DHW systems.

The system was installed at the LolieEccles Teen Home at the Salt Lake City

YWCA. This residential acility has thecapacity to serve 12 pregnant or parentingteen girls who are homeless or in statecustody. Although the deadlines or thisproject were tight, the system has beenproducing solar heated water at its design

capacity since startup.

The chapter members, representingmany companies, teamed up to donate thedesign, materials, and installation workwhile many others contributed nanciallyto the project (http://tinyurl.com/nt4dl).Based on early overwhelming support orthe project, the decision was made justweeks prior to the meeting to have the

system operational prior to the opening

session in June 2008. Unortunately, ourdecision to not ollow the original plan ocollecting the money at the annual meetingand then completing the project causedmany o our problems through its normaldesign and installation process. Rushingthe installation, we skipped submittal andmaterial reviews, which orced us to correctthe resulting problems ater installation.The design criteria is discussed here

and the installation process and commis-

sioning is described in detail to give thereader a sense o the various problems weencountered, the solutions we developed,and the important operation problems thatany solar system designer would want toknow. Included are system diagrams andWeb screen shots o the data reported bythe system, design, installation, and com-

About the Authors

Gaylen Atkinson is president of Atkinson Elec-

tronics, Inc., and Tom Colvin, P.E., is president

of Colvin Engineering Associates in Salt Lake City.

Lessons Learned

Solar Hot-WaterHeating System

This article was published in ASHRAE Journal, September 2009. Copyright 2009 American Society of Heating, Refrigerating and Air-Conditioning

Engineers, Inc. Posted at www.ashrae.org. This article may not be copied and/or distributed electronically or in paper form without permissionof ASHRAE. For more information about ASHRAE Journal, visit www.ashrae.org.


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S e p t e m b e r 2 0 0 9 A S H R A E J o u r n a l 4 5

missioning recommendations, and summer 2008 and winter2008 09 perormance history.

System Design

The rst task or the design eort was to dene and reduce

the load. Several dierent low-fow showerheads were testedby installing a sample at the existing acility to determinethat the showerheads would suce or the teenage girls. Theadministrator also assisted designers by providing some timedshower data rom the current occupants to accurately predicthot-water usage on a daily basis.

Based on administrator eedback, the predicted hot-water us-age or the acility was estimated to be an average o 75 gallons/day (284 L/day) on weekdays and 10 gallons/day (38 L/day) onweekends. The acility houses 12 teenage girls and their youngchildren. Now that the acility has been operating with the solarsystem or the past 10 months, the actual hot-water usage has beenmeasured and equates to 65 gallons/day (246 L/day) per person onweekdays and 5 gallons/day (19 L/day) per person on weekends.

Ater the number, duration and time period or daily showersand laundry had been dened, the optimum storage volume wascalculated. Using solar heating computer simulation sotware,the optimum solar panel size or this acility was determinedto be a our-panel array o 4 t by 8 t (1.2 m by 2.4 m) panels(Photo 1). This systems size was predicted to provide 63% othe annual hot water or the acility.

We decided to use photovoltaic (PV) powered dc pumping,a new trend in solar hot-water heating systems, rather than aconventional ac powered pump. This eliminated any parasiticcost o utility power because the pumping energy would be

provided by the sun. The additional benet is avoiding a glycolloss rom system boil-overs during power outages.

Monitoring and Control

An automation company donated a ull-eatured systemthat could be used or system monitoring, energy perormancecalculations, and dierential temperature control. Flow meters,temperature sensors, and other data points into the system weremonitored, and Web pages o live data were displayed at the2008 ASHRAE Annual Meeting (Figure 1). Custom automationprogramming was provided by Utah chapter members and rmsowned by local members installed the solar piping and controls.

Small solar hot-water systems typically lack the control sophis-tication o this project, but our automation has been essentialto identiying problem areas during system commissioning.

The secure project site required remote monitoring where westudied system perormance data logs and ne-tuned the con-trol sotware to obtain maximum energy collection (Figure 2).

Installation

The Teen Home was a good t or this project because o itsdaily domestic hot-water usage requirements. We decided toinstall two 80 gallon (303 L) preheat tanks with built-in glycolheat exchangers eeding into two existing commercial 80 gal-

lon (303 L) gas domestic water heaters. A local engineering

rm donated the design work and a local chapter membersrm donated the installation and successully persuaded otherrms to donate storage tanks and other accessories. Only thesolar collectors and mounting hardware were purchased, usingmoney donated by ASHRAE members and other Region IX

chapters. Excess contributions were given to the Louisvillechapter to seed its sustainability project or the 2009 ASHRAEAnnual Conerence.The revised chapter plan required the system to be installed

and running or the ribbon cutting on June 20, 2008, the daybeore the Annual Conerence opened. ASHRAE president KentPeterson attended, along with other dignitaries, local govern-ment ocials, and representatives o the media. Because thesystem and installation were donated, there wasnt a generalcontractor to enorce project schedules. The project was pro-ducing hot water, along with the live Web demo, barely in timeor the dedication. We would advise other chapters to allow theull year to accomplish their sustainability projects to avoidlast-minute completion headaches.

Getting the System to Work

The design engineering rm and mechanical contractor werewell respected or their conventional HVAC expertise, but theydidnt have a lot o recent solar hot-water system experience.As a result, we learned as we went. The rst major issue wediscovered during installation was that the company donatingthe storage tanks had mistakenly shipped the Florida version,which heated city water directly in the collectors without the gly-col heat exchangers required by the much colder Utah climate.Since the tanks were already piped, we decided to leave them

in place or the summer and correct the problem beore winter.We evaluated keeping the tanks by adding a separate heat ex-

changer or the glycol with a domestic water pump. This wouldhave required re-piping, modiying the controls with extra sensors,and more pump control. Fortunately, the company that donatedthe tanks volunteered to ship the correct tanks, and the installingcontractor agreed to re-pipe them when they arrived, leaving thesystem operating according to its original design (Figure 3).The correct tanks were installed several months later and the

glycol loop was isolated, but additional taps into the systemwere required to inject the glycol. In addition, once the glycolexpansion tank was returned to its isolated loop unction, we

also had to add an additional expansion tank to handle the extradomestic hot-water storage capacity.

We discovered the second major issue during the meetingwhile we were remotely monitoring the system. Because thehome used very little hot water during the weekend, the stor-age tanks were reaching almost 180F (82C). We ound theexisting tempering valve was not piped correctly to inject coldwater, because the original system piping schematic did notrefect the extra bypasses that we ound ater careully tracingall the old and new piping.

In addition, a check valve shown or the existing DHW re-circulating pump was missing, so we were back-eeding very

hot, solar-heated water into the system, bypassing the existing


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Figure 2: L ive solar collection hour ly Btu data log rom the project Web site.

water heaters and tempering valve. This became apparent a-ter we added control to the recirculating pump that had beenrunning continuously beore the solar water heating systemwas installed. We had to have the maintenance sta manuallydump solar-heated water several times to prevent a potential

hazardous situation.The installing contractor returned and re-piped the temperingvalve and added a check valve, ater which we were able to make thetempering valve unction properly in its saety role. To prevent utureproblems, additional temperature sensors were added to monitor thehot-water system. We also added a solenoid valve, so the automation Photo 1: The (our ) fat plate collectors with the solar pump PV panel.

Fi gure 1: Solar energy data on the projects li ve data Web site (www.utahashraesolar.tzo.com).

system could automatically dump hot waterto prevent overheating on light usage days.

The dc pump pushed about 2 gpm (0.13 L/s)o city water on peak solar days, or 0.5 gpm(0.03 L/s) per panel, alling within the col-lector manuacturers guidelines. However,this would probably not be adequate onceglycol was added to the system.

When the glycol heat exchanger tankswere installed, we ran the system on citywater or a ew days to discover any prob-lems beore we pumped in glycol. Thetanks were piped reverse-return or theglycol and domestic water sides, but wediscovered that one tank heated to 155F(68C) during the time that the other tankonly heated to 125F (52C). We had thecontractor re-pipe the glycol heat exchang-ers, counting every elbow and tee to make

sure each tank heat exchanger was exactlybalanced. The next day the two tanks weretracking within 0.5F (0.3C). I we hadbeen using a higher head and fow ac-powered pump, we could have correctedthe problem by balancing with an isolationvalve. This was not possible with the low-head and low-fow photovoltaic-powereddc pump that we were using.

Controls and Live Monitoring

The original control sequence called

or dierential control: starting and stop-ping the solar pump based on the collectoroutput and tank temperatures. However,with PV powered pumping, we startedwith the much simpler approach o thePV panel powering the pump wheneverthe sun was shining, with load-matchedfow during the collection period.

Because the automation system was calculating and loggingsolar hot-water Btu production, we shortly discovered that thepump operated or several hours beyond the 8 hour (4 hourso solar noon) thermal collection period. This resulted in the

fat plate collectors reradiating energy that had already been

collected in the storage tanks. The data logs showed the dailyBtu count decreasing, while the collector output temperaturewas lower than the input.

We had temperature sensors on the collector output, input,

and the tank, so we programmed the automation system to


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Figure 3: Or iginal pipi ng diagram rom the project Web site.

prevent the pump rom running unlessthe collector output was warmer thanthe tank. We wired the pump througha normally closed relay so that in theevent o a power ailure the pump would

continue to operate rom the PV panel.This turned out to be a wise decision asthe building experienced several powerailures and the PV-powered pump keptrunning during the outage, preventingoverheating and loss o glycol.

Another issue we ound was that i thetanks were hot, typically in the aternoon,and clouds rolled in, the collector outputtemperature would drop rapidly. I sub-stantial cold water entered the tank at thesame time, due to laundry, etc., the tanktemperature would be colder than the col-lector output, keeping the pump runningand reradiating collected heat. This wasalso noticed by our Btu count going down as the collector inputtemperature would be several degrees hotter than the output.We modied the control sequence to prevent the pump romrunning whenever the collector output temperature was lower

than the input, which corrected the problem. We experimentedwith time delays and minimum pump runtimes to prevent shortcycling the pump on morning startup and reradiation preven-tion shutdown.

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We noticed signicant burner runtime, making upstandby losses, which was especially irritating whenwe had much higher solar tank preheat temperatures.We added control to the domestic water heater burn-ers to shut them down (but with automatic restart

i the tank temperature dropped too low) wheneverthe solar tank temperatures were signicantly higher,saving a lot o gas. Because we had 160 gallons (606L) o additional solar preheat storage capacity, wehad the maintenance sta valve-o and operate onlyone domestic water heater at a time, which urthercut the standby losses.

Fine-Tuning the System

Ater we corrected the problem with the solarpump running too long, our fow meter, whichclocked one pulse per gallon, showed a thermo-syphon, which caused additional reradiation oalready stored energy. We suspected the solar loopcheck valve was stuck open (Figure 3). A newvalve was installed and the problem immediatelydisappeared. The Btu logs showed that we hadbeen losing up to 20% o the Btus collected eachday beore we stopped the PV pump extra runtimeand the thermosyphon.

Ater the overheat dump valve was installed,we never worried again about overheating on lowusage days because hot water was automaticallydumped to prevent reaching 180F (82C) storagetank temperature. The most eective sequence

was running the dump valve or several minutesany time the tank temperature hit 170F (77C)beore 4 p.m., dumping about an hours worth ocollection. We ound we could move the time toabout 3:30 p.m., which saved water, because theremaining solar collection time didnt allow thetanks to reach 180F (82C). We originally con-sidered stopping the pump when the tank reached180F (82C) but decided it was a bad idea or oursystem as the collectors would boil and lose glycol,shutting the system down until it was recharged.

since the tank transer pump was installed last all. The pump

has run or several hours each weekend during the hottest parto this summer.

When we changed solar tanks and added glycol, the solar loopdropped rom 2 gpm (0.13 L/s) on city water to 1.6 gpm (0.10L/s) on glycol, which proved to be a problem. Several weekslater the collector temperature was more than 240F (116C),there was no fow, and we suspected the glycol was boiling. Werushed to the project, ound steam coming out o the collectoroutput automatic vent, and covered the collectors with a tarpuntil we could correct the no-fow problem.The glycol expansion tank pressure gauge had dropped to al-

most zero. We could not remove the pump or inspection because

the isolation valves also included the expansion tank in the loop.

Figure 4: Piping or the tank tr anser pump rom the project Web site.

Ater observing that almost 100 gallons (379 L) o hot water

were dumped each weekend in September to prevent overheat-ing, we experimented by adding a pump to transer the solarheated preheat tank water into the gas domestic water heaters,pushing the lower temperature domestic hot-water heater waterinto the preheat tanks, allowing several more hours o solar col-lection in the aternoon (Figure 4). When we started the pump,we ound we also needed to add an external check valve as thedonated pump did not have one built in. Our sophisticated con-trol logic was not necessary because all we needed was to runthe tank transer pump or an hour ater 2 p.m. in the aternoon,whenever the pre-heat tank temperature exceeded 160F (71C).

We never dumped hot water again on a weekend last all. As

o Aug. 10, 2009, the emergency dump valve has not operated

Figure 5: Pump curve or dc solar pump.

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D5 Solar 710B/D5 Solar 720B


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We ound the dc pump manuacturer had a model with a dierentbody style that provided 1.5 times more fow at the pump curvepoint where we had been operating (Figure 5). We purchased andinstalled the new pump and a strainer with additional isolationvalves or easy uture removal. The glycol fow jumped rom 1.6

to 2.0 gpm (0.10 to 0.13 L/s) with the new pump.We suspect that the system simply air locked because wedidnt have enough fow with the thicker glycol at lower wintertemperatures beore the sun heated the panels. We installeda pressure transmitter at the same time so we could monitorglycol loop pressure. We tested the original, lower fow pumpin a bucket and ound it worked normally. We have not had anair locking problem due to insucient fow since.

The PV panel was originally installed at the same sun angle asthe thermal collectors. The gap we had allowed or snow removalproved to be insucient. In January, the sun was shining brightlyater a signicant snowstorm, and we noticed a very low PV volt-age, about 12V instead o 18, powering the pump, and the collectortemperature was above 240F (116C). We rushed to the projectto save the glycol that was boiling due to low fow. We ound thesnow had rerozen beore sliding completely o the PV panel sothe bottom solar cells were blocked (all the cells in a PV panel arein a series), preventing sucient current output to power the pump.

The thermal collectors had only melted away the top one-third othe snow cover but were suciently exposed to overheat and boilthe glycol. We immediately cleared the PV panel, and the resultantfow cooled the temperature. Fortunately, we only had to rechargea small amount o missing glycol. We remounted the PV panel on

stand-os so that snow would slide completely o beore the snowstarted melting on the thermal collectors. Upon questioning themaintenance sta, we ound that they had been clearing away snowater storms, but hadnt this time. We wanted to ensure that snowremoval would happen naturally and not cause a problem again.

In March 2009, we added a solar radiance sensor, so we couldcompare collector output with actual solar availability, to gainexperience with collector eciency in our geographic area. Theglycol pressure sensor was wired in so we could monitor systempressure. We added a dc current sensor to the PV-powered pump,so we could learn how it perorms versus its pump curve. Thepeak fow ran at 1.5 gpm (0.09 L/s) instead o the 2.0 gpm (0.3L/s) we had last all, we suspected either the strainer neededcleaning, or there was some other problem.

In April 2009 we noticed that the collector inlet temperaturewas much higher at night and during the day than had been thecase previously. It oten exceeded the collector outlet temperature,while the solar tanks were gaining in temperature during daytime



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5 0 A S H R A E J o u r n a l S e p t e m b e r 2 0 0 9

collection periods. This caused erroneous Btu collection data andshort cycled the pump as the controls program was trying to pre-vent a supposed reradiation problem. We commanded the pump torun continuously during the daily collection period until we couldget back to the project to determine the source o the problem.

The solar radiation sensor data energy potential showed that wewere not collecting the daily energy we had obtained previously.The glycol pressure sensor, which had also been wired just weekspreviously, showed that we had lost glycol rom over the winter.Returning to the project, we tightened unions where we noticedglycol drips, cleaned the strainer, which improved the pump fow,and replaced a aulty temperature sensor on the collector inlet,which explained the diminished Btu collection. We also rechargedthe glycol to make up the loss rom over the winter.

In early May 2009 the glycol boiled due to a pump vapor lockbecause the solar radiation was higher than it had been in thewinter. We suspected a problem with the glycol expansion tank,which was installed without access to the pressure port. We hadto remove the tank rom the line or testing where we measured50 psi (345 kPa), much higher than the 12 psi (83 kPa) listedon the label. We bled the pressure back to 12 psi (83 kPa), rein-stalled the tank, and charged the glycol and it has been operatingnormally since.

Because the automation system monitors all o the sensors,our live system data Web site (www.utahashraesolar.tzo.com)has become popular or those in the solar industry wanting tomonitor solar perormance. It is also used as an instructionaltool or schools and solar energy classes. We have learned a lotabout how solar hot-water systems operate, especially in the coldUtah climate, and are anticipating the perormance this summer.

Lessons Learned

We ound that with the daily average usage remainingsimilar, we can collect and store solar heated water thatis about 70F (39C) over ambient. In the summer ourdaytime temperatures average 80F to 90F (27C to32C), and we were achieving 150F (66C) regularly. Inthe winter with 30F to 40F (1C to 4C) in the daytime,we were storing 100F to 120F (38C to 49C) regularly.

This past winter, Salt Lake City experienced inversionsand cloudy days that diminished our solar collection.

The winter city water temperatures have dropped to thelow 40s and the summer city water has been almost 60F(16C).

The collector angles are at our latitude o 40, whichmeans peak collection should be in March and September. We are anticipating the comparison o the owners sum-

mer gas bills rom summer 2008 to summer 2009 tocompare with the estimated savings rom our logging othe solar-collected Btus.

The live data reported by the automation has allowed usto remotely diagnose many o the operational problemsthat we encountered. The energy calculations allowed usto enhance the solar system perormance.

From ollowing the live data Web site, we suspect that thediscrepancy between the accumulative solar-collected Btus andthe cumulative DHW usage Btus is the result o the heat gainrom the boiler room to the domestic water/tank piping system.

System Recommendations

Reduce the load with low usage xtures and determinethe daily load prole.

Size the system or a reasonable payback, not the entireload.

Design the system or the climatic conditions and evaluatethe structure or collector location and angle, wind load-ing, and pipe routing. Ensure adequate room to preventsnow buildup and allow natural snow removal.

I the system is being added to an existing building, care-

ully survey the piping, connection points, and valvingto ensure a simple interace. Just because the hot-waterheating system has been operational or several years,dont assume all o the components are working.

Veriy proper operation o existing water heaters, mixingvalves, circulation pumps, check valves and other keycomponents, as the solar heated water can be much hotterthan a conventional DHW system. You cant just turn othe sun when its convenient.

Since the solar collection capacity willlikely exceed usage at times during theyear, the design must include a means

o rejecting the excess heat along withensuring pump operation during poweroutages to prevent system boil-overs(PV pumping or emergency power).

Installation Recommendations

The plumbing contractor should be theprime contractor and be responsibleto coordinate the roong, electrical,insulation, and controls.

The piping schematic should be de-tailed, showing correct locations or

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all valves, sensors, glycol taps, with adequate isolationvalves or servicing each component.

Check the expansion tank pressure during installationbecause the label may not be correct. Make sure thepressure port is available or uture maintenance testing.

A preconstruction meeting with the plumbing contrac-tor and designer should emphasize the importance oequalizing the pressure drop through each circuit/tankto ensure balanced fow.

Commissioning and Operation Recommendations

The construction team should include a knowledgeableperson with solar control experience to veriy properlows, temperature dierences, and quantiied solarperormance.

Have a sun screen (tarp) available or the collectors socommissioning work can be perormed during the day-time by turning o the sun.

Educate the owner/operator to maintain clean collectorsand provide them a list o troubleshooting recommenda-tions. Request the owner to record operational problemswith date and time.

Snowy climates should include snow removal instructionsto prevent lost collection days.

Encourage the owner/operator to observe typical operat-ing temperatures and system pressures so that problemscan be identied i observed temperatures are out o thenormal range or a leak occurs.

Recommendations for Future Projects Dont let the joy o contributing to a great cause over-shadow the need to allow sucient time or good design,product submittals, sequential construction, and thoroughcommissioning.

I uture chapter sustainability projects are automated andlive Web data are available to all ASHRAE members, thecollective knowledge gain will be benecial to our industry.

The chapter host committee and Board o Governors shouldappoint a sustainability project committee with a strongchair, clear lines o authority, deadlines, monthly reporting,technical review, and undraising to implement the project.

Allow at least a ull year rom project inception tocompletion.

We hope the detailed report o our experiences will bebenecial to others as they embark on uture solar projects.We encourage readers to look at the chapter Web site (www.utahashraesolar.tzo.com) and provide comments or recommenda-tions or a collective learning experience.


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Solar Hot Water Heating System [1]