Nevertheless, people from all over the world continue visiting this site wanting information about solar thermal.  So I decided to gather my years of information from seminars, lectures, and webinars, including this web site, and put them into a book.

The result is Solar Engineering of Drainback Systems.  It is available directly from this site.  To order:

Your feedback and suggestions are welcome.  The table of contents is below.

Contents

Introduction

Chapter 1.  A (very) brief History of solar energy

Chapter 2.  Solar Collector Design

Chapter 3.  Solar Hot Water System Design

Chapter 4.  Collector Array Geometry and Piping

Chapter 6.  Controls

Chapter 7.  Project Analysis Methods

Appendix A.  Three Project Flyers

Appendix B.  Potential for Cross Connection between

Tank fluid and Potable Water in a

Non-Pressurized Drainback Solar System

Appendix C.  Load Side Solar Heat Exchanger Sizing

Appendix D.  Recommended Solar Tank Locations

Copyright 2020 by Gravely Research corporation

The post New Book on Solar Engineering appeared first on Dr. Ben's Solar Hot Water Systems.

We call the system CMR, for Control/Monitor/Report.  There are several flavors.  One transmits the data directly over a cellular telephone link to our server for uploading to the cloud.  The other dumps the data into a LAN, which can also be sent to the cloud hosting service.  The cellular link is useful if there is no ethernet link to the equipment, or if there is a military or prison security issue that will not allow a system to be hooked into it.  We also have 4 systems (soon to be 9) that have local dashboards not connected to the cloud.

Being able to see the system performance promptly allows us to do maintenance and performance troubleshooting daily.  Repairs for distant systems are therefore much easier and downtime reduced.  We have seen pump failures, sensor failures, power failures, and many other indications of improper operation.  The CMR system will restart automatically after a power failure and the program can be updated remotely if needed to change settings or parameters.

The CMR system has also been very useful in analyzing usage patterns, ground water temperatures, and other characteristics of system performance.  For example, a school may fail to report that it has no classes in certain parts of the year, or that it doesn’t use certain dorms during summer school.  This could lead to an error in system sizing, and indicates that the school needs to make sure to use the solar dorms for summer classes.  A wealth of historical data is available by summing parameters over six months, or a year.  A very valuable parameter that is simply not available in most places is the entering cold water temperature for every month of the year.  The CMR system makes this very easy to find.

One purpose of this post is to share a unique piece of daily data that appeared in our system.  One of the third party owned solar hot water systems is located at the New Hanover County Jail, in Castle Haynes, NC.  There are over 500 inmates, with an average daily hot water consumption of about 3500 gallons.  The solar system includes 1600 ft2 (gross) of collectors and a 2000 gallon Fluid Handling System.  For the period of Jan – May, 2015, the system delivered an average of 230 Btu into every gallon of water heated, or about 890 kBtu/day, ranging from about 400 to 1600 kBtu, depending on weather and usage.

However, on June 17, 2015, the system delivered 2.0 MBtu in one day.  That is the highest output we have ever seen, and we wanted to share it with you (see fig. 1).  There are several reasons this number is so high.  First, the 5317 gallons/day usage is way above the typical 3500 gallons/day.  The bigger the load, the higher the output.  Second, it was a bright, sunny day.  On the other hand, the system ran only 6.86 hours because the tank started out hot and the weather was cloudy until after 10:00.

There are many things that can be learned from this data, that are not explicitly shown.  For example, the highest Btu transfer rate was 31.12 kBtu over 5 minutes at 13:50 (see fig. 2).  This converts to an hourly rate of 373 kBtu/h.  The instantaneous efficiency of the heat exchanger can be determined by comparing the 41 F temp rise of the water going through the exchanger to the maximum possible rise of 45.11 F, or 90.9%.  The flow rate was 19.3 gpm.

We hope the future of solar includes more data reporting systems, so we can improve our system sizing and performance estimations, and verify the results.

Figure 1. Performance data for New Hanover County Solar Hot Water System on 6/17/15

Figure 2. Peak Heat Exchange Rate = 373 kBtu/h.

The post Cloud Hosting Performance Data appeared first on Dr. Ben's Solar Hot Water Systems.

We have been looking into variable speed pumping for some time now.  There seem to be obvious benefits from varying the speed of the collector pump based on the radiation level hitting the collectors, as indicated by the collector output temperature.  During marginal radiation, a constant flow system may cut on and off repeatedly.  The start/stop cycling is called short cycling.  The system collects little energy and just wears out the pump.

Some controller manufacturers try to minimize short cycling by having a 5 minute delay in startup/shutdown times.  The idea is to have the system ride through the morning radiation ramp up, or short (5 min) rain bursts, or cloud coverage, thus preventing short cycling.  At a minimum, it prevents short cycling any faster than 5 minutes.

Figure 1. Constant speed pump on a clear day.

Figure 1 shows a constant speed pump on a sunny day, and fig. 2 shows a constant speed pump cycling on and off on a very marginal day.  The blue line at the bottom of each graph is the pump runtime and the red line in fig. 1 indicates high limit shutdown when the tank reaches 180ºF.  Each data point represents a 5 minute average value.

Reducing the collector flow during low radiation allows the system to continue running and producing (reduced) energy.  We see these conditions daily on our monitored systems.  Variable speed pumping will not prevent all short cycling, but it can minimize it.  It can also extend the run time at the beginning and the end of every day.

To change the speed of a pump, a special electronic driver unit, called a Variable Frequency Drive (VFD) is used to power the pump.  The growing availability of Variable Frequency Drive units is likely to cause an increase in their use in solar systems, and some pump manufacturers are beginning to build VFDs on the side of their pumps.  However, most built-in VDFs are on small pumps, which are not well suited to large commercial systems.

Figure 2. Constant speed pump
on a cloudy day.

There are reasons to separate the VFD from the pump.  Vibration, heat, and electrical interference are some of them.  Most VFDs are located with other controls in a separate control panel.

VFDs are sized and priced according to the power and voltage of the pump motor.  The units come in single phase input/output, single phase input/three phase output, or three phase input/output.  The input and output voltages can vary from 110 to 440vac.

A specific type motor is required to work with a VFD.  Ordinary shaded pole and permanent spit capacitor types may work, but other motor types may not.  As the name implies, the VFD varies the frequency of the AC power going to the motor.  In our tests, we have run pumps from 100% (60Hz) down to 30% (18Hz).  Different pumps have different lower limits.

There are also permanent magnet DC motors with variable speed controllers.  In this article, we focus on VFDs for AC motors, which are the most common type used in solar systems.

The components of a variable speed solar pumping system include

• dT Control:  Typical solar controllers monitor the collector and storage tank temperatures and calculate the temperature difference (called delta T, or dT).  They turn a relay on and off in response to the cut-on and cut-off temperatures differentials.  However, typical solar dT controllers do not provide an output signal that can run a VFD.

• Voltage Ramp:  VFDs require a voltage signal that represents the delta T between the collector and storage.  As the temperature difference goes up, the pump speeds up.  As it falls, the pump slows down.  Usually the ramp signal is a straight line representing speed vs dT.  However, any line shape, such as an exponential, or simple steps, could be programmed to vary the speed of the pump.  The dT controller must have a variable output voltage signal to drive a VFD.

• VFD driver:  The VFD driver receives the voltage ramp from the controller and varies the frequency of the AC power going to the pump.  VFDs have a non trivial amount of programming needed to set them up properly.  There may be as many as 50 settings to control startup current, ramp up speed, scaling of input signals, max and min operating speeds, response times, etc.  The learning curve to properly program a VFD can be significant.

The first step is to get a dT controller that can supply a voltage ramp for the VFD.  We developed two systems that can control VFDs.

One is called a CMR system (Control, Monitor, Report) that controls the solar dT functions, collects data from 5 temp sensors and one flow sensor, calculates all the control logic, energy, and flow values, and sends the data to our web hosting site.  The CMR system comes in two flavors, wireless and LAN. (See http://cmr.holocene-energy.com/demo/systems for a tour of the wireless  cloud web hosting program).

Figure 3. 2500 gallon Fluid Handling System controls with VFD on right end.

The other device is a simple solar controller programmed to run up to two dT systems (collector and pool loops, for example).  It outputs two voltage ramps for two VFDs .

Figure 3 is a photo of a 2500 gallon system with a VFD (blue) installed on the right end of the controls.

The results can be seen in the graph of fig. 4. showing the operation of the system on a winter day after installation in Massachusetts.  The collector temperature starts at about 27F at dawn and rises to 95F by 9:20am when the pump cuts on at full speed (= 20 on the scale).

The tank water temperature of 57F chills the collectors down rapidly and the pump slows to maintain a positive heat gain.  The radiation level then begins to fall with the pump running slower and slower, until it finally cuts off about 10:45am.

The empty collectors heat up rapidly again and the system turns back on, but the solar radiation continues to fall, shutting the system off about 11:10am.  At 11:25am the sun comes out and the system starts again, running to the end of the solar day at 12:30 pm, when a cold weather front moves in and radiation levels remain low.

Figure 4. VFD operation on 2500 gallon Fluid Handling System in Massachusetts.

Over the run time of 3 hours and 10 minutes, the tank gained 46ºF,  which added  about 950,000 Btu to the tank.  The average pump speed was perhaps 50-60% of max during the run time.  This calculation does not take into account any energy that may have been removed from the tank, so the total energy produced could be much higher.

In summary, the results show that a VFD system can significantly increase the solar energy collected under low radiation conditions.  Determining whether a VFD system is worth the cost and effort is dependent on the climate where the system will be installed.  A sunny climate may see little overall benefit.  A cloudy climate will see the best results.

The components are not commonly available, so building a VFD control system can involve a significant learning process.  In addition, pump manufacturers may not endorse running their pumps at slower speeds.  The concern is that a reduced flow may cause overheating of the pump volute bearings, causing either leaks or seizing.  One major pump manufacturer I spoke with is still studying the question.  It would be wise to check with the pump manufacturer to be sure their pumps are suitable for VFD operation.

The post Variable Speed Pumping appeared first on Dr. Ben's Solar Hot Water Systems.

What methods/products do you recommend for the exterior pipe?  As you have stated in your blogs, one of the big advantages to db is the reduction of continual service/maintenance.  I want to make sure that the insulation will last the test of time.”

Nate Pembleton, Owner
Asheville Solar Company

______________________________________________________________________________________________________

Nate,

Good to hear from you.

Your question about pre insulated pipe has two answers.  First, I don’t care whether the pipe is SS or copper.  I know SS is very popular in Europe – that’s where this product comes from.

However, there are two problems with this product.

First is the insulation is the soft foam type.  It can melt and become a sticky goo on the hot return pipe of a DB system that produces steam occasionally.  I used to have a 1 ft section of copper pipe with rubatex melted all over it.  You couldn’t scrape it off with a knife.

I don’t know of any soft foam insulation that I can recommend.  I recommed isocyanurate (ISO) foam insulation, 1″ thick for solar water lines, with an aluminum jacket outside and the default paper jacket inside.  Aluminum is the only outside jacket that will last forever (SS will, but very expensive).  Fiber glass or mineral wool insulation of the same R-value is also acceptable.

Second is the flexible nature of the SS pipe.  It is not really that different from the soft copper coils that installers used in the ’70s and ’80s to get away from straight pipe.  The problem occurs when the pipe must be run any distance horizontally.  It will sag and cause a trap for the collector lines.  Installers have tried using wire hangers every several feet, and one used a 2 x 4 support to keep the lines from sagging.

Some dealers used a hybrid – soft copper pipe on the near vertical runs, and hard copper where sagging could be a problem.  You might be able to get away with rubbery insulation on the collector supply line up to the roof, then do the ISO outside.  I would do the return line all the way to the tank with ISO.

The better insulation pretty much kills the preinsulated piping idea, since it is the wrong insulation.

If you want a really trouble free system, the extra time, money and effort getting the piping and insulation right is worth the effort.

Rubbery insulation will rot and crack under ultraviolet and shrink away from the collector sensor, causing no end of control problems.  You get into a “it ain’t broke, but don’t work” dog-chasing-tail situation.

I recommend the expensive, hard route.  The trick is to make that a “feature” when you sell it to the customer.

Good luck!
Dr. Ben

The post Pre-insulated Pipe appeared first on Dr. Ben's Solar Hot Water Systems.

This is especially important for the non pressurized drainback type of solar system (single pressure, backsiphonage), which has been lumped into the category of double pressure systems (backpressure), causing unnecessary expense and performance reduction.

The goal of this paper is to show that non pressurized solar systems fall under the backsiphonage classification and should be treated as such in any codes.

In fact, non pressurized solar systems can be viewed as a special case where the probability of cross-connection hazard is so low that backsiphon equipment does not add to the safety.

A statement reflecting this fact is:

1) Non pressurized drainback tanks with unobstructed vent/overflow pipes and single wall heat exchangers meet the leak detection requirement and cannot be operated at a negative pressure differential sufficient to mix tank fluid with potable water.

A statement requiring the same backsiphonage equipment as conventional equipment is:

2) To guarantee that a negative pressure cannot be established across a single wall exchanger in a non pressurized drainback system, a backflow preventer (vacuum breaker) shall be installed on the street side of the heat exchanger.

While statement 1) is preferable, statement 2) is also acceptable.

Regards,

Dr. Ben

The post Cross-connection Hazards appeared first on Dr. Ben's Solar Hot Water Systems.

Steve,
The answer is more philosophical than technical.  We think the only honest thing to do is measure the energy that actually goes into the load.  When we bill people for the energy used, they expect to pay for the energy that went into their system.

Measuring the energy collected doesn’t tell us whether that energy went onto the load.  If the delivery system is low efficiency, for example, it has a lot of heat lost through poor insulation, incorrectly sized heat exchanger, etc, then the customer is not getting their money’s worth.

The counter argument is based on the utility model of selling gas/oil/electricity to a customer.  The customer pays for the gas/oil/electricity as it comes off the street, regardless of the efficiency of the boiler, or device using the energy.

The flaw in that argument, as I see it, is that the gas/electricity/oil costs ongoing money to produce, or capture, and deliver it to the customer, so they have to pay that cost.  That is a continuing cost of production.  Solar, on the other hand is a fixed, one time expense (ignoring maintenance), so the production cost is not relevant here.  Actually, the customer does pay the electric bill to run the equipment, which could be considered the production cost.

Also, the oil/gas/electric energy devices are a century old and well defined.  Solar is more like the wild wild west, with variable quality in design, engineering and installation.  We have seen systems where the energy captured was billed and the energy delivered was a fraction of that energy.

We don’t believe the production model of conventional energy is the relevant model for solar, so we only measure and sell the actual energy used in the application.

I hope this answers your question.
Dr. Ben

The post “Why do you monitor the BTU’s on the consumption side?” appeared first on Dr. Ben's Solar Hot Water Systems.

Therefore, a different approach must be used, and large safety factors applied.  For example, an 1800 gallon tank can store approximately 1.2 million Btu over the range 60-140F.  If this energy is removed during a 4 hour duty cycle, say two hours in the morning and two in the evening, then the energy transfer rate must be 1.2 millionBtu/4 = 300 kBtu/hr.  In addition, the tank temperature is falling continuously, in the case where there is no solar gain during the use cycle.  This is not the normal case if the load occurs during the solar day, but will be ignored in this example.

In the tank heater mode, a B&G TCW 1084 exchanger (100 ft2 area) is rated to heat 2490 gallons per hour of water from 40-140F, using 180F inlet water (140F delta T). That is equivalent to 2.07 MBtu/hr, or 8 MBtu over a 4 hour duty cycle. If we assume thermodynamic reciprosity in heat transfer from heating the tank to removing heat from the tank, and assume that the maximum output conditions are a tank temperature of 140F with cold water temp of 40F (100F delta T), then the output heat transfer rate would be a factor of 100/140 = 0.71 less that the stated rate above. That gives us a maximum heat transfer out of 0.71 x 2 MBtu = 1.43 MBtu/hr, which is 4.8 times the expected heat transfer.  We have ignored the fact that the system may gain over 1 MBtu/day from solar energy on any given day.  This calculation presumes that heat removal occurs at 140F tank temp.  Since the key characteristic of a solar system is that the tank temperature is never constant and can go from max to min in a short time, we have to presume a much lower average temperature.  So, the 4.8 times safety factor is actually a sliding scale depending on tank temperature.

Since the exchanger capacity is directly tied to the maximum energy that can be stored in the tank, which is 1.2 million Btu, the exchanger must be sized not only for capacity, but it must be able to carry the maximum flow rate through the cold water system without creating an unacceptable pressure drop.  We can now make a more rational specification for the exchanger.

1. The exchanger must have at least 0.04ft2 of area per gallon of water in the tank.  This should be considered a minimum.  A real world exchanger can have more area than specified.  If the exchanger is too small, energy will be left in the tank, raising the average tank temperature and reducing the collector efficiency.

2. The exchanger must have an acceptable pressure drop for the expected max flow rate.  This requirement frequently results in an “oversized” exchanger.  For example, a 1000 gallon tank may require a 40ft2 exchanger, but if the cold water line going through the exchanger is 3 inches in diameter, then an available exchanger with 3 inch ports may have 100ft2 or more.

This specification has been proven in over 2000 solar systems dating back to 1978!

Dr. Ben

The post Load Side Solar Heat Exchanger Sizing appeared first on Dr. Ben's Solar Hot Water Systems.

Dr. Ben

//www.youtube.com/watch?v=kiTjWlZzTzw

The post February’s Solar Thermal Webinar appeared first on Dr. Ben's Solar Hot Water Systems.

Raintree Subdivision

In the early 1980s, Omni Builders was planning a new single family housing development of seventy units in Jacksonville, NC. The houses were sized from 1400 square-feet  to 1600 square-feet on quarter-acre lots. These were single and two story homes for young families… starter homes.

Omni decided to look into providing a solar water heating system on each home as standard equipment and contacted me to investigate the possibilities. I reviewed the project layout and individual house plans and developed special energy conservation measures. The measures included upgrading the floor, wall, and ceiling insulation, combined with an appropriately sized solar thermal system, which supplied both domestic hot water and space heating.

The builder chose to install the same system on all seventy homes, standardizing the systems and lowering installation costs. Computer calculations showed energy savings of 60-70% for the smaller homes and 40-50% for the larger ones. Most importantly, the buyers were very happy and indicated that the standard solar water heating systems were a key factor in their purchasing decision.

Many of these solar systems are still in operation today, 30 years later. We’re starting to see a resurgence of this concept again today in North Carolina and many other states. Builders are recognizing that homeowners see the value that solar energy brings.

Let’s hope this trend continues!

Dr. Ben

The post The First Solar Neighborhood in NC appeared first on Dr. Ben's Solar Hot Water Systems.

Ending 2012 on a very good note, the solar thermal system for the New Hanover County Jail was completed and put into operation! This system was designed and installed by Holocene. It’s also owned and operated by Holocene. The upfront cost to New Hanover County was $0. Instead, the county simply pays for the hot water the jail consumes.

Understandably, this project made local news in Wilmington. Channel 6 (WECT) recently broadcasted a story covering the system. Take a second to check it out here.

Even better, 2013 is already off to a good start with several more interesting solar thermal projects in the pipeline. Stay tuned!

Dr. Ben

The post Solar Water Heating at New Hanover County Jail appeared first on Dr. Ben's Solar Hot Water Systems.