Interesting Temperature Response During Stagnation

I received a call the other day from a customer that was testing the stagnation response of a bank of solar collectors.  He filled the system completely with water and then just left the pump off while the sun baked the collectors.   He was doing a form of pressure testing of the system for leaks.  Since the system is ultimately going to be a closed loop glycol system he has a generously sized expansion tank connected to the system.  He is using an immersed thermal well with the temperature sensor for measuring the temperature of the solar collector array.  What he noticed was under stagnation conditions the temperature sensor went up to about 252 degrees Fahrenheit and then would drop down in temperature about 10 degrees for a period of time before it would build up to 252 degrees again.  The system was repeating this cycle over and over again and he wondered why.

What the installer was experiencing was the result of the fluid boiling, turning to steam, and then cooling as a result.  He confirmed that his solar system was operating around 30 psi when it was doing this.  You can see from the charts below that the boiling point of water at 30 psi is roughly 250 degrees.  This matters because any closed loop glycol system can exhibit the same behavior.

When the fluid in your system is undergoing this phase change it will experience significant amount of pulsing as the pressure spikes from the creation of steam.  During this pulsing you don’t want your pump running.  If the pump is running you end up rattling the pipes by continuously pumping cold fluid into the collector to experience boiling.  You also can damage the pump by having it operate when the shock waves of the fluid phase change are propagating through the system.  The issue comes in when you use the standard turn off point of a solar controller.  The different manufacturers of solar differential controls have different safety turn off points.  Steca uses 265 degrees as the point after which the pump will shut down.  Other manufacturers use 249 degrees.  All of these settings are adjustable.  To prevent customers complaining about the rattling of their pipes and to prevent damage to your installed equipment make sure that the turn off point for your differential control is set to keep the system off during these times.  I would recommend you set your turn off point for the system at 249 to be safe.


Comparison of Solar Site Evaluation Tools

When evaluating a site for a solar heating or photovoltaic installation, there are many features of the site to take into account. The latitude and longitude establish the sun’s path. The orientation of the collector array, the tilt and azimuth, defines the field of view of the sun. Shading from obstructions, such as trees and buildings, decreases exposure to the sun’s radiation. Local and regional weather patterns also must be taken into consideration.

Solar site evaluation tools use these factors in determining the best location for optimum solar exposure. Three of the leading solar site evaluation tools used by the solar energy industry are Solar Pathfinder, Acme Solar Site Evaluation Tool and Solmetric SunEye.

Solar Pathfinder includes a transparent plastic dome, instrument platform, base section, tripod legs and paper sunpath diagrams and is easy to quickly assemble. Solar Pathfinder Assistant software is also available for use in conjunction with the Solar Pathfinder unit. The sunpath diagram for the specific latitude of the site is placed on the instrument platform and under the plastic dome. Looking down onto the dome, the site evaluator can see a panoramic view of the site reflected on the surface of the dome. Since this view is a reflection instead of actual shadows, the Solar Pathfinder can be used anytime of day in any type of weather. Any obstructions to the sun’s rays are visible in the reflection. Openings in the side of the plastic dome permit the evaluator to trace the outline of the obstructions on the sunpath diagram, thereby indicating what areas of the site will be shaded. A digital photograph may also be taken and uploaded into the Solar Pathfinder Assistant software program. The software identifies the shading patterns, merges them with solar radiation data and local weather records (from the National Renewal Energy Laboratory) and mathematically calculates the potential percentage of solar radiation of the site.

Acme Solar Site Evaluation Tool, also known as ASSET, consists of a precise positioning system, a digital camera, and accompanying software. After the site evaluator levels the positioning system, the digital camera will take a set of photographs from nine positions. The software will then generate a composite panoramic photograph and add an overlay of the path of the sun. Using the historical solar radiation and weather of the site (from the National Renewal Energy Laboratory), the software will perform a shading analysis of the photograph and determine the quantity of solar radiation that can be predicted.

Solmetric SunEye is an integrated hand held electronic device and includes a touch screen interface, a fish-eye lens, a digital camera and measurement software. The digital camera automatically photographs a panoramic view of the entire horizon, identifying the obstructions. The site location’s sunpaths are created. After taking past regional weather into account, the Solmetric SunEye will generate reports and bar charts showing shading percentages and expected solar exposure of the site.

Which one of these solar site evaluation tools is the best? The Solar Pathfinder is the only one requiring assembly, which may be inconvenient for the site evaluator. It also may not be as accurate since site evaluators may not always position their digital cameras in the precise location over the plastic dome. The Solar Pathfinder does, however, have the advantage of being able to be used during any time of day or weather. The Solmetric SunEye, being a hand held device, may lack the stability of a stationary device and possibly not be as accurate. In addition, the weather data used is regional, not local to the site, and is not acquired from the National Renewal Energy Laboratory. The Solar Site Evaluation Tool seems to be the most convenient of the three for the site evaluator, although the quality of the photos may be compromised by sun and weather conditions. However, it is perhaps the most accurate of the three evaluation tools.

The costs for these units varies tremendously.  The Suneye is the most expensive coming in around $1,300.  The asset at just over $500 and the Pathfinder at around $300.  Each of the tools can do the job well but the only question is how easily they do the job.  If you are looking for gee whiz then the Suneye has it.  If you are just looking to get the job done professionally I would recommend the pathfinder with your own digital camera.


Solar Simulation Software

While solar site evaluation tools determine the best location for the solar collectors used in a solar heating or photovoltaic installation, solar simulation software provides the software tools to help design and simulate a solar energy installation and facilitates the design making process.

Three leading companies that offer solar simulation software packages to those in the renewable energy fields are RETScreen International, Vela Solaris AG and Valentin Energy Software.

At the onset of a new renewable energy project, each one of the three companies’ software package allows the user to input information regarding the site’s location and define the energy requirements for the project. The user may then select a template or configuration from the software’s database, and in addition, may select specific commercially available products to be used. The software will generate the simulation evaluation which will include financial feasibility and potential cost savings, calculation of weather data, energy production, expected energy savings and emission reductions of the project.

RETScreen International developed their Clean Energy Project Analysis Software with the input of experts from government, industry and academia from around the world. RETscreen is applicable to various categories of renewable energy, not just solar energy. The software consists of a series of worksheets to be filled in by the user, along with a comprehensive database that is the largest and most detailed of the three software packages. The user may omit any of the worksheets that do not apply to their project. It is suitable for large commercial and governmental projects, although it may also be used with smaller residential undertakings. RETscreen software may be downloaded free from their website.

Polysun Simulation Software was developed by Vela Solaris AG, a Swiss corporation. Polysun Simulation Software is a set of four software programs specifically created for the design of heat pumps, solar thermal, photovoltaic and cooling systems. Each software program is available separately or as a complete set of four. In addition, each program is offered in three user levels, light, professional and designer. The evaluation process provides detailed reports in PDF format, including colorful graphics, and is a significant feature of this software. A Polysun demo copy may be downloaded for examination on the Vela Solaris AG website. Polysun is also available for purchase on the site.

Valentin Energy Software, based in Berlin, Germany, offers two solar energy simulation programs. Their T’SOL software was conceived for solar thermal energy systems, while their PV’SOL is for used in the design of solar photovoltaic systems. Both programs are also available in three user levels, express, professional and expert. The company’s Meteonorm software, a global climate data database, is also available separately. Customized versions of T’SOL and PV’SOL may also be developed by Valentin for individual user objectives. Valentin Energy Software may be purchased through distributors or online on the company website.

What one of these is the best solar simulation software package? Our suggestion is to download the free copy of RETscreen and try it out. If RETscreen does not meet the needs of your applications, then download the demo copy of Polysun and determine if this software is a better choice for your company. In our evaluation of the T*Sol product we found that it didn’t compete effectively with the Polysun for a number of reasons.  Most notably was the lack of SRCC certified collectors and standard U.S. components.  If RETscreen or Polysun does not provide the requirements that you need, perhaps your company should consider a customized version of Polysun, T’SOL or PV’SOL.

Although the main objective of each of these software packages is to facilitate in the design process, the professional looking reports generated by them will also assist solar energy companies in the marketing of their products to potential customers.

Solar Installers

After reviewing dozens of websites, we have come up with a top ten list of our favorite solar water heating installer websites.

Each of these websites has introduced their company to the potential customer with content that is informative, clear, concise and up to date. Each one has provided a description of a typical solar water heating installation, some with illustrations. All of these websites have also included their company’s solar energy credentials and the reasons why they should be chosen as the customer’s installer. Some have incorporated photographs of their recent installations, with customer feedback. In addition each one has presented the information in a professional looking website that is easy for the user to navigate.

Since websites should be designed as marketing tools, each of these websites has been graded on the elements of aesthetics, content and navigation, and have been given a numerical score in each. Additionally each website has an SEO grade based on its grading from the website Having a great looking website is only part of the battle since you need the customers to be able to find your website as well.  If you notice our top website isn’t the best looking and also doesn’t have the best content but it does have the highest visibility to the search engines and is passable in the other respects.

Our top ten list of solar water heating installer websites is as follows:

#1 ECS Solar Energy Systems, Inc located in Gainesville, Florida

Excellent slide show of various types of solar panels installed on homes.

Aesthetics 5, Content 8, Navigation 10, SEO Grade: 91

#2 Dovetail Solar And Wind located in Cincinnati, Ohio

Extensive “frequently asked questions” area with important information.

Aesthetics 7, Content 10, Navigation 10, SEO Grade: 68

#3 Solar Assist, located in Eugene, Oregon

Impressive photo gallery of recent solar heating projects

Aesthetics 9, Content 7, Navigation 10, SEO Grade: 55

#4 Greenhead Solar, located in Spokane, Washington

Excellent descriptions of solar panels.

Aesthetics 5, Content 8, Navigation 10, SEO Grade: 52

#5 New England Solar Hot Water, located in Duxbury, Massachusetts

Informative explanation of solar basics

Aesthetics 8, Content 8, Navigation 10, SEO Grade: 36

#6 Enviro Plumbing, located in Santa Monica, California

Extensive photo gallery including pictures of workers installing panels

Aesthetics 8, Content 9, Navigation 6, SEO Grade: 34

#7 Boston Solar Living, located in Medford, Massachusetts

Great information on tax credits and rebates

Aesthetics 7, Content 10, Navigation 6, SEO Grade: 29

#8 Creative Solar USA located in Canton, Georgia

Read an open letter from the company’s founder on why he went green

Aesthetics 9, Content 10 , Navigation 10 , SEO Grade: 18

#9 Aladdin Solar, located in Excelsior, Minnesota

One of the few websites to give estimated solar water heating prices

Aesthetics 5, Content 7, Navigation 9, SEO Grade: 24

#10 Solar Heating Services located in Berlin, Wisconsin

Read interesting case study of one their latest solar projects

Aesthetics 8, Content 8, Navigation 8, SEO Grade: 12


If your solar water heating installer website is on our top ten list, congratulations!

If not, contact us with a link to you website and we will review it for our future top ten lists.


The Growth of the Solar Water Heating Industry

The U.S. solar water heating industry produced a record year of growth in 2008, with a 50% increase in capacity compared to 2007. By the end of 2008, approximately 139 MWTh (MegaWatts Thermal equivalent) was installed, bringing the total installed capacity to about 485 MWTh, according to a report released by the Solar Energy Industries Association.

One of the main reasons for the growth of the solar water heating industry was the extension of the residential and commercial solar investment tax credit. Additionally, the public has become increasingly aware of solar energy options available and more concerned about the overall energy crisis.

Even with the significant growth of the solar water heating industry in 2008, solar energy (solar water heating and photovoltaics combined) accounts for only 1% of the total U.S. energy usage. This small percentage is mainly concentrated in several states, possibly as a result of varying state incentives. The state of Hawaii, with its tax credit of 35% of the cost of a solar hot water installation, accounted for 37% of the total MWTh installed in the U.S. in 2008.

The U.S. currently ranks fourth in the world in installed solar energy capacity (solar water heating and photovoltaics combined). Germany ranks first, with Spain and Japan ranking second and third respectfully.

It is expected that the U.S. solar water heating industry will continue to grow in the coming years as the country confronts the issue of reducing solar energy costs to the same level as that of conventional fossil fuel energy.

Solar Blogs

There are many blogs on the internet devoted to the subject of renewable energy, and specifically to solar energy. In searching the web, the following six blogs are among the most unique and interesting ones that we found. Our ratings for each blog are also included.


A particularly noteworthy solar energy blog is Solar Energy Rocks!. This blog primarily focuses on informing homeowners about rebates, grants, subsidies and tax credits available in all 50 states and the District of Columbia. The writers of the Solar Energy Rocks! blog believe that the reason that residential solar systems are not more prevalent is because homeowners are unaware of and misinformed about these incentive programs. Along with blog entries, visitors to the blog can click on their state to retrieve this information which is presented in an easy to understand format. In addition,  Solar Energy Rocks! Has recently added two podcasts to their blog. This renewable energy blog can be found at

Quality of content: 10

Applicability: 10

Aesthetics: 9

Frequency of posts: 10


The Build It Solar is a relatively new renewable energy blog. The objective of this blog is to respond to questions that have been emailed to their website. The blog’s entries primarily deal with installation issues. Professional installers and do-it-yourselfers alike will benefit from the information offered by this blog. Recent posts regarding the building of a solar water heating system for $1000 and its continued performance will be of particular interest. Visit this  blog at

Quality of content: 10

Applicability: 9

Aesthetics: 9

Frequency of posts: 9


Clean Technica is a renewable energy blog published by Green Options Media Production. This blog’s intention is to provide the general public with information on renewable energy options available and to describe actual applications of this technology. The Clean Technica blog is divided into categories, one of those categories being “solar energy”. Written for the lay person, blog entries to the Clean Technica blog are contributed by knowledgeable experts in the field. Visitors to the blog are encouraged to add their comments or questions, to which blog writers will respond. A recent blog post describes how a California company installed a solar power system in its warehouse facility. Other recent blogs include the U.S. Army’s solar energy system at a California base and the Energy Information Administration’s recent news of the significant increase in renewable energy in the U.S. Users may click on to be directed to the solar energy category of the Clean Technica blog.

Quality of content: 8

Applicability: 8

Aesthetics: 10

Frequency of posts: 10


Powered By Solar Panels is the name of another solar energy blog. In addition to offering information about the benefits of renewable energy, this writer of Powered By Solar Panels is deeply concerned about the damage that the use of coal, oil and natural gas has done to the environment. Subjects of recent blog entries include a comparison between conventional and solar energy, an description of how solar energy works, how to generate solar house plans, and an explanation of why renewable energy solutions are necessary. Visit Powered By Solar Panels at

Quality of content: 9

Applicability: 9

Aesthetics: 9

Frequency of posts: 8


Get Your Green On is another renewable energy blog. The user will find recent posts pertaining to solar energy in the “green energy” category of this blog.  The Get Your Green On blog is geared towards the residential home owner who is looking for increased energy efficiency in their home. Written in an easy to read style, subjects of recent posts include selecting a reputable renewable energy company, the installation of solar panels, the recent technological advancement of solar panels, the functionality of solar panels in cloudy winter months, how a solar energy system adds value to a home, how to build your own solar panels and more. The Get Your Green On blog is available at

Quality of content: 10

Applicability: 8

Aesthetics: 6

Frequency of posts: 10


The Residential Solar blog is published by The Solar Energy Source. The goal of this renewable energy is to educate the community, especially those uninformed, about how carbon emissions can be reduced for a cleaner environment by the use of solar energy solutions. User feedback is promoted with an email address for questions or comments. Recent entries this blog consist of the steps to take for a solar system installation, the top ten reasons for using solar panels and more. In addition to The Residential Solar blog, The Solar Energy Source also publishes several other renewable energy blogs. Visit The Residential Solar blog at

Quality of content: 9

Applicability: 8

Aesthetics: 8

Frequency of posts: 6

Solar Water Storage Tanks – Pressurized or Atmospheric?

Solar water heating systems almost always require some kind of thermal storage for solar heated water. Tanks used in solar water heating systems are available in two basic types: pressurized and atmospheric.

Pressurized tanks, constructed of stainless steel or welded steel with a baked-on glass liner, are made to withstand city water pressure without rupturing. Polyurethane foam insulation is used in between the metal skin and interior tank to minimize heat loss. Pressure relief valves are included in the design of pressurized tanks. If the pressure builds up too high inside the tank, the relief valve opens and water is released. Pressurized tanks range from about $500 for an 80 gallon capacity to about $17,000 for a 1,000 gallon capacity. The life span of a pressurized tank around 13 years but can vary depending on local water conditions and tank maintenance (changing the anode rods regularly).

Atmospheric tanks are basically just non-pressurized containers to hold solar heated water. Since they are not completely sealed from the surrounding atmosphere, they should be well insulated. An atmospheric tank that is filled beyond its capacity will overflow. In addition, atmospheric tanks will lose water through evaporation. A variety of materials are used in the construction of atmospheric and include stainless steel, EPDM lined steel, EPDM lined wood, polypropylene and fiberglass. Tank costs and longevity vary with material selection.

A two tank solar water heating system consists of a solar storage tank (it can be atmospheric or pressurized) and a pressurized backup tank. A one tank solar water heating system uses a pressurized tank which serves both as the solar storage tank and the backup water heater.

Simply stated, when using an atmospheric solar storage tank in your design, heat from the collectors is delivered first to the atmospheric tank, then on to the pressurized tank. When no atmospheric tank is used in the design, with only a pressurized tank, the solar heat can be sent directly to the one pressurized tank.

Should you incorporate an atmospheric solar storage tank in your solar thermal design? Your solar equipment manufacturer should be able to help you decide what is best for your individual needs. However, if your tank capacity needs to be over 240 gallons, it may make sense to include an atmospheric tank for economic reasons.

Solar Pool Heating Systems

Solar pool heating systems are one of the most cost efficient uses of solar thermal technology. The cost of heating the average residential pool using a gas or electric heater is approximately $2,000 per year, according to the US Department of Energy. The cost of the average solar pool heating system is between $2,000 and $4,000. Although solar pool heating systems are exempt from federal and most state incentives and rebates, the payback period is usually between one to two years. After the payback period, owners of solar pool heating systems can enjoy their solar heated pool for free!

Solar pool heating systems work much the same way as drainback solar domestic hot water systems. In the collector loop, the pool water is heated in the solar collectors. The pool pump will be turned on and the flow of the pool water will go through the panels when the collector temperature is greater than the measured pool temperature. A pool heating system generally doesn’t include a heat exchanger.  When the pool panels are no longer hotter than the pool (or the maximum desired temperature is reached) the flow is no longer directed through the panels.  The panels (with the assistance of a vacuum breaker) would then drain the fluid back into the pool.  If a certain temperature is required then a back-up heater would be installed after the pool panels prior to the heated water reentering the pool.

If your home already is already utilizing a closed loop solar domestic hot water system, you may be able to integrate a solar pool heating system. A valve would be installed on the glycol mixture’s return line. This valve would be able to divert the heated glycol mixture to a second heat exchanger to heat pool water. A sensor and controller would be required to prioritize the two loads. The domestic hot water system would receive first priority. The heated glycol mixture would be diverted to the pool heating system, the second priority, only after the required temperature of the domestic hot water system had been met.

The above integration of a solar pool heating system into an existing solar domestic hot water system may not be employed in an open loop system.