The flow rate and piping size are important considerations when designing and installing a solar hot water system.

The flow rate, measured in feet per second (fps), is generally recommended to be between 2 fps to 5 fps for a solar hot water system. If the flow rate is at the high end of this range, the heat exchanger will be more efficient and less scale will be created in the heat exchanger. A flow rate of greater than 2 fps is needed to entrain air through the piping. This is critical in a glycol system since a glycol solar water heating system will use some form of air elimination.  In order to make effective use of the air elimination feature in the system the air needs to be carried to the device that will capture and release the air.  If the flow rate is over 5 fps, excessive flow noise may be detected.  When you get beyond 8 fps erosion corrosion may be produced inside the piping as well as noise.  This internal corrosion of the pipes will ultimately lead to the system springing a leak.

Where solar collector manufacturers certify their product at a given flow rate solar collectors will operate well over a wide range of flows.  If you understand the trade-offs between; 1) entraining air, 2) noisy/corrosive flow, and 3) pump energy you will be able to select the appropriate line size.  The smaller the line the greater the pressure drop at a given flow rate.  The smaller the line the lower the cost for the line set as well as the insulation.  For most residential solar hot water systems, the inside diameter piping size should be between 1/2 inch to 1 inch. In addition to flow rate, piping size should also be determined by the length of piping needed, the type of pump used, the capacity of the collectors and whether the system is an open or closed loop.  As a general rule the following is the maximum flow rate you should plan on for different size copper pipe.

Generally, in designing a solar hot water system, using a larger pipe size will give you lower pressure drop.  The lower pressure drop will result in less pump required to overcome the pipe resistance.  This may (or may not) result in lower energy consumption for the pump.   However, using the minimum pipe size will be the most cost effective.

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.

Expansion Tank Sizing for Solar Collector Circuits
by John Siegenthaler , P.E.
April 1, 2009