Tilting of a Collector

One of the issues that we see come up from time to time is when people need or want to install a flat plate collector flat on a flat or very low pitch roof. While this is easier to install it brings with it it’s own set of problems.

Most flat plate collectors on the market today have some gasket or seal around the glass. This gasket or seal has on top of it a cap rail or a portion of the extrusion. This build up of material on top of the collector (gasket and top rail) creates a lip that will hold a puddle of water. Pretty much every collector on the market has a sealing system that isn’t designed to keep water out when it is fully submerged in water for a period of time. What this translates into is more than a normal amount of fluid entering the collector box.

Flat plate collectors should have ventilation holes that will enable the collector to breath and exhale any excess moisture that gets into the box as a result of humid conditions. The ventilation holes rarely can accommodate the extra moisture that would end up in the collector as a result of the collector sitting flat on its back in a rainy environment. If too much moisture gets into the collector box then the collector will have a tendency to sweat on the inside as the moisture evaporates and then condenses on the glass. Over time this will leave dirt and contaminants on the glass making the glass dingy and difficult to see.

For those of you that think this spells an advantage for evacuated tubes you would be mistaken. Evacuated tubes that use heat pipes require the collectors to be tilted up at 35 degrees in order to allow the heat to move successfully from the bottom of the tube to the heat exchanger at the top.

The long and the short of it is. Tilt your collectors if you want them to last and not have crud build up on the inside that could hamper future performance. We recommend a minimum of 8 degree tilt.

Flow Rate and Piping Size for a Solar Hot Water System

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

Pipe Diameter

Max Flow rate (gpm)

3/8”

2

½”

3

¾”

6

1”

12

1 ¼”

18

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.

Flow Rates of Solar Collectors

The efficiency of solar collectors is dependent on several features in a solar hot water system. In addition to the amount of solar radiation absorbed by the solar collector and the temperature of the ambient air, the efficiency is also determined by the flow rates of the heat transfer fluid in the solar hot water system.

At it’s most basic level any solar collector is nothing more than an air to liquid heat exchanger.  The sun provides the heating on the outside of the collector (air side) and the fluid flowing through the collector picks up the heat as it passes through.  In any air to water heat exchanger the amount of heat that is transferred over time increases as the flow rate increases. Along the same lines, any heat exchanger that the solar fluid passes through (either in a tank or external) increases it’s rate of heat transfer with higher flow as well.  So, higher flow rates increase both the amount of heat that is extracted from the collectors as well as the amount of heat that is passed into storage.  In general, the efficiency of solar water heating systems improve as flow rates increase. The reason all systems aren’t pumped at the maximum flow rate is because as the flow rate increases the pumping power required generally increases as well.  At a certain point the increased efficiency you achieve through higher flow rates is offset by the greater pumping power.

While we get the question all the time “what is the right flow rate for this collector?”  The real answer is hidden in the details.  We do not like to see systems that are pumped at a fluid velocity beyond what the piping can support (see previous blog).  That being said adding a flow meter to a system so you can make sure it matches exactly the “recommended” flow is counter productive.  Flow meters to confirm flow make sense.  Flow meters to control flow don’t.  Pumps come in a finite number of sizes and the best answer is to choose a pump that matches your system design.  When in doubt choose a larger pump (but not beyond the flow limits of the pipe) and the little you pay extra in pump energy will more than be made up in system output.