Solar Thermal Hot Water Collector

Solar Thermal Hot Water Collector

  Solar hot water 026 

There are many ways to reduce energy demand while maintaining a high standard of living.  Technological innovations and their commercial applications (with its appropriate lag time or gestation period) changes our environment and in turn the new environment changes us.  The web site, “Build it Solar” (BIS) has lots of great inexpensive do-it-yourself wealth creating solar projects (http://www.builditsolar.com).  There are many detailed projects on the site for the learning and doing.  They range from low to high budget designs and from small scale to whole house designs. 

A solar thermal hot water collector collects sun (solar) energy (in this case thermal) and transfers it to a domestic hot water tank for direct use or as a per-heater for the regular gas/electric hot water heater inlet water.  If the system is large enough it can also be used to heat the house in part or in whole.  The solar thermal hot water collector combines, the physical (material) and metaphysical (know how, scientific principles) to build real wealth (synergetic).

 

Collector

 

Figure 1.  30 unit evacuated glass tube collector.

Figure 1. 30 unit evacuated glass tube collector.

The basic function of the collector is to capture the incoming sun energy as heat and transfer that heat to a primary loop that will, in turn, transfer its heat to the domestic hot system.  Each glass tube is evacuated, in vacuum, and has mounted at the center and through out its length a sealed copper pipe/tube with some water in it.  The copper tube has attached to it a thin metal fin on each side that runs the length of the evacuated tube.  The fins and the copper tube nearly fill the diameter/cross section of the evacuated glass tube. At the top of the glass tube and penetrating about four inches above the evacuated enclosure is the upper end of the internal copper tube.  This copper end section is a little larger in diameter than the internal copper tube and looks like a bulb. This bulb section is inserted into the primary loop header hidden by the stainless steel cover at the top of the collector.  The header is designed so that the copper bulb connects to the header (surface to surface) to transfer heat through conduction but in no way mixes with the primary loop fluid.  

Figure 2.  The centered copper tube and attached metal fins.

Figure 2. The centered copper tube and attached metal fins.

The purpose of the evacuated tube design is simple, no sense in wasting heat energy on air molecules and also to keep water vapor out. The fin’s surface area collects heat energy and transfers it to the copper tube.  The water in the copper tube (negligible amount) super heats into steam and through convection raises to the bulb to transfer its heat to the primary loop whereby becoming relativity cooler and denser thus descended back down the copper tube, a passive self regulating design. 

Since the collector is roof mounted you need to figure out the angle above the horizon that works best for you.  You can get started by going to the BIS web site in the solar site survey section (http://www.builditsolar.com/SiteSurvey/site_survey.htm) and printing out the sun chart for your area (free from the Un of Oregon).  Since this collector is in the Seattle area the angle selected coincided with being near perpendicular (best efficiency) with the afternoon sun  in October and March (larger angle to the horizon) since the sun is low on the horizon.  The angle of the collector isn’t a major factor between March and October, less efficient, because there is so much sun energy getting through.  It made sense to maximize the efficiency in the not-so-sunny months between October and March. 

Primary Loop  

The function of the primary loop is to transfer and move heat form the collector (via heat exchange) to the solar hot water tank to be absorbed via a heat transfer coil inside the solar hot water tank.  Since, in this case, the collector is above the hot water tank a pump and computer controller is required to move the primary loop fluid around.  Also since the Seattle climate does experience freezing weather the primary loop fluid is a food based glycol (non-freezing) vice water.  

Figure 3.  Pump and controller for the food based glycol primary loop

Figure 3. Pump and controller for the food based glycol primary loop

It’s important to note that the food based glycol primary loop fluid circulates in its sealed loop and never leaves it.  The insulated primary loop (made of copper at the header and stainless steel everywhere else) does all its heat transferring through conduction.  

Figure 4.  The computer controller for the primary loop pump.

Figure 4. The computer controller for the primary loop pump.

The computer controller measures the temperature at 1) collector header (in the picture it shows 149 degrees), 2) lower, and 3) upper portions of the solar hot water tank and determines when the pump is turned on and varies its speed accordingly.  The screen display shows the basic set up flow of the system on the far left.

 

Solar Hot Water Tank

The solar hot water tank (special 80 gal design) has a stainless steel coil in the lower portion of the tank where the primary loop fluid is circulated through to transfers its heat via conduction to the solar hot water tank.  In this particular set up the solar hot water tank feeds the regular 40 gal house hot water tank.  In this set up the solar hot water tank is a pre-heater for the water entering the house hot water tank since the sun doesn’t shine regularly in the Pacific Northwest (specifically winter).  

Typically in the Winter months the 80 gal solar hot water tank will go from 58 degrees (incoming water temp) to 90 degrees on a sunny day, 77 degrees on a average day and stay at 58 degrees on most rainy days. On a sunny early Fall and early Spring day the tank will go to 109 degrees and to 125+ degrees the rest of the year.     

From mid Spring to mid Fall the water entering the house hot water tank is mostly above the temperature setting on the tank so the house hot water tank doesn’t kick on all that much.  In the other months the water entering the house hot water tank is almost always above the incoming water temp of 58 degrees so you still save energy with the pre-heated water. 

Figure 5.  80 gal hot water tank

Figure 5. 80 gal hot water tank

 A Better design

The best position for the collector to collect the most heat energy is to be perpendicular to the sun through out the day.  In the above case the collector is roof mounted/fixed which means very rarely is it perpendicular to the sun.  A better design would be to have the collector on a pole mounted sun tracker array system where it stays perpendicular to the sun through out the day. The tracker tracks the sun declination and moves the array both horizontally and vertically.  This greatly increases your heat energy capture to where you could heat more water at higher temps (~25% more heat energy for the same 80 gal tank)  If you had two collectors in series on a sun tracker array system you could produce enough heat energy for your hot water and heat the house in part or in whole.  In the above system the solar collector is above the water tank so a pump is required to circulate the primary loop.  A better design is to place the water tank above the collector to take advantage of convection or natural circulation.  The heated water exiting the collector is hotter/less dense and will rise naturally to transfer its heat to the water tank whereby becoming relativity cooler/less denser thus descended back to the collector.  It becomes a passive self regulating design.  You could still add a small solar heat pump to aid in the circulation and still be completely passive. 

We were born into the current social/economic structure and depending on where we checked in at, our sense of what’s normal/base case evolves.  At each time step we collectively tend to maintain the inertia of what’s normal until such time that it’s to far removed form the current normal and is abandon. Its not that we don’t have enough energy resources and need to get more and more (more geopolitical warring) it just that we don’t use what you have efficiently. We are highly inefficient.  In part, some of these inefficiencies are purposeful to keep volume sales/cash flows growing, some are due to special interest, some to job security/fear (trade union, etc) and some from not being informed of commercial off the shelf (COTs) technologies that are available and of many basic scientific principles.  Since it’s more effective, rewarding, and empowering to change things within our control, our first step should be to advance our own individual efficiency and the rest will follow.  

The total energy efficiency (work produced divided by energy input) of every day life in our society is in the 5-10% range.  If energy efficiency were to double then there would be twice the current amount of energy to operate the world. Think about it. This doubling is well within current capabilities.  Once a corporation sinks capital cost into a production line it is almost impossible to get new technologies/innovation upgrades integrated (like in electronic), the conditioned reflex is to “Milk It” (the Lawyers don’t allow the CEO to do anything that jeopardizes share holder return).  Turbine engines are twice as efficient as the current reciprocating car engine. 

The estimated annual cost for a 40 gal gas hot water tank is around $400 and $600 for an electric one (costs and usage varies geographically and with family size).  The solar thermal hot water system above has reduced annual costs by at least 60% in the Pacific Northwest.  The sun extracted energy is significant enough, it’s not 100% efficient, that it more than doubles current hot water efficiencies.  In this case we are not improving the efficiency of the hot water tank but augmenting energy inputs with sun energy.  Looking at the amount of non-direct sun-energy used to heat the same annual amount of water, we see it’s clearly more efficient.  What is your return on investment (ROI) ? ……..  Real Wealth.

 

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