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Which is Superior—Passive Solar, Hybrid Systems, or Active Solar?

The paper below is something I researched and wrote about at age 19 in college, and although we have made great strides in technology since then, I am amazed at how relevant the information still is in 2015, some 25 years later (boy am I starting to feel old, haha). However, I definitely have a few comments to add, especially in regards to hybrid solar systems.

I was a new student at the community college in 1990, and also a new student in the solar energy industry. Just in the few months of becoming active with the El Paso Solar Energy Association as a new recruit, and then elected onto the Board of Directors, and finally being voted in as Secretary, I had learned a few things about solar energy… namely, that passive solar was superior to active solar. In this case, my research paper was about solar water heating, and also touched on passive solar home design.

As you read the short paper below, which I have incorporated into this article, with an intro to explain a few things, I think you will see how the information is still applicable today, but after the paper I have added some commentary in a discussion section as to why there are times that hybrid systems may even surpass some passive systems in quality and quantity, without raising the price too significantly.

So what is the difference between passive and active solar, you might ask?

Furthermore, what is a hybrid system?

direct gain

Passive Solar House

Passive Solar

Passive solar has no moving parts, therefore it “runs” passively, using no electricity or mechanical features.

An example of a passive system is a any house with windows. It just sits there and does not move. Sunlight shines through the windows and warms the floor and interior, and passively heats it. Although some houses do this better than others (based on southern exposure/orientation, window area, thermal mass ratios, and overhang specifications), it works by sheer design. The house itself becomes the solar collector and has no moving parts in order to “work.” If a house has windows that let the sun in (and most all of them do) then it is a passive solar house.

Active Solar

Active solar is the opposite, where electricity is typically involved and it also runs by mechanical means.

An example of an active solar system is a solarized toy car. Perhaps you have seen these, where the small solar panel on top of the toy puts energy directly into the car’s mechanisms so that it scoots across the floor. Although active solar can accomplish a lot of work in a short period of time, it is usually more expensive up front, has mechanical parts that eventually fail or need maintenance or replacement, and so cost more in the end as well.

Hybrid solar systems

Hybrid solar systems are a combination of the two systems named above.

An example of a hybrid system would be a solar PV (photovoltaic) array—solar panels that utilize sunlight and turn it into electricity, which is usually stored in batteries. PV panels themselves are passive in nature—they just sit on your roof or on the frame and quietly do their thing. However, most of these systems are designed to “run” something, such as a water pump, or a fan, or other devices, and so although the mechanical moving parts are considered “active,” the system as a whole would technically be a hybrid system of both passive and active parts.

With this foundation in mind, you can now read the paper I wrote at such a young age, and hopefully you will glean some knowledge. Remember, I will have a discussion at the end, after the cited works, so be sure to scroll down to the very bottom after reading…

PASSIVE-Hybrid-ACTIVE
The Superior Passive Solar!

Written April 1990

When active solar attempts to match passive solar, the differences yielded between them are great. One aspect in which this shows true is when solar water heaters are concerned. Not only is a passive solar water heater more simplistic in design but it is longer lasting and more efficient than an active solar water heater.

There are two pamphlets that are free from the Texas Energy Extension Service. One is “Active Solar Water Heating” and the other is “Passive Solar Water Heating”, which can be acquired by writing the Governor’s Energy Management Center, P.O. Box 12428, Austin, TX 79711. Active solar water heaters are basically one of two kinds. There are direct (open-loop) systems and indirect (closed-loop) systems.

The direct systems heat your household water inside the collector itself. Because of this, they must be protected from freezing. Also, these systems have a problem with scale build-up. The build-up of scale concentrates in the collectors and all the piping, which is in essence, the entire active system.

The indirect system has fewer problems with scale except in areas with hard water. They don’t freeze as readily as the direct system because a separate fluid circulates through the solar collectors and then through the heat exchanger which heats the household water. These fluids are usually a type of antifreeze, distilled water, etc.

The problem with the antifreeze fluids (especially in the phase-change system) is that they sometimes corrode the collector and eat through them, thereby contaminating the water or leaking into the atmosphere. The pamphlet also concludes that these systems are less efficient as a whole, and are poor candidates for home-owner installation.

The active solar water heaters easily cost around S2,000 or $4,000 installed, plus an average of one to four percent annual maintenance cost of the initial system’s price tag!

The water heaters- according to the ”Passive Solar Water Heating” pamphlet- of passive systems have well up to a 50% ability in providing the necessary hot water needs of a household. The advantages to a passive design are obvious as stated in the paper, ”Relative simplicity and economy are attractive features of passive solar water heating systems. These systems involve few mechanical parts and are generally uncomplicated in design and construction.” (Texas 1) The hardest part is simply making sure that there isn’t any shade in the way of the water heater’s sunlight and directing it due south.

The sunlight’s maximum intensity occurs during the hours of 9 a.m. to 3 p.m. The direction should be to the true south but it may vary up to 20° to the east or the west and still yield good results.

Now that the positioning of your system has been established, it is important to know what types of solar water heaters there are to choose from. Basically, there are two kinds: The batch solar water heater and the breadbox system.

Batch systems are the least complicated of all solar water heaters because the solar collector and water storage tank(s) are combined into one unit.” (Texas 1) The paper also mentions that the water tank(s) are painted flat black- to absorb 95% of solar radiation for heating the water.

The most common type of batch system is the “breadbox”. These breadbox water heaters can be purchased from the manufacturer ready-made or they can easily be assembled by anyone with basic handyman skills and the necessary tools needed. (Texas 1-2)

If you have to buy the materials: new- glazing, water heater tank, 2-by-4 framing, plywood, caulking and insulation – a single tank system can usually be built for less than $650. This amount can drop considerably if some of these items are already on hand.

The insulation that is used is often times for the outside pipes. If the pipes tend to freeze in the colder regions of Texas, electric heat tape may also be used, but the large mass of hot water in the breadbox system keeps the water from freezing in all but the most severe climates.

There are even further ways to prevent heat loss. Besides heavy insulation in the box behind the glazing there should

be a reflective interior facing the tank. As an extra, a hinged, reflective and insulated lid can be made to be put over the glazing during the nighttime.

The “Passive Solar Water Heating” pamphlet also quotes, ”The main appeal of passive solar water heating is its simplicity and relative low cost. In addition, passive systems are generally very reliable because there is little that can go wrong. In an area where conventional energy prices are high, passive systems can be cost effective, especially if you can do the work yourself,” and it goes on to say, “Owners who understand the limitations and proper operating characteristics of passive solar water heating are usually happy with their systems and consider them a good investment.” (Texas 4)

Lastly said about passive solar water heaters is, “Moreover, many people take satisfaction in knowing that they are using an environmentally safe, non-polluting, renewable source of energy – regardless of its cost efficiency relative to conventional fuels.” (Texas 4)

Another area that should be covered besides passive solar water heating is passive solar heating for inside the home. The most known and used form of heating a home is the burning of fuel. Charles Wing calls these systems “Fuel Heat”. This illustration shows the different types of heating options to the home owner. (Cole and Wing 188)

Charles Wing says that even with active systems they have to use “auxiliary fuel-heat systems” anyway and that they “often provide more than half the heat as well.” Passive solar heating systems “also have auxiliary fuel-heat systems,” he goes on to say. But the major difference- even though both types have a back-up fuel-heating system – is that same argument between which active and passive solar heating boils down to. (Cole and Wing 189)

This is what Mr. Wing has to say, “It is possible, through the application of vast arrays of plumbing and money, to collect, transport, store and distribute solar heat. The heat is generated in glazed rooftop collectors, transferred in water or air, stored in a basement tank or rock bed and distributed upon the command of a thermostat. Those systems are called active solar.” (Cole and Wing 189)

After all this, it’s concluded, “Passive solar systems have none of the controls, plumbing, pumps, blowers, or thermostats of an active system. In a very real sense, passive solar buildings are simply solar collectors in which people live,” (Cole and Wing 189)

The three major types of passive solar heating systems are Direct-gain, Indirect-gain (mass-wall), and Isolated-gain (sun space).

Direct-gain systems allow sunlight directly into the living space in which most of the sunlight is absorbed by the first surface it strikes. Charles Wing calls this absorption of sunlight “sensible heat” because “80 to 90 percent of the sun’s energy is absorbed by the surfaces of the floors, walls, ceilings, or furnishings.” (Cole and Wing 189)

Furnishings and lightweight fabrics soak in heat rapidly and warm the air around them. This quick-releasing heat helps warm the house during the daytime. The sunlight that strikes massive surfaces like masonry floors and walls is “absorbed without great temperature change and later released over a long period.” The massive heat storage is usually used to keep the building warm through the sunless night, and sometimes through the next day also. (Cole and Wing 189)

This brings us to Indirect-gain or mass wall systems placed between a glazing and the living space. Quoted on this is “warm air generated in the narrow space between the glazing and the mass wall is vented into living space for daytime heating, and the remainder of the heat is stored in the mass wall for later release.” (Cole and Wing 191)

In an eight-inch thick mass wall, the heat should release in about eight hours after the peak solar gain. This is true because in a mass wall, the heat penetrates the concrete at a rate of about one inch per hour.

The third type- Isolated-gain (sun space) – enlarges the space between the glazing and the mass wall thereby creating a living space that is pleasant with heating capabilities similar to the regular mass-wall system. If the back wall in the sun space isn’t a heat-absorbing mass wall, a fan that is thermostatically controlled may be used to blow excess heat into the house and frost-preventing nighttime heat back into the sun space, which can also be used as a greenhouse.

Greenhouses promote a rise in humidity which may show a good amount of sharing of warm air between the living space and green- house.

Charles Wing suggests, “of the three types of heating, I prefer passive solar, and of the three types of solar systems, I prefer direct-gain. The reason is quite simple and consistent with my philosophy: It doesn’t cost much!” (Cole and Wing 190)

Showing the energy-juggling act of the next illustration [image not available], you can see three building heat losses – conduction, convection, and radiation – compared to the three free heat gains – animal, utility, and solar gains – aren’t constant. Seasons and cloudiness sometimes interfere with these applications, but overall – especially when the design of the building is involved – the outcome of the benefits are surely worth it.

The last area that is to be expounded upon is solar building design. Ralph M. Lebens mentions towards a design philosophy on active vs. passive on the approaches to low-energy home design.

He tells about the ”solar collecting panels, storage tanks or bins, an energy transfer mechanism and an energy distribution system. It is known as an active system and always employs one or more working fluids which collect, transfer, store and distribute the collected solar energy.” (Lebens 1)

The more familiar passive design “seeks to reduce the house’s energy budget by close attention to orientation, insulation, window placement and design, and to the subtleties of the energy transfer properties of building materials. Since solar gains are present in every building, all buildings are passively solar-heated to some extent.” (Lebens 1)

The simplicity of passive solar design and cost savings is put ·forth as he goes on to say, “Although often less spectacular or graphic, this second approach produces better results in terms of energy conservation and money saved as a result of initial expenditure. An active space heating system will have difficulty in producing less than a 30-year payback figure. With passive systems, payback periods of anything from 1-13 years are usual. The durability of passive systems further increases their economy. They require little maintenance and will last the lifetime of the building. Thus, they will not require replacement, as may be the case with certain parts of active systems.” (Lebens 3)

Intermittent sunshine or overcast skies when using an active system will cause it to shut down but a passive system will continue collecting.

The AIA Research Corporation published a book called ”A Survey of Passive Solar Buildings”, which summarized many constructed passive solar projects. In the survey they mention that, “the definition of a passive system, then, is determined by the dynamics of its operation rather than any static rules or any aesthetic image. This expresses a functionalist approach to architecture, in which the passive solar system is an intrinsic part of the building, and requires the designer to broaden his concept of the building.” (AIA 17)

As in isolated gain, where a greenhouse is usually present, they say that “To improve the efficiency of passive solar systems, it is sometimes helpful to assist the natural energy flow with mechanical means, a small fan for example where mechanical means are used to assist energy flow from the collector to storage or from storage to the conditioned space, but not both, the system is called hybrid, employing elements of both passive and active systems. The adjacent chart [chart not available] helps distinguish between active, passive, and hybrid solar systems.” (AIA 17)

It is obvious by viewing the chart that not even the hybrid is considered priority above the basic passive system. Sticking to the most natural and efficient system is the best and cheapest way to go in solar home building design.

When designing a home and applying solar heat gain, it should be known that the emission affects buildings in two ways. This is depicted by S.V. Szokolay:

  1. Entering through windows, absorbed by surfaces inside the building, thus causing a heating effect
  2. Absorbed by outside surfaces of the building, treating a heat input into the fabric, which will be partly emitted to the outside, mostly by convection, but will be partly conducted through the fabric and subsequently emitted to the inside (Szokolay 51)

Both of these effects – if they can’t be determined- can be at least influenced. “The transmission through windows is determined by”:

  1. Orientation of the window (thus the intensity of radiation incident on its surface)
  2. Size of the window
  3. Type of glazing (clear, heat absorbing, heat rejecting, or photochromatic glasses)
  4. Shading devices, either external (grilles, louvres, canopies, awnings, shutters) or internal (blinds, curtains) (Szokolay 51)

Most windows should be faced due south to receive the most amount of sunlight possible. But the standard house has terrible window positioning as Charles Wing’s chart shows [no chart available].

There is so much more that can be discussed where active and passive solar applications are regarded. But whether it’s solar water heating, solar heating and cooling or the most important of these, solar building design, it is most obvious that the passive solar design is indeed the most efficient, profitable and economic of all.

Not only is passive solar environmentally safe, easy to apply and install, wonderfully effective and a major savings for the years to come, but it will help keep the fuel consumption in this country down as more and more people are implementing passive systems in their homes. The best thought in this conclusion is that passive solar designs of all types are truly the wave of the future and also the hope of the future’

[Scroll down for discussion]

Works Cited

AIA Research Corp. A Survey of Passive Solar Buildings. Washington D.C.: The American Institute of Architects, 1978

Cole, John N. and Charles Wing. Breaking New Ground. Boston, N.Y.: The Atlantic Monthly Press

Lebens, Ralph M. Passive Solar Heating Design. London, England: Applied Science Publishers Ltd, 1975

Texas Energy Extension Service. Active Solar Water Heating. Austin, TX.: Governor’s Energy Management Center, 1989

Discussion

I will be honest, although I may have tweaked a few things had I written this paper today instead of at age 19, it is almost entirely still applicable today, meaning that I still prefer passive systems to active or hybrid systems overall. However, there has been a switch in my thinking at different times, and occasionally I find that hybrid systems, if kept to a very minimal standard for mechanical or moving parts, can at times be a boon to the homeowner, and may actually be superior.

Examples?

Sure… one is a radiant floor heating system, and another is a commercial sized solar water distiller.

Hybrid radiant floor heating

Radiant floor heating is usually made by running heated water through tubes in the concrete or other flooring, preferably thermal mass (which conducts heat) rather than an insulator like wood (which retards heat transference). In a hybrid system you might find a passive solar water heater and insulated storage tank coupled with a hot water heating backup (could be gas, electric, tankless hot water heater, or woodstove, boiler, etc.), and then a small pump that circulates the water through the floor, all temperature controlled by a thermostat.

I stayed at the straw bale Studio room at the Black Range Lodge one time in Hillsboro, NM that had a radiant floor heating system and I was sold on this type of heating method from that point forward. Where a passive solar system might keep it warm throughout, a hybrid system or even an active system would allow on-demand heat, and with R-50 insulated walls it does not take much to heat up the space very quickly.

A passive solar heating system example is those big flat rubberized panels that sit on a house roof to heat water for a swimming pool. Many of those are passive, which means they heat and then circulate via convection, which is a very slow process. To heat a house this way would be harder to control, temperature-wise. A well-designed passive solar house, with a hybrid heating system, might be the best option, depending on the circumstances and design of the building and heating system.

Hybrid commercial solar still

I have installed over 100 solar water distillers in the field, which means at peoples’ homes in southern NM, west TX, and even colonias in Mexico. Solar water distillers take contaminated water and use the sun to purify (through distillation, which is evaporation and condensation) the water to make it potable. Passive solar stills use no electricity whatsoever and do not require running water (water pressure) either. You simply put water in (gravity feed or a hose) and let it sit in the sun and it works by itself. At the end of the day you have purified water, basically for free. The water is usually a neutral pH, and tastes great.

solar still

Solar Water Distiller

Alternatively, I have been the not-so-proud owner of a forced-distillation countertop water distiller that plugged into the electric outlet. It used enormous amounts of electricity, boiled the water and condensed it, and although it was fast, the water tasted metallic and it had an acidic pH, which over time wreaked havoc on my stomach. We had to let the water cool, put it into bottles, and then aerate it and let it sit a while longer before drinking it, sometimes adding baking soda to make it more alkaline or neutral, and to make it taste better. We had a contaminated well at the time so having this distiller was crucial to my family’s health, but it would have been far superior if we could have had a passive solar water distiller instead.

Fast forward about 10 years, when I met Hill Kemp, who is the owner of Suns-River.com, which is a company that builds hybrid commercial solar still systems. Hill Kemp actually placed his prototype in my backyard in El Paso for a year or so, which helped him glean some initial findings for his still project. After removing the still, he moved on to build a bigger one in Alamogordo, NM.

This kind of system is far too large for a single family dwelling, but it uses concepts that I had learned over time that far outdo the standard passive solar still, at least in way of the output of product water. In fact, hybrid solar stills can generate up to 4-5 times the amount of distillate using the same amount of sunlight per square feet as a classic/passive solar still, simply because it utilizes some hybrid features, such as a fan, etc., and is engineered to function in a certain way.

Today, although I am still a major fan of passive systems, especially for their simplicity and lowest expense factor, there is no doubt that sometimes a hybrid system comes available that is superior in some way, but only if it is not fully an active system, which is easy to spill over into if one is not careful. People in general, I have noticed, somehow love to make things far more complicated than they need to be.

As for me, and my house, I will always choose passive solar first, definitely hybrid systems on occasion, and active last.

 

 

 

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