Household heating and cooling account for 55% of total residential energy consumption. Including refrigeration, 63% of household energy consumption goes into moving heat around, something that can be accomplished without ever having to burn fossil fuels or convert heat into electricity.
So whats the problem?
If an insulated panel could be constructed entirely of cheap thermoplastics, it would cost significantly less than current systems. Thermoplastics, as there name implies, melt if exposed to high temperature, and that's a real problem. Efficient capture of heat in a panel when the ambient temperature is low (i.e. the winter when its need most) requires insulation or glazing. Insulation prevents heat from escaping back into the air once it has been captured, but it also causes a problem. If heat is not removed from the panel, the temperature can rise to very high levels. Traditional panels are capable of withstanding high stagnation temperatures. A plastic panel could never handle these temperatures. The problem in a nutshell is figuring out how to build a solar thermal panel out of inexpensive plastic that is both insulated but also never overheats.
A Solution?
A particle panel for heating applications is oriented mostly vertical so that it is perpendicular to the sun, which is low on the horizon in the winter. Liquid flows up through the panel. Small black particles trapped inside the panel by two wire filters are pushed up by liquid flow, but are also pulled down by gravity. When the liquid is flowing, the particles distribute themselves over the panel and become an efficient light absorber. When the flow stops the particles sink to the bottom, occupying a substantially lower cross-sectional area. A mirrored or otherwise low-absorbing surface behind the panel then reflects the light away and the panel stays cool. The ability to turn off when not in use prevents the panels from exceeding the upper working temperature of thermoplastics, allowing for the construction of an all-plastic, light-weight, insulated panel that can be manufactured and shipped for very low cost.Prototype
The first prototype particle panel. Water from a garden hose enters from the bottom and exits at the top. The flow causes the panel to turn on, distributing the silicon carbonate particles and turning the panel black.
To validate the concept we constructed a prototype particle panel from double-wall polycarbonate and acrylic plastic. The panel turns on in ~10 seconds at household pressure, and turns off in approximately one minute. The power output was measured on a sunny November day in Santa Fe, NM and ranged from 750-830W/m2.We melted 45 micron stainless-steel wire mesh into the top and bottom of a double-wall polycarbonate sheet and used 65 micron silicon carbonate particles. These particles are manufactured for use in the abrasives industry and are available pre-sifted into a multitude of grain sizes for relatively low cost.
Side View of prototype during construction. The 45 micron stainless steel wire filter can be seen melted into the end of the panel. 0.25 inch acrylic plastic was laminated together to form a manifold. I later glued on end caps.
Innovative Features
Besides a substantial cost reduction, which is the primary innovation, the physics of the panels may lead to high efficiencies. In a traditional flat solar thermal panel, heat is absorbed onto a solid black plate, where it must travel upward of 10cm before transferring into water flowing through a pipe. This bottle neck can result in heat build-up, which translates to lower efficiencies due to heat loss. Although silicon carbonate has a lower heat conductivity than copper by a factor of 100, the distance that the heat must travel to reach the water is only the particle radius, which is a factor of 3000 smaller. Thermal conduction favors the particle panel by a factor of 30. A great deal of further optimization could be done, starting with the particle material and size.
Turbulent flow is clearly seen on the sun-facing side of the panel during operation. The visual effect is quite stunning, resembling something like a sparkling black flame.
During its on-state convection currents were observed in the prototype, with turbulent flow moving the water up the front side and laminar flow down the back. Similar emergent phenomena is observed in Bénard Convection Cells, where circulation currents spontaneously appear in a liquid layer when heat is applied from below. Initially all dissipation through the fluid occurs via conduction and molecule to molecule interaction. When the gradient reaches a critical level the transition to highly organized convection occurs. Accompanying this transition is increased heat transfer. Emergent liquid-particle dynamics in the panel act to maximizes efficiency. It appears that this could be a fundamental innovation to the solar thermal industry.
Downward laminar flow develops on the side opposite the sun.
Solar Cooling
Absorption coolers use a heat source rather than electricity to power a cooling cycle. The minimum temperature needed to drive an absorption cooler is 88 °C. Solar thermal cooling systems are already being produced. Ethylene Glycol, a popular heat transfer liquid, is chemically compatible with polycarbonate, and the upper working temperature of the plastic is 115°C. Whereas the prototype particle panel utilized gravity for continuous operation, a modified design using convection currents could be constructed. This would allow the panels to lie horizontal, thus maximizing energy absorption in the summer months to power a cooling unit. It is reasonable to believe that plastic particle panels could be used as an energy source for air conditioning systems.Shipping Costs
Currently available panels are quite heavy, at 38 lb./m2. Shipping in the continental United States ranges from $0.30/lb. to $1.00/lb., depending on the distance. This does not include a crating charge, which amounts to $33/m2. Therefore, it costs from $44.00/ m2 to $71.00/ m2 just to deliver a traditional solar thermal panel. A plastic particle panel would be constructed of thin, durable and lightweight plastics and foam and weigh approximately 1 pound. Its size, weight and superior durability means it can fit in a standard cardboard box and be shipped via standard mail.Installing a solar thermal heating system with existing panel technology is difficult and expensive. The high stagnant temperatures of the panels translate to high pressures, resulting in expensive and technical installations. By providing a mechanism that eliminates overheating, the complexity of particle panel installation could be reduced to fittings with inexpensive vinyl plastic tubes: No cutting, routing and soldering of expensive copper pipe. The result is that the average home owner could forgo the $1000+ installation cost and do it themselves.
Space Heat
Space heating is the largest consumer of residential energy, yet solar thermal has traditionally been limited to water heating. The introduction of low-cost solar thermal panels with temperature regulating abilities could open up a significant market in the U.S and around the world, a market that is enormous and completely untapped.The biggest mistake in thinking about solar energy is that it must provide for 100% of the energy demands. The sheer volume of energy required to heat a house and warm its water means that savings can be attained if even a fraction of this energy can be replaced with solar. Lightweight, maneuverable panels designed to be easily installed in expandable systems means that solar heat can be incrementally introduced into homes and public spaces.
More then enough solar energy falls on the roof of a house to supply all of its heating needs. To realize this full potential, energy storage is needed to act as a buffer during nights and low-light conditions. Water has the highest volumetric heat capacity of all commonly used materials and can be used very effectively to store heat energy. For a room kept at 25ºC, 50 gallons of water at 75ºC could store 11 kWh of energy. This is equivalent to the energy output of an average electric space heater operating continuously all night long. To put this in perspective, a volume of water 2ft on each side, a pump and 5 m2 solar panels will heat a room all day and all night. The payback on such a system could be as little as two years, or 50% annual return on investment. Large-scale solar thermal heating systems can be readily achieved by tapping into existing hydronic heating systems, or in the case of a remodel or new construction, large capacity insulated thermal tanks can be incorporated into the structure. This type of system would be similar in effect, but far less expensive, then a geothermal installation.
