Published on April 22nd, 2018 | by James Ayre0
Solar Chimneys & Solar Updraft Towers 101
April 22nd, 2018 by James Ayre
Solar chimneys are an ancient passive solar technology whereby natural heat convection processes are harnessed to provide ventilation, amongst other possibilities.
Sometimes referred to as a thermal chimney or a heat stack rather than a solar chimney, this type of ventilation and/or cooling system has seen widespread use in the region encompassing and surrounding Greater Iran, throughout the Middle East and North Africa, and in Roman Europe throughout the millennia.
To keep it simple here, the basic idea behind solar chimneys is to increase the turnover rate for air within a building by amplifying natural convection processes through the use of (typically external) vertical shaft that heats up rapidly when exposed to sun.
Picture a room with vents near the roof leading to an external “chimney” made with (or painted) a dark-colored material and vents near the floor (preferably from a shaded side of the house), and you basically have the idea. Thermal mass can be used in association with dark coloring as well, to improve heat retention. An example of a thermal mass might be stone flooring that the sun shines on during the day warming it up, which then lets off that heat during the cool of the night. Ventilation exchange rates are increased due to the heating of air within the solar chimney (thus an updraft is created since warm air rises), even though some ventilation would be occurring anyways.
Generally speaking, the solar chimney is located so as to maximize solar gain — that is, it’s located on the part of the room that gets the most sunlight. Also notable is that the chimney vents should be faced away from the direction of the wind (if not straight upward).
Due to the air being drawn in from ground level and from an externally shaded area, there’s also a pronounced cooling effect with the use of such a system. Alternately, the air being pulled into the structure can be drawn through cool/earth tubes installed in the ground as a means of further cooling the air, if wanted. This is referred to as geothermal cooling.
As those who read the earlier article on wind catchers are probably now surmising, solar chimneys are largely less effective in many regions than a well designed and well placed wind catcher is. In other regions, though, solar chimneys can come out ahead. In particular, regions with only low levels of wind, and particularly those where the summers are often the less windy time of year, are often well suited to the use of solar chimneys.
To again keep it simple here, there are 3 basic design components in a basic solar chimney: 1) solar collecting elements (dark coloring, thermal mass, possible insulation, etc.); 2) the ventilation shaft itself (design will later and/or limit possible updraft rates); and 3) the vents themselves where air inflow and outflow occurs (this further limits and shapes updraft rates and quality).
Solar Updraft Towers — Renewable Energy Power Plant Design
Solar updraft towers are, generally speaking, a type of renewable energy power plant that utilizes the above-discussed amplified convective processes achieved via solar chimneys in combination with large surrounding greenhouse-like structures and integrated electricity generation turbines in the tower itself.
The large greenhouse-like structures under and around the solar updraft tower itself heats up and in turn creates a large temperature differential with the surrounding area, thereby generating convective flow that can be harnessed to generate electricity through turbines.
Alternately, solar updraft towers can be used to passively filter large amounts of polluted urban air through the integration of filtering technology. Such an approach is now being trialled on a large scale in some cities in China — reportedly with good results.
Going back to the use of solar updraft towers as power plants, despite the inherent sensibility of such an approach, the design is generally unable to compete with simpler technologies that require less in the way of initial capital investment. Solar photovoltaic (PV) technology and conventional wind energy projects (whether onshore or offshore) tend to undercut solar updraft towers by a fair margin.
That reality hasn’t stopped projects from occasionally being developed, but these projects have largely not proven economical. This is partly due to the investment costs, and partly also due to the amount of otherwise valuable land that’s required — solar PV is much more flexible and unobtrusive with regard to installations, and offshore wind energy projects aren’t competing for land space at all.
To those wondering about potential hours of operation, it should be stated clearly here that solar updraft tower power plants can easily be designed to function 24 hours a day simply through the integration of large amounts of thermal mass — whether simply in the ground (dark soil or stone) or in water of some kind (highly saline water probably being the best option in many places).
That being the case, solar updraft towers are notable for being a solar energy technology that can function essentially 24/7 without the use of battery-based energy storage. Also notable is that such systems could be tailored to suit the needs of high latitudes quite effectively, simply through distortion of the associated collection areas (altered to target the sun path of the regions in question).
That said, scaling up the design would require large amounts of space — with some estimates placing the needs for a power plant with a 100 MW nameplate capacity at: a 1,000 meter tower and a 20 square-kilometer thermal collection area. Estimates vary quite a bit, though, and real-world experience is limited, so who knows what the exact design requirements would be to successfully build a 100 MW solar updraft tower power plant.
Overall solar updraft tower power plants make for an interesting idea, but it’s unclear whether or not they will every be utilized on a large scale.