This was a competition entry for the Land Art Generator Initiative’s 2014 competition in Copenhagen. I worked on this project with David Shimmel and Ian Mackay. We ended up getting shortlisted (we were in the top 25 entries of over 300) and were included in the competition book: New Energies Copenhagen and in a gallery show at the Danish Design Center. You can read about the project below
SunBath
As a city and port Copenhagen exists at the threshold between land and water. It is startlingly appropriate that the city’s symbol, the Little Mermaid, also exists at that threshold: a creature between worlds, half man half fish, caught between land and sea. Copenhagen’s rich bathing culture partakes of this liminality as well, inviting visitors to shed their daily skins and partake in the delights of a warm public bath, or a bracing swim in the harbor. SunBath is a project that attempts to harness the innate capacity of the Refshaleøen site in order to generate electricity, provide a civic amenity, and to give Copenhagen an enduring symbol of its liminal position, culture and leadership in forging a path toward carbon neutrality.
What is it?
A constructed piece of land, the Refshaleøen site is both vast and exceedingly flat. With its southern edge ideally oriented toward maximum solar exposure, the site is a natural fit for a solar energy installation. However, the site’s visible and public nature also demand that the installation be visual, accessible, and publicly engaging. To that end SunBath offer a series of large public indoor and outdoor baths, both hot and cold, ringed around and actively participating with a working power plant. The site thus becomes another type of liminal space, blurring the distinction between land and water while also challenging the false dichotomy of a working landscape vs. a pleasure landscape.
How does it work?
Energy is generated at the Sun Baths through the interactions between five key elements
- The Solar Pond
At the heart of SunBath lies a 14,700 square meter salt gradient solar pond. Solar ponds occur in nature, and have been utilized in electrical generation since the mid 1950’s. Solar ponds collect and store solar heat within a super saline layer of water that forms at their bottom. The extreme salinity of the water stops the normal process of convection, and thus solar heat is trapped within the lowest and saltiest layer of the pond and insulated by the layers of water above it. This is known as a temperature inversion — the water at the bottom of the pond is warmer than the water at the top. The bottom layer of a solar pond can reach temperatures as high as 100 degrees Celsius, but more typically produce temperatures around 80 degrees Celsius (Tabor 1981). The Solar Pond is covered by a greenhouse to prevent disturbance of the water and to aid in the maintenance of its internal temperature. The greenhouse roof is oriented to allow for maximum solar exposure. The northern wall of the greenhouse is pitched at a 60 degree angle and mirrored in order to increase the pond’s insolation.
- The Catalyzing pool
In a typical solar pond installation heat from the pond is used as part of a district heating operation or passed through a standard heat engine in order to produce electricity. At SunBath, heat from the solar pond is transferred to a catalyzing pool via a series of heat pipes. Heat pipes are simple devices that efficiently transfer heat over long distances without requiring external power and with no moving parts.[i] Cold water from the harbor is pumped into the Catalyzing Pool via the Minto Wheel and used to continuously cool the condensing end of the heat pipes. It is at this threshold between the hot pipes and the cold harbor water that electricity is generated. The Seedbeck Effect is the conversion of a temperature difference into an electric current. This is accomplished via a series of Seedbeck Thermo Electric Generators (TEGs).[ii] Assuming an average harbor temperature of 9.75 degrees Celsius and an average solar pond temperature of 70 degrees Celsius the TEGs could produce roughly 4.4 Megawatts of electricity over a limited (four hour) time frame. This capacity could allow SunBath to act as a peaking plant – operating at maximum capacity for peak consumption hours (roughly noon to 4 o’clock).
- The Minto Wheel
Cold water is pumped into the catalyzing pool via a Minto Wheel (C).[iii] SunBath uses the Minto Wheel as both a pump and semaphore. The Minto Wheel pumps cold water into the catalyzing pool where it is warmed via excess heat from the solar pond.[iv] Partially warmed water then flows from the catalyzing pool to the attenuating pool. Beyond its functional role, the Minto Wheel provides a visual demonstration of the physical principals at work within the project – specifically the action of heat on fluids under pressure. The sheer scale of the Minto Wheel makes it a visual symbol of the SunBath and a new, highly legible, icon of sustainability on Copenhagen’s skyline. Finally, the Minto Wheel acts as a signal to bathers, the speed of its rotation indicating the relative temperature of the water in the Attenuating Pool.
- The Attenuating Pool
The Minto Wheel pumps cold water from the harbor into the Catalyzing Pool where the water is partially heated via excess heat from the solar pond. Partially warmed water then flows from the Catalyzing Pool to the Attenuating Pool. Water in the Attenuating Pool is further heated by the archipelago of dark “hot rocks” which, because of their low albedo and thermal mass, absorb solar heat and release it slowly into the water. The heated water of the Attenuating Pool powers the Minto wheel. This cycle creates a self-regulating system: the hotter the Attenuating Pool becomes, the faster the Minto Wheel pumps water from the harbor to the Catalyzing Pool. That cold water then moves to the Attenuating Pool where it slows down the Minto Wheel. This internal regulation ensures a relatively steady temperature difference between the Solar Pond and the Catalyzing Pool and thus a relatively steady power output.
- The Harbor
The Harbor, and the consistently cold water fed to it by the Øresund are the true source of power on the site. This is so because the mechanism for producing electricity at SunBath is not purely solar heat, but rather the difference in temperature between the warm side of the Thermoelectric generator (the Solar Pond) and the cold side (the harbor water). The colder the harbor is, the more electricity the SunBath generates.
What does it produce?
Unlike a normal Photovoltaic array, a solar pond essentially acts as both collector and battery, allowing power to be drawn of as needed. Assuming an average harbor temperature of 9.75 degrees Celsius and an average solar pond temperature of 70 degrees Celsius the TEGs could produce roughly 4.4 Megawatts of electricity over a limited (four hour) time frame. This capacity could allow SunBath to act as a peaking plant – operating at maximum capacity for peak consumption hours (roughly noon to 4 o’clock). Operating at this rate would draw down the temperature of the Solar Pond. Producing 4.4 MW would draw the solar pond down by 10o Celsius. Alternatively, SunBath can produce a more modest amount of electricity without drawing down the Solar Pond’s heat supply. This can be calculated as simple function of insolation rates during various times of the year. So, in June, when insolation peaks, SunBath could generate a continuous 675 kW of electricity, (24 hours a day), while maintaining a constant internal temperature. In December, when insolation is lowest, SunBath could only produce 64 kW of electricity without drawing down its heat supply.
What does it do for Copenhagen?
Beyond the significant amount of power generated by the installation, SunBath also create a vast public pleasure ground for the citizens of Copenhagen. The 3700 square meter heated attenuating pool is the moment of interface between SunBath’s utilitarian and public faces. The hot rocks provide warm islands and climbing surfaces for sun bathers and swimmers. The partially warmed water of the Attenuating Pool stays ice free year round. The Cinema Pool provides 1850 sq. meters of contained swimming where night visitors can swim while catching an outdoor movie (projected onto the western wall of the bathhouse). In winter the Cinema Pool ices over and transforms into a public ice-skating rink. The 6,000 square meter Bathhouse contains a large indoor pool, showers, changing rooms, a café, saunas and the 1500 sq. meter, solar-pond-heated Caldarium where bathers can relax year-round in the warm solar waters. In keeping with Copenhagen’s grand tradition of harbor baths, 265 linear meters of boardwalk extend into harbor, providing space for bathing, professional swimming, diving, water sports, and play of all kinds.
What are its potential impacts?
SunBath has minimal environmental impact. The absence of moving parts other than the Minto Wheel in the generation of electricity creates little chance for accidents, breakdowns or contamination. Precautions should be taken to ensure that water from the Solar Pond does not escape into the harbor, as its levels of salinity would be temporarily harmful to aquatic life. Similarly, the heat pipes and Minto Wheel contain relatively small, sealed, amounts of the flammable fluid Isobutane. If one of these pipes were to leak it could cause limited contamination. These pipes are small and independent, a breach in one would not compromise the others in any way, and so large scale contamination from the working fluid is extremely unlikely. Isobutane is considered safe enough that it is commonly utilized in household refrigerators and other retail appliances.
Works Cited
Lee, HoSung. Thermal Design: Heat Sinks, Thermoelectrics, Heat Pipes, Compact Heat Exchangers and Solar Cells. Hoboken: Wiley, 2010.
Mother Earth News. “Is This the Breakthrough Solar Engine We’ve All Been Looking For?” Mother Earth News, no. 38 (March 1976): 96.
Rabl, A. and Carl E. Nielsen. “Solar Ponds for Space Heating.” Solar Energy vol. 17 (1975): 1-12.
Tabor, H. “Solar Ponds.” Solar Energy 27, no. 3 (1981): 181-194.
Notes
[i] Heat pipes work by utilizing a fluid with a low boiling point such as Freon. This fluid vaporizes when brought into contact with heat, and the vapor travels along the length of the pipe to the cooler end where it condenses and heat is rejected (Lee, 180).
[ii] A thermoelectric generator (TEG) converts thermal energy (heat) to electrical energy using the Seedbeck Effect without moving parts (Lee, 107).
[iii] The Minto Wheel is a form of heat engine which, like a heat pipe, relies on a working fluid with a low boiling point. As the fluid comes into contact with the warm water bath it vaporizes, rises up the tube, and condenses on the opposite (cooler) end of the tube. As the vaporized moisture collects at the top of the tube it unbalances the wheel causing a slow rotation with significant torque (see diagram on board 2). The Minto Wheel was invented by Wally Minto in 1976 and can function on a temperature difference of around 2 degrees Celsius. The wheel used here is 42.6 meters tall and is expected to generate around 4 kilowatts of mechanical power while making approximately 1 rotation per minute.
[iv] Our calculation assumes a thermoelectric generator operating at a conservative efficiency of 20%. This means that 80% of the heat energy transferred by the heat pipes is transmitted to the water in the catalyzing pool and then moves into the attenuation pool – helping to power the Minto Wheel.