The next generation of passive solar for cold climates

by Katie Daniel | December 5, 2017 2:56 pm

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By Mark Driedger
As a child in the early 1980s, this article’s author had a clubhouse he called the ‘Millennium Falcon’—standing 2 m (6 ft) off the ground, it was a simple A-frame held in place by four old telephone posts, complete with a fireman’s pole and pully system for escape when under attack by the Imperial Starfleet. However, it was also an experiment in solar energy storage.

In its former location, the ‘Falcon’ had been filled with stones from the surrounding fields, and its south façade was fully glazed, allowing the sun to access the stones. The theory was the sun’s heat could be harvested from the stones long after it had dropped below the horizon.

The author’s parents’ plan was to build a house that was extremely energy efficient, and this was the prototype to provide the heating energy. In the early ’80s, natural gas pipelines had yet to reach the farm, and heating with electricity or fuel oil were the only options. With rising energy costs, it became important to save money, and using glazing to access passive solar energy was heavily featured in the media.

With the input of an American architect who had experience with passive solar design, traditional south-facing windows were chosen for the house, and a well-sealed staggered-stud enclosure was employed rather than the more exotic thermal mass method. Unfortunately, the chosen system simply was not balanced to provide additional energy to the house—although some sunlight was harnessed, the reality was the low thermal performance of this particular glazing caused much more energy to be lost than gained.

The glazing was also not large enough to collect sufficient heat, but increasing its size would also increase conduction losses during the cold winter nights, as glass is inherently a terrible insulator. Increasing the thermal properties of the windows reduced these losses, but it also reduced the sunlight that was allowed through. The system may have worked in a more temperate climate, but in Kingsville, Ont., the structure was destined to become the Millennium Falcon.

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Some 30 years later, as an architect, this author’s interest in the problem remains. Why are architects not doing more to harness the sun? Yes, photovoltaics (PVs) have become more efficient, and large-scale battery installations are being considered, but battery storage still has a long way to go. Given a large portion of the energy used in buildings by Canadians is for heating (Figure 1), it would be very difficult and expensive to build a battery-storage system that would power a house through the winter. (This data was obtained from National Resources Canada (NRCan). For more information, click here[3].) It does not help that the cost of natural gas is still affordable at the moment, and it is cheaper for Canadians to run pipelines from Alberta than convert furnaces to electric heat and build solar panels with huge battery banks. Technology has changed rapidly over the last 30 years, but the equation still comes down to how energy from the sun can be efficiently stored. Research seems to be focused on the complex transformation of solar energy using batteries, rather than the simple transfer of energy into a thermal mass.

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Transforming energy involves a high amount of waste
The first law of thermodynamics states energy cannot be created or destroyed, although it can be transformed from one form to another. When energy is transformed, there are always inefficiencies—usually in the form of escaped heat. In fact, most energy systems involve inefficient transformations of energy. Photosynthesis, for instance, transforms light into chemical energy, yet around five per cent of the solar energy received is actually converted into energy for the plant, which compensates for this low efficiency by increasing the surface area of collection to survive. (This information comes from Photosynthesis by David O. Hall and Krishna Rao, published by Cambridge University Press in 1999.) It is amazing that with such low efficiency, photosynthesis can support the Earth’s food chain. Another example is the combustion engine—approximately 80 per cent of the energy released in the typical auto gasoline combustion process ends up as waste heat, while the remaining 20 per cent is used to power the vehicle.

PV technologies[5] have been steadily improving, but the numbers are still lacking. The average solar cell converts around 15 per cent of sunlight into electricity. Therefore, if 1000 watts of sunlight (i.e. a clear summer day in Toronto) are received on a 1-m² (11-sf) solar panel, only 150 watts get to the battery system or grid. Further losses occur in the transformation of the electricity into potential energy within the battery, and in the process of extracting the energy from the battery. As with the plant, the surface area of collection must be substantial to make the system feasible.

Typically, off-grid PV systems with batteries are not used for heating. Heating simply requires a massive battery system. A typical 1000-watt space heater, using the solar array listed above would need just under 7-m²  (75-sf) of solar panels in cloudless summer sun to make it work. As winter months tend to be very cloudy with limited daylight hours, even with battery systems, it just does not make sense at this time to use PV systems for heating.

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Transferring energy is simple and efficient
We can take a cue from nature for storing thermal energy. For instance, large bodies of water typically moderate the temperatures of adjacent areas. Ocean and lake water warms surrounding areas in the winter, while cooling the air in summer. The water captures the radiation from the sun as heat, keeping energy in its original state. Similarly, a black latex paint wall exposed to the sun will convert 96 per cent of the sun’s energy directly into heat. There is very little waste in simple transference (Figure 2) In 2011, with the help of Lawrence Technological University (LTU) in Detroit, this author took another look at what caused the A-frame solar project to become the family treehouse, with the goal of transferring rather than transforming solar to heat energy.

The weaknesses of the Falcon were addressed in the LTU experiment in the following manner:

Increasing the energy available to the system
By increasing the amount of sunlight hitting the thermal mass with multiple heliostats (i.e. reflectors), more energy can be made available for the system. As an example, a plant uses multiple leaves to maximize its receiving area—the system brings the concentrated sunlight directly into the insulated thermal mass rather than exposing the conditioned water to the exterior environment, as would happen in a typical solar hot water system.

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Reducing the glazing loss
Decreasing glazing area decreases energy loss. By making heliostats adjustable and concave, more energy can be obtained through a smaller glazed opening or inlet accessing the thermal mass. To reduce energy loss at night and in cloudy periods, the glazing exposing the thermal mass could be covered with an insulated shutter.

Maximizing storage and spanning the seasons
One should take advantage of the Canadian summer sun, increasing the thermal mass and viewing the process as more of a long-term storage solution than a day-to-day one. This means heat would be collected in the summer, then stored in a highly insulated thermal mass and used in the winter. Water would be used instead of field stones because of its higher heat capacity, and circulation could be employed to transfer heat and minimize stratification (Figure 3).

The HelioArch
The team named the experiment the ‘HelioArch’—the next generation of active/passive solar for cold climates. The approach focuses on small-scale, dwelling-based installations. It is important to note, however, this method is not a novel one—solar thermal plants generating heat have been around since the early 1970s. Using water as a thermal mass is also a common concept. The critical element here is the system is designed to be integrated into the architecture of the building, balancing gains from the heliostats, the storage capacity of the thermal mass, and the losses routed to the dwelling.

In 2013, the team created the balancing software for the system, which modelled the thermal dynamics of a simulated environment. A computer model was created over a six-year period, using climatic data recorded by Toronto Pearson International Airport. (To view a sample of the software and continued research, visit www.helioarch.com[8].) The HelioArch software produced during this research models the hourly operation of the system over those six years, and the present version takes several issues into account, including:

• the area and number of heliostats;
• the type of heliostat;
• sky conditions (i.e. whether energy is available from the sun at that particular time and whether it is cloudy);
• whether the gains of the system outweigh the losses at a particular time;
• heat loss to the dwelling due to infiltration under the varying temperatures and wind from the climate data;
• heat loss to the building’s exterior due to conduction and convection under varying temperatures and wind velocities;
• the heat loss through the glazed inlet under the varying temperatures;
• heat gain from the heliostat;
• temperature limitations of the mass, as the charging of the mass is maintained between room temperature and the boiling point of water; and
• the heat loss of the assemblies under varying temperatures.

The program summarizes these components into a single heat loss/gain calculation, ultimately showing the final temperature of the thermal mass at the end of each hour over a simulated six-year period. The output is a graph showing the temperature of the thermal mass at any one time within the six-year period. It is critical the thermal mass and heliostat(s) are sized correctly to ensure the energy can be made available when it is needed. Figure 4 displays the output of the HelioArch software.

In Figure 4, one can see the heating and cooling of the thermal mass over time. The house modelled is approximately 167 m² (1800 sf); thus, the thermal mass is close to 125,000 L (33,000 gal). The heliostat size must also be balanced in area to provide enough energy to span the seasons. The software simulates the shutdown of the heliostat system if the temperature approaches the boiling point of water, thereby avoiding the pressure issues associated with the phase change of water turning to a gas, and will also display a flatline graph if the system is undersized or oversized at any point.

Once the mathematical model had been reviewed by the team, prototyping and real-world testing commenced. In 2013, with the assistance of Toronto’s George Brown College, the team began creating scale models of the system to test the accuracy of the software. George Brown assembled the heliostat, as well as the tracking logic and mechanism to allow the Helioarch to follow the sun (Figure 5).

This led to a ¹/₈ scale working model test, completed by architecture professor Ramani Ramakrishnan and the fourth-year building science studio at Toronto’s Ryerson University in 2016. The experiment, conducted over eight hours on a sunny winter day, showed an increase in water temperature, using a 600-mm (23-in.) diameter concave heliostat and a 900 x 900 x 900-mm (35 x 35 x 35-in.) insulated thermal mass. The test mimicked the results of the software.

This winter, a ¼ scale model complete with an automated shutter and heliostats will be tested. In this model, the heated water will be circulated through a simulated dwelling using hydronic pipes in the floor. Cooled water will be returned to the energy storage tank in the building’s basement.

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Conclusion
Although the calculations in HelioArch are only theoretical at this point, and more scale models are required to verify the concept, it appears the technology may be a viable alternative to fossil-fuel heating systems in cold climates. The capture efficiencies are presently approximately double (about 30 per cent) of PV and battery systems (less than 15 per cent), and are rising as the design and testing of the system progresses.  The concept is designed for cold climates, where water is plentiful and there is enough full sunlight to carry the building through the winter months. The system is tightly integrated into the architecture of the building, leaving behind the present model of using mechanical units to overcome the weaknesses of the architect’s design. In this case, the architect must use the HelioArch software early in the design process to correctly balance the system while orientating and designing the building on the site.

However, items still need to be addressed, such as keeping the thermal mass free from algae and the maintenance of the heliostats. If the thermal mass was integrated into the structure of the building, moisture issues with the surrounding architecture would need to be adequately dealt with. If a critical component of the system were to fail, a backup heating system would be required to carry the system and building into the spring.

The system could be adaptable to many different configurations and building types. The model shown in this article takes a very simplistic approach to design to find the lowest common denominator; based on this work, the concept can be scaled up to fit more complicated building types, as long as there is room for the heliostats and a higher-capacity thermal mass, which is an obvious problem in urban areas. Integrating the system on building rooftops could be one option on tight sites.

Architects are aiming for all new buildings to be carbon-neutral by 2030, but this goal is still very far away. Today’s design practices will need to undergo a fundamental change to reach such an objective, especially in cold climates such as Canada. It is this author’s opinion thermal mass technologies like HelioArch are crucial to achieving this goal.

By keeping the system simple and minimizing both the transfer and transformation of energy, it is possible to efficiently harness the power of the sun. With further testing, a properly designed HelioArch may provide the means to satisfy all heating energy requirements of a building. By reducing these requirements for a dwelling, separation from the grid via small PV and battery-storage systems becomes more possible.

Mark Driedger is an architect with ATA Architects Inc. in Toronto and Oakville, Ont.  He is also an adjunct professor with Lawrence Technological University in Detroit. Driedger continues to integrate science with design. He can be contacted at mark@ataarchitectsinc.com[12].

Source URL: https://www.constructioncanada.net/next-generation-passive-solar-cold-climates/

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