Solar on the brink
For perhaps the first time in Canada, solar technologies are catching the interest of conventional building owners and developers. Whether it is because solar serves as a visual flag to show that a bu...
For perhaps the first time in Canada, solar technologies are catching the interest of conventional building owners and developers. Whether it is because solar serves as a visual flag to show that a building is green, or because solar is an investment opportunity thanks to Ontario’s Feed-in-Tariffs (FIT), more and more engineers are being asked to integrate solar technologies into building projects.
Solar photo-voltaics (PV) to generate electricity have caught the imagination of building owners and financiers in Ontario for good reason. The FIT program is poised to make Ontario the Germany of North America as a centre for solar technology, with the attendant benefits of green collar job creation. PV systems are easy to understand, implement and maintain. Further they make great sense as a tool for reducing peak electrical demand in urban areas, which eliminates the need for utilities to build peaking electricity plants and upgrade distribution networks.
Efficiencies of thermal vs. photovoltaics
What is often overlooked, however, is that in the absence of subsidies PV remains the least efficient solar technology from the perspective of energy generation, CO2 offset, and economics. From an energy perspective, solar thermal (capturing the sun’s energy to produce heat) is typically three to five times more efficient than PV in converting the sun’s rays into useful energy.
Solar thermal technologies are five to 12 times more cost effective per equivalent kWh (ekWh) of energy generated for a comparable $100,000 invested. Electricity is, however, a more valuable fuel to offset, and so thermal’s advantage over PV from an economic point of view is reduced by half in Ontario. In provinces with cheap electricity, that half reduction may be negated, meaning there is an even clearer financial case for solar thermal from a pure dollar-for-dollar investment perspective.
Engineers, who are given the task of identifying solar options, should review the merits and potential for solar thermal before committing to PV.
What’s my load?
Canadian buildings can have significant ventilation air heating loads that may not be served economically by heat recovery. Solar thermal technologies are one of the simplest and most cost-effective means of pre-heating ventilation air. The most basic system uses transpired air collectors (perforated, dark-coloured metal cladding installed over south-facing exterior walls). Air is drawn through pin holes in the cladding and gains heat from the metal as it passes. Additional heating is gained from the back of the panel and the building facade (heat that is usually lost to the atmosphere through the wall) as the air is drawn up to the ventilation fan inlet. The system requires a large wall space and an architect or owner sympathetic to the look of dark siding on a south-facing wall. A sloped or flat roof can also serve for backpass types of solar air collectors, which are less efficient. It is important that make up air systems are relatively close to the solar collectors in order to minimize the need for expensive insulated duct-work. In addition, products are available that integrate PV with solar air and give improved efficiencies.
Another energy-efficient solar technology is solar domestic hot water (SDHW) pre-heating. Solar collectors on the roof heat up in the sun. A fluid circulating through the collectors transfers its heat via a hydronically separated heat exchanger into a thermal storage tank containing water. A controller compares the temperature inside the collectors with the storage tank; when the collectors are hotter, a circulation pump is activated to begin transferring heat. When that temperature drops, the pump shuts off. A well-designed system exceeds 50% efficiency in converting the sun’s energy to useable heat, making it the most efficient of all active solar technologies. However, a regular water heater is unfortunately necessary for the days when the sun isn’t shining.
Solar domestic hot water systems require dedicated storage to store energy for when it is needed in the evenings and early mornings. The building design team therefore needs to allow for space for the tank. Also, the solar and main hot water heating systems must be integrated. As storage temperatures even in February can exceed typical set points, a tempering valve is required after the solar heat exchanger and before the regular hot water heating system.
Considering how to tie collectors into the building struc- ture takes care; roof interconnection costs can exceed the costs of the solar collectors themselves.
Good applications for solar water heating are buildings with large water loads such as hospitals, hotels, apartment buildings, and recreation facilities.
Working with architects and solar experts
An architect typically holds sway over a building project so he or she needs to understand early what are the aesthetic impacts of solar systems. A correctly sloped roof surface can save on structural costs, and solar can be integrated into the building’s faade. However energy generation fundamentals will likely dictate an installation angle requirement of 20-60 up from horizontal and a need to account for the impacts of accumulations of snow.
If the goal is energy self-sufficiency, it is very difficult to achieve this aim without using available south-facing surfaces. Note, however, that since solar generation peaks in the months of March to September when the sun is often north of the east-west axis, solar can still be viable for a building that does not face to the SE or SW.
Energy modeling is going to be a key component and so alerting the team early on to the realities of carbon neutrality and to potential energy sources is vital.
If some or all of the above is new information, chances are you should bring in a consultant familiar with solar who has the proper understanding of how the system should be designed, and how it will impact construction and integrate with the building as a whole. An accurate model of how the system will operate must be in place, and you should take advantage of construction economies by coordinating the system’s installation with the roofers, structural engineers, plumbers, and so on. Make sure that the control systems integrate the renewable energy systems.
Making solar worthwhile
It is important that the solar expert guiding the design is also responsible for commissioning. Commissioning is hugely important for systems whose performance varies significantly throughout the year. Real-time monitoring for all solar system types is available at low cost and should be a vital part of the design; after all, owners and engineers need to know if it is working as expected. The project costs should account for operational reviews throughout the first year.
As with all aspects of building technology, it is vital to understand the factors affecting solar investment decisions. A solar PV system selling energy at 60-80 cents per kWh to the grid (such as Ontario’s FIT program establishes) can provide returns on investment in excess of 10-15%. But without such a selling rate, the pay- backs on PV systems are in excess of the product’s lifetime.
A solar thermal system’s economic performance can only be understood in relation to the operational efficiency of the building’s heating plant, the projected heating fuel price, as well as the constantly changing incentives available. These factors need to be well understood within the context of the system’s life expectancy.
Finally, once a project is commissioned, it is very important to leave the knowledge and resources required for system maintenance in place. Owners and facility managers need to understand the equipment, performance reviews, standard maintenance tasks and schedules, and simple trouble-shooting. Maintenance budgets and preferred maintenance and repair contractors are best identified ahead of time. Nothing gives solar energy a
bad name like a system that ceases operation after a few years simply because no one knows that it isn’t working properly or does not know where to go to fix the problems.
Ian Sinclair, P. Eng., LEED-AP, is manager of existing building services with Enermodal Engineering’s Toronto office.
Annual Solar Radiation in Toronto
Flat Roof -3.57 kWh/m-year
Vertical wall facing due South -2.71 kWh/m-year
Vertical wall facing due SW or SE -2.65 kWh/m-year
Vertical wall facing due West or East -2.27 kWh/m-year
Vertical wall facing due NW or NE -1.69 kWh/m-year
Vertical wall facing due North -1.35 kWh/m-year
Approximate optimal angle for Toronto: sloped at 30 degrees from horizontal and due south -3.96 kWh/m-year