Bioenergy, Hydropower, Solar

Making solar thermal fit in: Ann Arbor’s 5000 solar roofs programme

Issue 5 and Volume 10.

The challenge set by the Mayor of the US city of Ann Arbor, Michigan, to provide 20% of the entire city’s energy from renewable sources is an ambitious goal. It has the Energy Commission looking at every opportunity to convince the citizens to use renewables. One programme being initiated by the Commission is an effort to get solar domestic hot water (SDHW) systems on at least 10% of the dwellings in Ann Arbor – 5000 systems by 2015. Wayne Appleyard and David Konkle explain how the programme is going so far.


The initial stages of the programme have included an in-depth look at the barriers to the purchase and installation (both real and imagined) of SDHW. Additionally, there has been an exhaustive study to determine what constitutes a good solar roof in Ann Arbor and how many of them exist in this city.

Climate change is now accepted as fact. The US federal government has been slow to act, but Ann Arbor is one of the cities trying to increase its use of renewable energies while reducing its production of carbon dioxide. The movie An Inconvenient Truth can leave individuals feeling a little overwhelmed and at a loss as to what they can do, so the importance of providing opportunities for each person to reduce their carbon footprint can not be overstressed, for with helplessness comes inaction.

Although solar domestic water heating systems are among the most cost-effective renewable devices, using solar energy on a daily basis throughout the entire year, they are often overlooked in favour of less cost-effective devices such as photovoltaic arrays. Yet they are within the budgetary possibilities of most homeowners and also offer the best return on investment in renewable energies.

The Mayor’s challenge

In November 2005, Ann Arbor’s Major John Hiethje presented the City of Ann Arbor’s Energy Commission with a set of challenges. First, convert 20% of the municipal government’s entire energy usage (electricity, natural gas and transportation fuels) to renewable energies by 2010 and, secondly, convert 20% of the entire city’s energy use to renewables by 2015.

The City’s Energy Coordinator, Dave Konkle, and the Energy Commission reviewed the municipal government’s existing renewable energy diet and reported back to the Mayor that it was currently almost at 20%. They believed that 30% by 2010 was reachable. The city – through its hydroelectric and landfill gas generation, and its use of alternative fuelled vehicles – is well on its way to meeting that goal. Additional purchases of renewably generated electricity and some additional use of higher amounts of biodiesel are all that is needed.

Enticing a substantial portion of the city’s citizens to voluntarily convert to renewables is a much more daunting task. The city is working closely with the University of Michigan (Ann Arbor’s largest user of energy) to help improve the carbon footprint. The city is also working on partnerships with its privately owned, profit-making utility – DTE Energy – to make it easier for citizens to buy electricity generated by renewable sources. With a voluntary programme and no municipally owned utility to provide incentives (unlike Austin, Sacramento and virtually every other city with a major renewables programme), Ann Arbor is left with few tools to reach this goal. Education becomes a key element.

Ann Arbor’s 5000 solar roofs programme

While developing a list of what each and every citizen might be able to do to help meet the Mayor’s challenge, it became clear that after doing whatever conservation initiatives made sense, a solar domestic hot water system was an excellent choice.

Solar domestic hot water (SDHW) was the mainstay of the solar boom of the 1970s. Some of the Energy Commissioners (and the Coordinator) had direct experience with SDHW back then and still have systems in their homes. SDHW has been found to be the most cost effective solar application available for the homeowner today. The technology is simple and not difficult to install.

In the United States, SDHW systems have, however, been largely overlooked in the recent resurgence of interest in renewables. More people seem to be interested in photovoltaic systems than SDHW. This is somewhat puzzling given the fact that the return on SDHW is between six and 10 times that of a photovoltaic system (at least in Michigan). This lack of interest in SDHW is so prevalent that in January 2007, a workshop was held in San Diego called Expanding Solar Thermal in the US (see www.swhmarketexpansion.com) to explore this phenomenon.

The US government had a programme called 1 Million Solar Roofs (now Solar America Initiative), so it seemed fitting that we set a goal in the name of the programme. Ann Arbor has about 49,000 residential water meters – an approximate guide to the number of homes in the city – so it was decided that getting SDHW on 10% of those homes would be an ambitious goal. At that time, there were probably 15 residential SDHW systems in Ann Arbor. There was some talk about going for 500 solar roofs, but the energy impact seemed too inconsequential.

It is important to look at this effort from the perspective of the Mayor’s Energy Challenge (20% city-wide combined electricity, natural gas and transportation fuels). If 5000 SDHW systems are installed in Ann Arbor, they will offset about 1% of the city-wide energy use.

Development of the programme

The taskforce working on the Ann Arbor 5000 Solar Roofs (AA5KSR) programme identified several tasks and initiatives that could foster the installation of solar in the city.

System rebates
A US$500-1000 rebate to reduce the initial cost of the system has been proven in other cities to provide a dramatic boost in the number of installed systems. Adding this to the $2000 Federal Tax Credit reduces the cost of the average system to $3500. The prospect of getting the city to find $2.5-5 million for such a programme in these times of government under-funding is low. The city is requesting a Solar Cities Grant that would provide some financial incentives.

Low-interest loans
The next best thing to free money is low-cost money. We are exploring the possibility of partnering a local lending institution to provide lower interest, home-equity style loans that do not require much paperwork.

Pre-qualified installers and systems
Providing a list of installers and systems that have a proven track record can take some of the fear of purchasing and the risk out of lending. The Great Lakes Renewable Energy Association (GLREA) educates and certifies installers.

Bulk purchasing
If a large number of orders could be combined, some reduction in the materials price can be achieved. The GLREA is in the third year of its Go Solar Ann Arbor Program, which saves purchasers small amounts because of group purchasing.

City-wide education and promotion
Education is perhaps the best avenue for achieving the goal of AA5KSR. The belief is that people really just don’t know how well SDHW works and that it can work for them. GLREA’s Go Solar Ann Arbor Program is already raising the awareness of individuals about the virtues of SDHW (and photovoltaic systems). The GLREA got between 50 and 100 people at each of the two seminars it hosted in the city. The Commission is planning to have more such events, as well as displays, at the annual Earth Day and Green Fair events. Additionally, an SDHW system is being installed on the fire hall next to the Hands On museum, which will have a display with actual monitoring readouts at the museum.

Broadening the programme
Although AA5KSR originally focused solely on SDHW systems, it will be broadened to include photovoltaic systems. Although they currently have a much smaller return than SDHW, the idea of generating electricity seems to have greater appeal to some consumers. Broadening the programme will allow those enamoured by photovoltaics to further their education about solar thermal and allow the city to count any photovoltaic systems installed as part of AA5KSR.

Testing the premise

Was Farrington Daniels wrong?
The question of whether there were 5000 roofs suitable for solar in Ann Arbor (known as the tree city) as well as what really qualified as a suitable roof needed to be addressed. Identifying those homes that did have a solar roof was also one way of focusing in on prospective solar purchasers. It was first important to determine what constituted a good solar roof. As Dr Farrington Daniels put it in his book Direct use of the Sun’s Energy, in 1964: ‘In the northern hemisphere they are faced south and tilted at angle with the horizontal equal to the latitude. In the winter it is recommended that the flat-plate collectors be tilted at the angle of latitude plus 15°, and in the summer at the angle of latitude minus 15°.’ That statement is one that any solar installer has ingrained in their head and which they use to declare the potential success or failure of a proposed solar installation. Yet it deserved another look.

It is a good rough rule but might be too restrictive and could be one of several impediments to mainstreaming solar installation. One has to remember that Daniels made that statement in 1964, long before computer simulations and easy studies of the significance of orientation and pitch on solar collector performance. One also has to remember that Daniels also lived and worked in Florida, a state with a climate significantly different from Michigan’s. Michigan has a significant amount of cloud cover in winter, although its annual solar input is still 78% of that in places such as Sacramento and Austin, which have significant solar programmes. This cloudy winter weather could alter what would be the optimal orientation and pitch for solar thermal equipment.

One can only imagine how many people have looked at the roofs of their homes and declared: ‘Solar won’t work for me. My roof doesn’t face due south and is at too small a slope.’ If we were to put SDHW on 10% of the homes in Ann Arbor, they would be going on many roofs that are a lot less than optimal and might end up being propped up and angled so as to be unsightly. It was important to quantify what penalty was to be paid in reduced production by installing the equipment parallel to the existing roof at whatever angle and orientation it happened to be at.


A group of graduate mechanical engineering students from the University of Michigan agreed to take on the task as their term project. Under the supervision of Wayne Appleyard, Energy Commissioner and Architect with Sunstructures Architects, they used German solar thermal computer simulation program TSOL Pro 4.3 to simulate a basic two-panel SDHW system’s performance in the Ann Arbor climate under various angles of pitch and orientation.

Both flat plate and evacuated tube collectors were simulated. Evacuated tube collectors perform better in periods of low ambient temperature, while flat plate collectors work better in the summer months.

Simulation results

Table 1 shows the simulation for two flat plate collectors at various combinations of pitch and orientation. Farrington Daniels was right! The optimal pitch and orientation for year-round use is south facing, pitched at an angle equal to the latitude. The important fact to note is that for less than optimal pitch and orientation there is much less of a penalty than believed.


Figure 1. How annual solar fraction varies with orientation and pitch

A collector angled at a 4 in 12 pitch (common in Ann Arbor) that faces south will capture 90% of the ideal pitch of one angled at 12 in 12. A collector facing due west with pitch 12 in 12 still gets 75% of the ideal south-facing system. Figure 1 shows the same data in graphical form. Note that the 45° and 60° curves are one over the other.


Figure 2. Performance of flat plate collector through the year with a solar fraction of 62%, two collectors, 45° tilt, 0° azimuth

As predicted, the flat plate system worked better in the summer than the evacuated tube panels. The evacuated tube panels worked better in the winter than the flat plate collectors. Figure 2 shows the monthly average solar fraction for the flat plate panels and Figure 3 shows the output for the evacuated tube system.


Figure 3. Performance of evacuated tubes collector through the year with a solar fraction of 68%, two panels at 45° tilt, 0° azimuth

 

Interpretation

It is clear that orientation and pitch play less of a factor in performance than most have previously thought in a climate like Michigan’s, with a significant amount of cloud in winter.

It is important to understand that shading and snow cover were not factored into this simulation. Most experts consider that a 30° pitch is necessary to ensure that snow slides off in a relatively short period to limit losses. This is something that should be studied further. There also seems to be a difference between the flat plate collectors and the evacuated tubes as to snow shedding. Some studies and some observations indicate that evacuated tubes, because of their shape and heat retention, take longer to shed snow.


Neigbourhood mapping. Key: full sun = optimal orientation, no shade • green tree = good orientation, little to no shade • yellow tree = medium orientation, some shade • red tree = east-west facing orientation, some shade • red octagon = no exposure

These results indicate that the majority of homes in Ann Arbor that are not shaded by trees or other buildings are adequate candidates for SDHW systems. It is very important to realize that this is very climate specific. It is equally important to understand that these climate-specific studies should be completed for every municipality trying to use solar energy.

Identifying good candidates for SDHW

The students also conducted neighbourhood drive-throughs to try to verify pitch and shading for a few sample neighbourhoods. They created a set of symbols that were added to a GIS database in an effort to create a map of solar candidates. The photograph (previous page) is a sample GIS photograph with the symbols indicating solar potential. The concept is to provide a database that would verify the solar potential of neighbourhoods and individual residences. It would also be possible for home owners or prospective purchasers to evaluate a home’s solar potential. The students came to the conclusion that it would take hundreds of hours to evaluate the entire city of Ann Arbor using the neighbourhood drive-through model.


Flush-mounted collectors look like skylights

The city is looking into methods of evaluating shading, which seems to be the most important factor, via satellite photos for direct entry into GIS without the neighbourhood drive-throughs. Work continues on the GIS mapping project.

In summary

The AA5KSR concept appears to be feasible, although the goal is an ambitious one considering the limited number of solar thermal systems currently in use and the current number of qualified installers.


An unattractive installation

Perhaps the most important finding to date is that orientation and pitch are less critical to performance than expected. This will allow several important outcomes. First, it means more homes can have solar roofs. Equally importantly, more homes can have the panels located parallel to the roof surface, which improves their appearance and acceptance by both the home owner and the neighbourhood. This would allow Ann Arbor to have more SDHW installations that look like that in the photograph above left and fewer like that in the photograph above right. Widespread user acceptance requires proper attention to aesthetics.

Wayne Appleyard is an Energy Commissioner and Architect at Sunstructures Architects, Ann Arbor, Michigan, US.
e-mail: [email protected]

David Konkle is City of Ann Arbor Energy Coordinator, Ann Arbor, Michigan, US.
e-mail: [email protected]

This article is based on a presentation at the American Solar Energy Association event held in Cleveland, Ohio, in July 2007.

Reference

 

  1. Water Tubes? … Not in My Living Room, Homeowner Attitudes Towards Passive Solar Aesthetics, Eighth National Passive Solar Conference Proceedings, 1983.