Is it better to have a green roof or one covered with solar panels?

Featured Debate 21

Of course there's been discussion of trees v. solar panels, but what about the use of your roof?  Money considerations aside, what do you think is better?  To have a green roof?  Or a roof covered in solar panels?

This debate topic comes courtesy of dana1981.  Remember, folks, if you've got a debate topic, just let me know!

I'm going to have to do some pretty serious research before I come up with an answer to this one.  A green roof will reduce your home energy consumption by adding insulation and the vegetation will absorb some CO2.  It also reduces the urban heat island effect, and eases strain on stormwater systems by absorbing rain water.  On the other hand, solar panels can provide a home with a significant amount of carbon-free energy.  I'll have to crunch some numbers to see which provides the greater benefit, but I think it's a fascinating question.

Great question!

Like dana1981 said, I would need to do some research- and not to miss the whole point of the debate- but what about  a combination?  Green roof over your house, and solar panels over your garage?  Green roof and solar panel siding?  (Can they do that?)

Or skip the solar panels, and do a green roof with mini wind turbines?

Yeah actually I was thinking about the possibility of a combination green/solar roof, too.

So far I've found that a square meter of vetiver would absorb about 5 kg of CO2 during a year's growth.  That's a big, deep-rooted grass, so let's assume grass grown on a green roof would abosorb half as much.

A green roof will reduce ~10% on summer energy consumption from its insulation - something which a solar panel will not improve.

Solar panels can provide 130 watts per square meter of power, and useable panel output will be ~150 kWh/m2/year.

without research figures to back me up, one question that arises in this debate is what type of climate the house is located in.  if one of the significant pros of a green roof (per dana1981) is its ability to insulate, then this 'pro' may carry less weight in a more neutral climate than an extreme climate.  in this case, perhaps the solar paneling would be more useful.  

Lola has a good point- if you live somewhere that tends to be overcast, a solar panel roof won't make much of a difference.  But I am sure you can find green roof plants that don't mind shady conditions.

Okay so just doing a quick and dirty calculation using a house like mine as an example:

Green Roof: My roof has ~1000 sq. ft. of area = ~ 93 square meters.  Let's call it 100.

So over 1 year, if I had a green roof, the grass would absorb 250 kg CO2.

It would also reduce my summer heating bill by ~10%, saving me ~180 kWh.  There would also be some winter savings from the added insulation decreasing heating needs, so let's call it a total of 300 kWh per year.

My utility provider is PG&E, which emits 0.5 lbs CO2 per kWh, so that 300 kWh reduction would save 150 lbs of CO2 emissions.  1 lb = 0.45 kg, so that's 68.2 kg.  Add the 250 kg of CO2 absorption, and you've got ~320 kg of CO2 emissions saved per year.

Solar Roof: 150 kWh/m^2 * 100 m^2 = 15,000 kWh of solar energy generation.  Except I only consume about 3,000 kWh/year.  Whoops.  So I guess I don't need to cover my whole roof with solar panels.

Let's just say I get enough solar panels to cover 75% of my energy bill, or 2,400 kWh/yr.  On the PG&E grid, that accounts for 1,400 lbs of CO2 emissions saved, or ~500 kg of CO2 emissions saved per year.

So they're pretty similar results on the scale of my house, using the PG&E power mix.  Solar panels save a bit more CO2, but I neglected the other benefits of a green roof - it lasts a lot longer than a conventional roof (less raw materials, overall energy for construction, etc.), puts less strain on the stormwater systems, and reduces the UHI effect.

If a bunch of homes used green roofs, it could reduce the UHI effect enough to reduce their A/C use even further and save even more energy.

As alluded to earlier, one good solution might be to combine the two.  For example, since I could get most of my energy needs from solar panels covering just one-fifth of my roof, a possible solution would be to build a green roof and embed some solar panels in it.  Of course, that would be a very expensive proposition, but potentially a very green one.

And yes, certainly the local climate will be a factor in determine which roof is better.  If there isn't much sun year round, then a green roof would be better.  Also if you live in an area with dirtier power production than CA (which is most areas), the benefits of saving energy by using solar panels increase.

So basically in a sunnier area, solar panels are probably better.  In a cloudier area, a green roof is probably better.

Isn't it only practical to place solar panels on the south facing side of your roof? You could easliy do a green roof on the other side. Also you could even, if you were crafty (or determined), do a green roof under your (elevated) solar panels with some moss or something else that thrives in the shade.

Personally, I like the UHI created by Milwaukee, it helps eat tornados that come near...

Does anyone know what plants would work well on a semi-shaded green roof in the mid-Atlantic region? 

I'm not sure exactly, but this resource list was put together by the Mid-Atlantic EPA and maybe some of those sites have suggestions!

Wow, I'm amazed at how much power the solar panels would generate. Think what you could do with that extra power - you could use it to... *drum roll* .... power the nations fleet of electric vehicles rather than building more coal and nuclear power stations!

I wonder how much infrastructure would need to be built for a distributed power grid like that? Perhaps instead of building giant stand alone solar power stations, they would be better of retrofitting a neighborhood? It would add to the value of your property, reduce the amount of electricity lost in transporting the electricity, and would save a huge amount of money and land needed to build a a large solar power station.

Were you assuming maximum performance from the solar panels each day? How does that translate to the real world? We live in MN, so I assume we wouldn't have much production in the cooler months.

I wonder how green roofs perform in northern climates. If they are frozen how would that affect their ability to insulate the building?

(The City I work for was thinking about putting in a green roof, but chose not to because one person was concerned they would have to mow it!! I can't believe they didn't look into it. At least they are using 100% recycled shingles.)

The assumption was just an average solar panel performance for Australia, just because that's the scenario I was able to find in a quick internet search.

In theory, decentralization of energy production would be ideal: No transfer; energy would be produced where it's consumed. There is, however, a force that runs counter to this: We are a highly specialized society. Not everyone wants to be an energy producer.

I think that very few people are against the idea of having a solar panel (or solar shingles?) on their roof that reduce the cost of their energy bill.

I don't ever see this happening though because the energy companies will do everything they can to protect their income If they can't make money from providing power that they generate, they will just charge more for the energy you draw when local generation isn't an option (ie night time).

One has to wonder what role the central government will play in this as well. Is it possible that the government would assume the role of maintaining the national grid and supplying energy at night? I personally can not see that happening here, because of political views on the role of government.

(Somehow it's believed that a private company that makes millions of dollars of profit is more efficient than a not for profit government organization... yeah right).

Imagine a world without power companies.... I'd bet it would reduce your power bill...

I think that for what you're describing to become reality, solar or wind (or some other form of) energy production has to become really cheap and as easy as pie, so easy that anyone can do it. In other words, the barrier to entry has to collapse.

For as long as there's a barrier to entry, there will be an industry

That's a very hard question and I don't think it can be just answered by comparing carbon footprints for the one house, you have to look at the whole picture and all the indirect advantage and disadvantages.While it may be true that more CO2 may be injected into the atmosphere if using a green roof instead of solar panels, don't forget that people do have an option normally to subscribe to renewable energy sources such as wind energy. You also have to look at the overall impact of decreased green space when choosing the solar panel option. This have a very negative impact such urban heat island, and increased water runoff which drains pollutants into waterways (more details at  www.hapcity.com/greenLiving/pages/greenRooftop.jsf if you're interested).

Considering these impacts on surrounding environment, I'd say it's better to go green than with solar panels.