Solar Energy
We will develop an index of solar energy systems, a pro/con table for each, and pointers to representative suppliers.
In the Energy 101 section we look at “energy” with a little more theory. Of course most of the energy we use is ultimately derived from the sun. Even fossil fuels are solar – just on a longer time scale. This section is meant to be practical, so things like wind, waves and wood, even if they are ultimately solar, are treated as a separate sources.
Passive solar
Where we use solar energy directly, without further intervention.
- Solar mass
- Direct sunlight
Solar thermal
Solar energy used to heat some sort of fluid or gas, and use that in turn to heat or cool the house.
Solar thermal also covers the large, industrial scale solar installations that use sun light to produce steam (or some other gas) and in turn drive turbines to generate electricity. This technology is not really feasible for an individual house, so we cover the subject under “world”.
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Solar air heating systems
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- Description: A relatively simple enclosure, with a black background, an airspace and a glass cover. Sunlight heats the air and generates a convective flow. The warm air can be piped directly into a room, or multiple rooms via ducts. The system is best mounted on a south facing wall, but can be roof mounted. More technical details here, here and here.
- Pro: Simple, inexpensive, can do-it-yourself, forgiving in terms of construction and leaks (leaks do degrade performance but do not cripple the systems), can be designed for standalone use – no need for electric fans or pumps.
- Con: Needs a relatively large south facing area, ideally a wall, not shaded by permanent plants or neighboring structures. Not very attractive in a suburban setting – the neighbors will not love you for presenting them with blank, black wall spaces. Not the most efficient use of solar energy.
It does not have to look bad. Here is a nicely executed refurbished house from Germany. The collectors are integrated with upgraded exterior insulation. The deciduous tree is a nice touch; provides shade in summer and in winter, stripped of leaves, allows sun to hit the collectors.
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Solar hot water system
Solar hot water systems have been a feature of Australian houses since the mid 1960’s. Because freezing temperatures are so rare the systems can be simple, just water and a roof-mounted collector panel and tank. They are not very attractive, but do the job at a reasonable price.
Roof mounted solar hot water system, Kendenup, Western Australia
In the US they are much less common. In the North-East, and anywhere where there are freezing temperatures they also need to be more complicated, with a anti-freeze solution in a primary heating loop, which then heats the water in a storage tank to supply the house. It is an ideal setup to supplement an existing electric or gas-fired storage tank. Electric tanks are great because they are better insulated than gas-fired tanks, as they do not need the central “chimney” where the burning gas heats the water. There are some good incentives to either add solar to existing tanks, or replace the existing tank altogether. At the time of writing this update (October 2010) the NJ incentive of $1200 is only available for conversions from natural gas fired systems. The funds for electric systems have run out.
Photo-voltaic
Turning sunlight directly to electricity. I don’t go into much detail here because solar PV is such a popular topic with millions of web sites, hundreds of suppliers and installers.
Here (the solar-estimate.org home page) and here (the estimator) is a good site to estimate the cost and benefits of photo-voltaic systems. I have also explored the options and commented in the energyrealist blog; on how not to sell PV, and the importance and risk of SRECs.
In each case the initial cost of the system is in the $60 to $80 thousand range. Payback, with the SRECs (Solar Renewable Energy Certificates), is in the range of 5 years, with pure profit beyond that. If you have the deep pockets for that sort of investment then it’s better, and more certain, than most other investment opportunities out there. If you are at all interested contact a local supplier and look at their proposal – it may well make sense!
On the technical side there is always room for debate and individual decision.
A couple of e.g.’s:
Integrated Inverters. The integrated inverters mean that the panel delivers AC directly, without another unit to mount in the house and lower losses in “piping” the power from the panel to the house supply. Also each panel is self sufficient, failure of an inverter loses just one panel, not all of them. The cons are multiple; cost, lack of experience (will those inverters hold up to the environmental extremes up on the roof?), theft etc. It’s a space worth watching.
Placement of panels. Most times the only place to mount panels is on the roof and you are stuck with the orientation and pitch of the roof – unless you are building new and/or are willing to put some more elaborate frame work up there.
If you can mount on the ground there is the usual discussion:
- Pro: easy to maintain and clean, able to orient exactly, house retains appearance.
- Con: space requirement, appearance, more difficult to get approvals from authorities, theft, damage by cars and yard equipment.
But if can mount on the ground it is probably worthwhile to allow the panels to tilt with the seasons. Experience seems to indicate that fully tracking systems are not viable (yet).
The diagram shows the theory behind changing the tilt in the panel – here for 40 degrees N (New York city and surroundings). It’s important that the panels face true south, not magnetic (which is currently some 13 degrees west of south in the NYC area).
With a fixed panel you lose some 25% of the power available over a year. With a tilting panel there are a number of places that give advice on the optimum angle. In the diagram above the equinox angle (E) is the same as the latitude (measured from the horizontal up) and the summer angle is 23 degrees lower, while winter is 23 degrees steeper. Those angles are not ideal – because the summer sun only rises that high on noon on the 21st of June and only sinks so low on December 21. Also, unless you have some sort of automatic system I assume you will only want to change the angle a few times a year. The diagram below shows a possible mechanism with a threaded rod and a simple home made tilt indicator, calibrated by month or season. In addition to the rod the panels will need to have some secure framing to hold the heavy panels against wind and leaning people.
For much more on placement and tilting of solar PV panels I like these sites:
www.macslab.com/optsolar.html and www.freesunpower.com/solarpanels.php
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Such forward-looking statements are any statements made that are not
historical facts. The trading of REC s traded clearly indicates that there has been little attempt to expand them to augment the national grid – up to a maximum of five megawatts.
We need to be sold at the same time an equally active group of entrepreneurs are working
to advocate for a price on carbon — which I believe will revolutionize the world we live in.