Solar Energy(s)

[juliettelucie]

I've just found a rather interesting article (in French, sorry everyone) on a new solar powerplant in Nevada that doesn't use PV panels but mirrors. It seems a pretty good concept to my (rather uninformed eye):

I didn't even know there were two types of solar energy. Is there a solar expert (cough *PI* cough) who could tell me how good this system is?

[Pikey]

I`m guessing it`s pinpointing the radiation/heat from the sunlight to generate steam which in turn can drive turbines or maybe focussing it directly onto heat exchangers and using the hot water/steam in other ways?
My french is very poor.

[juliettelucie]

Originally Posted by Pikey

I`m guessing it`s pinpointing the radiation/heat from the sunlight to generate steam which in turn can drive turbines or maybe focussing it directly onto heat exchangers and using the hot water/steam in other ways?
My french is very poor.

Yeah, that's the general idea. i was just wondering if this was one more thing that looks great when it's presented by a representative of a company, but turns out to be completely useless, or if it's really a good source of energy.

[Pikey]

Oh no it`s a perfectly GOOD way of collecting `waste` energy and using it.
After all the sunlight is just radiation and will be absorbed or dissipated naturally anyway.
I think it really is a very good idea.

[Bethrezen]

There you go.. see what people can do if they want to? And once that facility is built, it produces continuously with the only cost of mentainance.
Green AND cheap on long term (not to say free, the mentainance and the wages of people working there will cost a few bucks, but other than that, it's simple, SAFE, clean and free.
US only: -It may be the right time to redirect the money used to "pacify" the arabs in exchange for their oil, to do something for gaining energetic independence, finally.

[P.I.]

Originally Posted by juliettelucie

I didn't even know there were two types of solar energy.

Oh! There are a quite a few ways to use solar energy.

If you are looking for electricity, then best known ones would be photovoltaics, solar thermal plants (like this one) and solar towers.

Photovoltaic works (simplified explanation) with sunlight photons with certain wavelength striking a semi-conductor and knocking some electrons loose. Those electrons are “collected” resulting in DC current being produced.

Concentrating thermal plants work, as mentioned by Pickey, with using mirrors to concentrate the sunlight to a point where it warms a heat transfer fluid. That very hot fluid is then used to generate steam which is then circulated in a turbine to generate electricity. The fluid can also be circulated directly into a Sterling engine (I believe this method hold the record for highest efficiency for a large plant). These produce AC current.

There are many variations on use of this principle. One of them is the solar power tower. In this case, the mirrors focus the heat on a tower which contains a material/solution that can store a lot of heat at high temperature. This heat can then be recuperated and used to generate electricity with a turbine. The fact that the heat is stored greatly reduce intermittence (the fact that the sun shines when it does and not whenever we demand electricity) problems.
http://en.wikipedia.org/wiki/Solar_power_tower

A solar updraft tower is mostly a huge greenhouse with a chimney in the middle. As the sun warms the air in the greenhouse, this air goes up the chimney. The chimney contains turbines that convert the air movement to electricity. This is a little bit like a wind turbine where we create our own wind using the sun. It is also relatively easy to counter intermittence by placing dark containers of water (or anything cheap with a good thermal mass) in the greenhouse. The water gets warmed when the sun is at its peak and then it releases the heat back into the greenhouse after the sun goes down.
http://en.wikipedia.org/wiki/Solar_updraft_tower

There is also a solar downdraft tower.
http://en.wikipedia.org/wiki/Solar_downdraft_tower

There are advantages to each of these technologies.

Photovoltaics are great/cheap for small applications, like your calculator or to mount on the roof of a house. For DC applications, like your calculator or long distance high voltage DC lines, the current does not need to be converted. For roof mounted ones, the current must be converted to AC.

The thermal alternatives produce large scale electricity cheaper, and almost twice as efficiently, than currently mass produced photovoltaic technologies. But photovoltaics have quite a bit of room for improvement and impressive new technologies are on the way.

Thermal plants also make storage of energy to compensate for intermittence easier as it is much simpler and cheaper to store heat than electricity. You can store the heat, and then send it to the turbine when there is electricity demand. Usually, for a renewable plant to generate baseload electricity, we aim at designing a storage system that will allow the plant to cover 90% of the load. The rest of the load is fulfilled by natural gas or diesel generators. Above that 90%, the required storage system is just too big to make sense economically. For that particular plant Juliette linked, they did a great job of storage as the natural gas will only cover 2% of the load. That's what you can do when working with heat.
http://www.renewableenergyaccess.com...story?id=43336

Another advantage of thermal plants is that the waste heat can have other applications like water desalination and production of cold/ice using sorption chillers.

I think I’ve been long enough, just tell me if there are questions. I believe it would also be a good idea if I started a thread of other ways and technologies to use solar energy at home. I’ll see what I can come up with during the weekend.

You should take something for that cough Juliette. We don't want you sick.

[P.I.]

The follow up. Again, this is not meant to be an in-depth explanation, barely an overview of things out there and how they work to give people an idea of the possibilities.

There are basically two ways to use the sun in a house, the passive way and the active one. The passive one is generally incorporated in the design of the house itself and/or its surroundings while the active one uses specific systems. Also, the passive way is only concerned with managing the heat/cold while some active systems are concerned with electricity. Since houses consume mostly heat and cold, a lot of savings can be achieved passively.

Passive use of the sun

Shading

A house should be oriented with most of its windows facing the sun: facing south if you are on the northern hemisphere and north if you are on the southern hemisphere. The sun is higher during summer than during winter. So it is possible to dimension the eaves on top of windows to block the sun during summer and let it in during winter to help with the heating. See:
http://www1.sedo.energy.wa.gov.au/pages/windows.asp

Leafy trees make wonderful shading devices. They block the sun during summer and let it through during winter. Another advantage of vegetation is that the soil absorbs water which is evaporated on the leaves of the trees when the sun strikes them. That evaporation cools the air around the house during summer, resulting in a more pleasant environment.

Other shading devices include the different varieties of shudders and blinds. When using those, one should know that anything in the path of the sun will absorb heat and release it later. So this type of shades should be located outside the house during summer and inside during winter. Yes pickey, our grand parents really knew what they were doing when using shudders, it’s a shame we don’t anymore.

Reflection

Because of the relative position of the sun previously discussed, the sun strikes the roof more during summer and the façade more during winter. So it is possible to place a radiant barriers, which is basically a metal foil, in the attic of a house to reduce radiant heat exchange there. This will also reduce the radiative heat losses through the roof during winter.

The colour of the exterior also plays a big role. A dark material absorbs lots of radiation while a light coloured shiny one will reflect more. That is why houses in warm countries tend to be white.

Absorption

As mentioned before, materials absorb heat and release it later. Different materials have different capacity to absorb a certain quantity of heat. For example, a certain amount of heat will result in a larger temperature increase in a small paper sheet than in a large block of concrete. So, using a wall with a large thermal mass will result in a smaller temperature increase during the day and the heat will be slowly released to the space during the night. That is one of the reasons why houses in Spain for example have massive concrete walls.

There are new materials called PCM (Phase Change Materials) that can be used to improve the thermal mass aspect of walls. A PCM is a material that switch from solid to liquid and vice versa at a desired temperature. A lot of heat is absorbed during liquefaction, as a lot released during solidification. Both these processes occur at a constant temperature. So, for example, if I use a PCM that changes phase at 22ºC, this material will absorb heat during summer when the temperature is above 22 and will release it when the temperature will be below that, so it will reduce the load on a cooling system by helping maintain the temperature below the cooling setpoint (generally around 24ºC). They also are useful in a similar way during winter.

Active systems

Active systems can be classified in two categories depending on the type of energy required: electricity or heat. Converting solar radiation to heat is much more efficient than converting it to electricity. So, if a house is located in a cold climate, where it requires a lot of heat, it is preferable to use thermal collectors. In hot climates, it is generally preferable to use a small thermal collector to produce domestic hot water and more photovoltaics to produce electricity.

Electricity

Residential solar electricity generation is mostly done with PV (photovoltaics) which were described in the preceding post. In general, there are two types of systems: independent or grid connected. Independent systems need to include a storage system in the form of a battery to compensate for intermittence and a backup generator, something that increase the cost of a system and the maintenance needs. So they should be mostly used for remote locations. Grid connected systems return electricity to the grid when there are excesses and take some from the grid when there is not enough, so there is no need for storage and backup. In general, the utility company will credit the electricity returned to the grid.

Heat

Again, the active solar heating systems can be classified in two categories, either they work with air or liquid.

Air

As we make houses that are more and more energy efficient, these need to also be more and more airtight (have less infiltrations). So there is need to control their ventilation, bring in fresh air, to maintain a healthy indoor environment. The main advantage of controlled ventilation over infiltrations is that we can recuperate energy from the flow of air exiting the house and add it to the incoming air.

The most common way to use solar energy on ventilation air would be the use of a solar wall. A solar wall is simply a dark layer added to a wall on the south or north (depending on your hemisphere) face, but separated a few centimetres from that wall. The incoming air is circulated in the gap between the solar wall and the house wall before it goes into the house. The sun strikes the solar wall, heating the air. This hot air layer also reduces the heat losses from the house on the face where the solar wall is located. Evidently, these are used only during winter and a bypass is used to bring the air directly during summer.
http://www.solarwall.com/home/images/brochure.pdf

Liquid

A hydronic solar collector is mostly a dark surface, called absorber, which absorbs solar heat. A liquid circulates in the collector and is warmed by contact with the absorber. This hot liquid is then used to produce domestic hot water or to warm the house. If the temperature of he liquid is high enough, it can also be used to produce cold using an absorption or adsorption chiller.

Basically, there are 3 types of thermal solar collectors. Each has advantages and inconvenients and is best suited for specific applications.

1- Unglazed Flat Plate Collectors are basically composed of the absorber and the fluid circulation system. They are very cheap, light and rugged. But they are also uninsulated, and therefore lose a lot of heat, something that limits them to low temperature applications like pool heating. They perform poorly in cold climates.

2- Glazed Flat Plate Collectors are similar except that there is a glass in front of the absorber and insulation at its back. This considerably reduces the heat losses. They are the most commonly used type of collector for hot water and space heating applications.

3- Vacuum Tube Collectors. In this case, the absorber is located at the center of a tube in which a vacuum is present. The heat is brought to the heat transfer fluid by capillarity using a heat pipe. That completely eliminates the convection heat losses, so these collectors perform well for high temperature applications like solar cooling or in cold climates. They tend to be expensive and fragile though.

A nice description of these collectors, including images, can be found in the following presentation.
http://www.retscreen.net/download.ph...Course_swh.ppt

Hybrid panels

There are solar collectors that handle both the generation of electricity and heat. Basically, they are PV panels with a fluid circulating at their back. The fluid takes the heat produced by the PV, cooling the panel and improving the performance of the electricity generation. Unfortunately, the amount and temperature of the heat obtained is not always enough to justify investment in a fluid circulation loop and the pumping costs. But there is quite a bit of research work going on to improve their performance.

[johanes balontia]

P.I i've seen the video...i'm sure its gonna cost lots of money...
but it is good...

[juliettelucie]

Here I ask you a question, and I didn't even thank you for your answer...
You answered perfectly. (and yes, I'm working on that couch)

[P.I.]

Originally Posted by juliettelucie

(and yes, I'm working on that couch)

So you're working on your cough from your sofa?