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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    •  
      CommentAuthorPaulT
    • CommentTimeJan 26th 2009 edited
     
    P in M
    - I am using the book "Renewable Energy" (Bolye et al ) - an Open University course book as my source

    They state "Geothermal is the only for of renewable energy that is independent of the sun"

    The most important word in my statement is "net"; From a core temperature fo 7000 to space there is a net flow of heat. The Solar radiation is a localised flux in this heat flow
  1.  
    Posted By: PaulTThey state "Geothermal is the only for of renewable energy that is independent of the sun"


    Ah, this is because we're mixing up terms here. Geothermal energy is different from that used in a GSHP - even though people often refer to GSHPs as "geothermal heating". A GSHP is a Ground Source heat pump and gets its heat from the ground, which is heated by the sun to a temperature that's equal to the annual average air temperature. True geothermal is from tectonic sources so as plate friction (as in Iceland) or other areas where there's a magma chamber that causes heating of the deep (1 to several km) ground - but this is not directly core heat. However, plate motion is basically convection currents from the earth's core so it is correct to say this is independent of the sun (though the Sun's gravity has an effect on heating the core due to tidal forces).

    The system in Southampton is geothermal (I cannot remember the source of the rock heating there), but my two GSHP systems are definitely not - they're just ground exchange.

    Hope this clears things up - you're not noted for posting incorrect information!

    Paul in Montreal.
    • CommentAuthorShepherd
    • CommentTimeJan 26th 2009 edited
     
    <blockquote><cite>Posted By: Fostertom </cite> Heat shd be recycled, many times in rank order of the grade of heat truly reqd for each process, so its grade gradually works down the scale by stages from high-grade requirement (cooking) to medium-grade requirement (dhw) to low-grade requirement (space heating) until finally exhausted as genuinely no-longer-useable low-grade heat. That's the way to achieve spectacular demand reduction right across the scale, from steel making through power generation to domestic and horticulture - use x amount of liberated heat three or four times over, instead of throwing it away and starting again four times over.</blockquote>

    Yet another re-enactment 17th century living parallel - ovens of the period were brick/stone structures built against the chimney breast. They were heated up by putting in hot coals plus new fuel and a fire burned in there for a couple of hours. Once really hot, you started baking. There was a baking cycle - starting with whatever needed the hottest temperature (bread if I remember rightly) and working down until you put in something that needed only tepid temperatures.
    •  
      CommentAuthorPaulT
    • CommentTimeJan 26th 2009
     
    Hi,

    I do have my moments, good that we can edit!

    The Southampton system is small scale (Compared to eg Paris).
    THe bore hole is 1800m deep into a fromation Known as "Sherwood sandstone" - a "Sponge" with water(brine) at 70C

    Pressure takes the water to 100m from the surface and is pumped to the surface.

    The brine is discharged into the sea and so is not re-charging the aquifier and so will have a limited life.

    The pump is now driven by electricity from a CHP which also feeds the district heating system (neat).

    -------------------------------------------------------------------------
    Around 50% of geothermal energy is radioactive and this is one of the reasons why there are hot spots (along with the other reasons you give)
    •  
      CommentAuthorfostertom
    • CommentTimeJan 27th 2009
     
    PaulT, I'm largely with you on this - the earth has never in its history been a nett gainer of electromagnetic (incl heat) energy from the sun, or anywhere else. The sun does not 'warm the earth', tho subjectively that's exactly what it seems to do. The earth has always been a nett loser of energy, as it's always been generating nuclear energy at its core, which has to be disposed of - a constant nett flow of energy outward, through the surface, to space. Even the moon, whose nuclear fuel is long spent (if it ever had any) does not gain nett energy from the sun.

    It's true that the presence of the sun's radiation, combined with the atmosphere as insulation blanket and 'greenhouse' does change the shape of the temp gradient that exists between the super-hot core, and outer space, with the result that the earth's av surface temp is warmer than it would otherwise be. I always think that's more thanks to the atmosphere, than the sun. The moon, which has the same sun but lacks atmosphere, has a very different av surface temp.

    Anyway, that is not repeat not a fancy way of saying that 'the sun warms the earth' in the sense that there's an inward nett flow of energy that, for instance, will penetrate the surface to replenish what GSHP takes out.

    The natural state is that heat is flowing outward, through the zone where the GSHP coils are, to the surface. There may be local fortuitous replenishment e.g. by groundwater flow, but that's not systematically relied upon by the GSHP boys - they really do believe that replenishment is by the sun from the surface. And they're right, in a way - replenishment does of course happen, but only following and as a result of the creation of an artificially exagerated reverse temp gradient by refrigerating the ground. No refrigeration, no nett flow of solar heat into the ground.

    Talk about fighting the natural order - and then calling it eco!
  2.  
    Posted By: fostertomPaulT, I'm largely with you on this - the earth has never in its history been a nett gainer of electromagnetic (incl heat) energy from the sun, or anywhere else.


    Tom, go and google "thermodynamic equilibrium" and you'll understand why there isn't a nett gain or loss of heat. If there was no sun, the temperature of the earth at the surface would be around 50K, with the sun it is 289.35K and is in dynamic thermal equilibrium.

    Posted By: fostertomThere may be local fortuitous replenishment e.g. by groundwater flow, but that's not systematically relied upon by the GSHP boys - they really do believe that replenishment is by the sun from the surface.


    Yes it is because, to a large extent, it is. Of course, there is some heat from the core, but that is a small fraction.

    Posted By: fostertomTalk about fighting the natural order - and then calling it eco!


    Sigh. It works because of the "natural order" of the laws of thermodynamics. It's hard to think of a form of heating that is more eco that's not purely passive.

    Paul in Montreal.
  3.  
    OK, here's some references:

    http://esrc.stfx.ca/pdf/grl-1.pdf This paper describes the heat flux into the earth's surface in Eastern Canada over the past 1000 years. If there wasn't positive heat flux into the earth, the global temperature could not be increasing. In fact, this paper notes that the flux diminished at certain points in the past (corresponding to the Little Ice Age).

    And this powerpoint presentation goes into great detail about the core heat of the earth (of the order of 10^13W) and how it is dissipated (mainly as convection current which move the tectonic plates around) and contrasts it with the energy received from the sun (of the order of 10^17W - i.e. about 10,000 times more than the core energy). http://www.eas.slu.edu/People/LZhu/teaching/eas437/heat.ppt

    Tom, you may of the opinion that the sun's energy doesn't contribute, but the raw physical facts don't support your hypothesis.

    Paul in Montreal.
    •  
      CommentAuthorfostertom
    • CommentTimeJan 27th 2009 edited
     
    It's a classic confusion between temperature and heat. The sun does raise surface temp:
    Posted By: fostertomthe sun's radiation, combined with the atmosphere as insulation blanket and 'greenhouse' does change the shape of the temp gradient that exists between the super-hot core, and outer space, with the result that the earth's av surface temp is warmer than it would otherwise be
    but does not contribute nett electromagnetic energy to the planet (I suppose it may slow earth's nett loss of energy).

    If earth were continually gaining nett energy, from the sun or anywhere, why hasn't it superheated and evaporated long ago? Where does that continuous energy gain end up?
    • CommentAuthorpmcc
    • CommentTimeJul 1st 2009
     
    In another thread (http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4154&page=3) doubts were expressed about the accuracy of some of the material in Prof MacKay's book and the validity of some of the assumptions. To reopen this discussion:

    1. Is there a better synthesis of our energy options out there?

    2. If not, then do those with the expertise to correct Prof MacKay have a moral obligation to do so, thereby making this important resource more effective for the public at large?

    3. What specifically would you like to improve in the book?
    • CommentAuthorpmcc
    • CommentTimeJul 1st 2009
     
    Overall I think it's a remarkable work, written in a witty and lucid style. However, like Keith I do find his advocacy of air source heat pumps grates a little (page 153), especially when he promised at the start of the book to keep his opinions to himself!

    However, no matter how much we may dislike such a technology-based solution, the numbers do add up. Let's say burning gas in a modern boiler is 85% efficient and all electricity is generated using coal at 35% efficiency. The ASHP needs a CoP of 2.5 to use less net fossil fuel for the same level of heating. If we use the gas to generate electricity rather than burning in home boilers, then at CoP of 2.5 the ASHP uses around two thirds of the energy for the same effect. As renewables are factored into the electricity generation mix, the saving multiplies.

    Consider the embodied energy of the equipment. Will an ASHP last longer than a gas boiler and be cheaper to make? I don't have ready figures for this but suspect that a fan + compressor is pretty well established technology and ASHP prices will plummet in the coming years.

    This raises the interesting possibility that heat pump units will be sufficiently cheap to be combined with solar thermal panels as an integrated solution.
    • CommentAuthorGBP-Keith
    • CommentTimeJul 1st 2009
     
    Thanks for this PMCC. By the way, do you have any ties with this book or the author?

    PS: I don't recall sayingd that I don't like air source heat pumps. The truth is I don't like all heat pumps full stop! In my opinion, the human race is far to naive to be playing with a technology full of false promises and potential for compromise!

    I'm no scientist but I would like to add that I believe that McKay is guilty of changing statistics facts and basic scientific assumptions to suit his own discourse. He admits it on page 27 and elsewhere in the book which for me immediately makes his book into a work of fiction.
    • CommentAuthorpmcc
    • CommentTimeJul 1st 2009
     
    "By the way, do you have any ties with this book or the author?"

    None whatsoever, other than my surname also begins with 'Mac' :). I just like the way he explains things and have not seen any other single work covering this subject matter of remotely similar breadth or clarity. I'm also a physicist by training so perhaps think in the same way as the good prof.

    "the human race is far to naive to be playing with a technology full of false promises and potential for compromise"

    Perhaps we agree on this. My main objection is that we're steadily becoming more dependent on a complex and potentially fragile network of technology. Take away any key part of the network and there will be a swift and arresting crash. The problem is essentially one of scale - what works on small scales doesn't necessarily work when applied to everyone everywhere (which it must to be a viable alternative to fossil fuels). That said, heat pump units themselves are basically just fridges, technically really quite simple stuff as far as I've understood it! I imagine that heat pumps are pretty much viable in the long term providing sources of locally generated electricity exist, heat is sufficiently important and there's not enough biomass around to burn instead.

    "I'm no scientist but I would like to add that I believe that McKay is guilty of changing statistics facts and basic scientific assumptions to suit his own discourse. He admits it on page 27 and elsewhere in the book which for me immediately makes his book into a work of fiction."

    Mmm, I don't read it like that. Page 27 (picky details section) is IMO a pretty clear explanation of how energy sources can be compared. Not even objectionably chatty :). A book like that must use a consistent approach, otherwise it's hard to compare things meaningfully. One of the things the book does very well is to rationalise the confusing mess of units into a relatively simple set of expressions. MacKay is doing what physicists do, which is remove the noise to see the underlying simpler truths.
    •  
      CommentAuthordjh
    • CommentTimeJul 2nd 2009 edited
     
    In another thread - http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4154&page=3#Item_13 -
    renewablejohn said:Our tracking panels are fixed it is only the focus tracking reflector that moves so the area does not increase so my point is valid.

    I'm not sure what you mean - do you have some photos or drawings?

    I look forward to your £100 donation. If your argument was correct than there would be no life at the bottom of the ocean as there is no light penetration however the ocean bed is full of plants and animals reliant on heat and nutrients not sunlight.

    My bet was in relation to the calorific value of rhubarb! Please don't try to move the ground.

    There are indeed organisms that live at the bottom of the sea that don't rely on light - but they aren't plants:
    http://en.wikipedia.org/wiki/Plant
    For example, animals in general don't rely on light, they eat other organisms or their waste: http://en.wikipedia.org/wiki/Animal
    There are as you say organisms that rely directly on chemicals from hot vents, mainly bacteria:
    http://en.wikipedia.org/wiki/Hydrothermal_vent#Biological_communities
    and there are organisms that rely on radiation or just about any other type of energy gradient.

    I didn't claim that all life depends directly on the sun! But the thing that is the main distinguishing feature of plants is that they do; they photosynthesize. And rhubarb in a dark forcing shed is 'eating' itself. Like a caterpillar turning into a butterfly. Or an egg into a chicken. Or a seed into a plant.
    • CommentAuthorpmcc
    • CommentTimeJul 2nd 2009
     
    Oops, should have posted to this thread...

    John

    "Mckay quotes useable solar energy at 50w/m2 and then extrapolates this to solar panels biomass etc when in the real world uk tracking solar panels achieve in excess of 100w/m2 and southern europe 200w/m2."

    The book is about the UK, so no point mentioning elsewhere. The figure 50w/m2 comes from approx average usable insolation of 100w/m2 * 50% capture device efficiency. He's talking about flat solar collectors on all south facing roofs in the UK. If your tracker technology could double this performance by capturing closer to 100% of available insolation, and if you feel it could be scaled out to fit on all roofs in the UK, then MacKay needs to know about this so he can update the book.

    Regarding the 100w/m2 estimated usable solar energy, remember that the book is looking at feasibility and using round numbers to make it comprehensible. The UK spans a range of climate zones and Sutherland is quite different from Surrey. However, if you feel 100 is a significant under-estimate please explain how the calculation could be improved.

    "He also assumes plants only grow during hours of sunlight. I would suggest he travels to the rhubarb sheds in Yorkshire to disprove his theories"

    Are you referring to chapter 6? If so, I'm baffled. Please re-read the first paragraph (after point 4) on page 43. How can this be misinterpreted as under-estimating biomass production? If anything he wildly but consciously over-estimates it.
  4.  
    djh

    What yourself and Prof MacKay fails to realise is that light is not the major influence in plant growth. As a commercial grower i know that plants in a polytunnel which only receive 80% light will grow better than plants in an open field. We have taken this one step further by using 70% shading windbreak to produce far better crops than crops on an open plot. We have also proved that crops could be grown all year round with the existing UK light levels and it is heat not light which is the limiting factor.
  5.  
    pmcc

    The point is if McKay was right with his solar figures then you would not be able to obtain 100w/m2 from a tracker system as the best panels are only 20% efficient therefore his fundamental assumptions must be wrong as it does not tally with the actual performance data.
    •  
      CommentAuthordjh
    • CommentTimeJul 3rd 2009
     
    Posted By: renewablejohnWhat yourself and Prof MacKay fails to realise is that light is not the major influence in plant growth. As a commercial grower i know that plants in a polytunnel which only receive 80% light will grow better than plants in an open field. We have taken this one step further by using 70% shading windbreak to produce far better crops than crops on an open plot. We have also proved that crops could be grown all year round with the existing UK light levels and it is heat not light which is the limiting factor.

    You do keep shifting the ground, don't you? Never respond, always try to move to another point. Your arrogance in assuming what either I or Mackay, or anybody else for that matter, knows about a subject is astounding. Neither you nor I know what Mackay knows except for what we've read, heard or seen.

    One thing you have read, I hope, is Mackay's statement "The most efficient plants in Europe are about 2%-efficient at turning solar energy into carbohydrates, which would suggest that plants might deliver 2 W/m2; however, their efficiency drops at higher light levels, and the best performance of any energy crops in Europe is closer to 0.5 W/m2." on p43. So he agrees with you that light is not the limiting factor. He also provides references for all his figures. The point is that light is the energy source and deriving a light-efficiency is therefore a sensible way of measuring potential energy output from biomass. But he invites you to do your own sums. What efficiency do YOU think is reasonable, if you disagree with his estimate or method?

    Heating increases growth rates in cold climates and allows cropping at times of year that are otherwise unusable, which is valuable for cash crops. Conventionally, the heat is often supplied nowadays by gas, and gas burners are even used in hot countries to increase the CO2 levels in the greenhouses, since this is another growth-limiting factor. But burning gas to grow an energy crop needs careful thought - it's not the most obvious thing to do!

    Using water storage to increase thermal mass in greenhouses is a well-known technique, of course. You might be interested in the solar-heated bubble-insulated greenhouses that are used in Canada or the straw-bale insulated one in New Mexico, since these techniques use a lot less energy. I don't know if you've come across them?

    Cheers, Dave

    PS You are still talking about your tracker system, but you still haven't explained it. Do you have pictures or drawings? Or can you explain it in words? I'd be interested to see the data behind your 100 W/m2 yield, too.
    • CommentAuthorpmcc
    • CommentTimeJul 4th 2009
     
    "The point is if McKay was right with his solar figures then you would not be able to obtain 100w/m2 from a tracker system as the best panels are only 20% efficient therefore his fundamental assumptions must be wrong as it does not tally with the actual performance data."

    John, are you talking about solar thermal or electric? Solid state physics tells us that around 30% is the theoretical maximum efficiency that a single semiconductor material can convert light into electricity. Practical devices today have not got much above 20%, and mass-produced devices around 15% or less. Layers with different materials hold out promise of somewhat greater efficiency in future, but there are many engineering problems to solve.

    http://www.solarserver.de/wissen/photovoltaik-e.html
    http://www.lbl.gov/Science-Articles/Archive/MSD-full-spectrum-solar-cell.html

    The 50w/m2 in MacKay's book was for solar thermal, based on 50% efficient collectors and 100w/m2 average incident radiation. Most commercial collectors claim to do better than 50%, and no doubt Surrey in summer gets more than 100w/m2. But the point is that unless these numbers double, they provide a reasonable rough estimate of the likely large-scale performance of this type of technology. Like Dave I would be interested in any comments you have about the reasonableness of the 100w/m2 assumption.
  6.  
    pmcc

    type in 2 axis tracking in the following model and you will see how unrealistic the assumptions made by Prof McKay are in reality.

    http://re.jrc.ec.europa.eu/pvgis/apps3/pvest.php
  7.  
    <blockquote><cite>Posted By: djh</cite><blockquote><cite>Posted By: renewablejohn</cite>What yourself and Prof MacKay fails to realise is that light is not the major influence in plant growth. As a commercial grower i know that plants in a polytunnel which only receive 80% light will grow better than plants in an open field. We have taken this one step further by using 70% shading windbreak to produce far better crops than crops on an open plot. We have also proved that crops could be grown all year round with the existing UK light levels and it is heat not light which is the limiting factor.</blockquote>
    You do keep shifting the ground, don't you? Never respond, always try to move to another point. Your arrogance in assuming what either I or Mackay, or anybody else for that matter, knows about a subject is astounding. Neither you nor I know what Mackay knows except for what we've read, heard or seen.

    One thing you have read, I hope, is Mackay's statement "The most efficient plants in Europe are about 2%-efficient at turning solar energy into carbohydrates, which would suggest that plants might deliver 2 W/m2; however, their efficiency drops at higher light levels, and the best performance of any energy crops in Europe is closer to 0.5 W/m2." on p43. So he agrees with you that light is not the limiting factor. He also provides references for all his figures. The point is that light is the energy source and deriving a light-efficiency is therefore a sensible way of measuring potential energy output from biomass. But he invites you to do your own sums. What efficiency do YOU think is reasonable, if you disagree with his estimate or method?

    Heating increases growth rates in cold climates and allows cropping at times of year that are otherwise unusable, which is valuable for cash crops. Conventionally, the heat is often supplied nowadays by gas, and gas burners are even used in hot countries to increase the CO2 levels in the greenhouses, since this is another growth-limiting factor. But burning gas to grow an energy crop needs careful thought - it's not the most obvious thing to do!

    Using water storage to increase thermal mass in greenhouses is a well-known technique, of course. You might be interested in the solar-heated bubble-insulated greenhouses that are used in Canada or the straw-bale insulated one in New Mexico, since these techniques use a lot less energy. I don't know if you've come across them?

    Cheers, Dave

    PS You are still talking about your tracker system, but you still haven't explained it. Do you have pictures or drawings? Or can you explain it in words? I'd be interested to see the data behind your 100 W/m2 yield, too.</blockquote>

    <blockquote><cite>Posted By: djh</cite><blockquote><cite>Posted By: renewablejohn</cite>What yourself and Prof MacKay fails to realise is that light is not the major influence in plant growth. As a commercial grower i know that plants in a polytunnel which only receive 80% light will grow better than plants in an open field. We have taken this one step further by using 70% shading windbreak to produce far better crops than crops on an open plot. We have also proved that crops could be grown all year round with the existing UK light levels and it is heat not light which is the limiting factor.</blockquote>
    You do keep shifting the ground, don't you? Never respond, always try to move to another point. Your arrogance in assuming what either I or Mackay, or anybody else for that matter, knows about a subject is astounding. Neither you nor I know what Mackay knows except for what we've read, heard or seen.

    One thing you have read, I hope, is Mackay's statement "The most efficient plants in Europe are about 2%-efficient at turning solar energy into carbohydrates, which would suggest that plants might deliver 2 W/m2; however, their efficiency drops at higher light levels, and the best performance of any energy crops in Europe is closer to 0.5 W/m2." on p43. So he agrees with you that light is not the limiting factor. He also provides references for all his figures. The point is that light is the energy source and deriving a light-efficiency is therefore a sensible way of measuring potential energy output from biomass. But he invites you to do your own sums. What efficiency do YOU think is reasonable, if you disagree with his estimate or method?

    Heating increases growth rates in cold climates and allows cropping at times of year that are otherwise unusable, which is valuable for cash crops. Conventionally, the heat is often supplied nowadays by gas, and gas burners are even used in hot countries to increase the CO2 levels in the greenhouses, since this is another growth-limiting factor. But burning gas to grow an energy crop needs careful thought - it's not the most obvious thing to do!

    Using water storage to increase thermal mass in greenhouses is a well-known technique, of course. You might be interested in the solar-heated bubble-insulated greenhouses that are used in Canada or the straw-bale insulated one in New Mexico, since these techniques use a lot less energy. I don't know if you've come across them?

    Cheers, Dave

    PS You are still talking about your tracker system, but you still haven't explained it. Do you have pictures or drawings? Or can you explain it in words? I'd be interested to see the data behind your 100 W/m2 yield, too.</blockquote>

    djh

    If you cannot follow the thread from light levels in a rhubarb shed to growing crops under reduced light conditions then it is pointless me continuing. As for arrogance I will requote my original statement which I still stand by but obviously does not appear on this thread.


    The problem with McKay's book is that large sections are just wrong. Fortunately some of us have actual experience of Biomass and Solar and can workout were his assumptions are wrong. The difficulty arises when people with influence and no practical experience quote from his book as gospel. Just because he is a Professor does not make his hypothesis correct.
    • CommentAuthorpmcc
    • CommentTimeJul 6th 2009
     
    "The point is if McKay was right with his solar figures then you would not be able to obtain 100w/m2 from a tracker system as the best panels are only 20% efficient therefore his fundamental assumptions must be wrong as it does not tally with the actual performance data."

    John, thanks for the link - very interesting. Looking at the figures for electricity generated in the simulator for Edinburgh in March (so we're comparing the same thing as much as possible):

    Fixed panel tilted at 35 deg = 2.45 kWh
    2-way tracker = 3.10 kWh

    Now, we need to know the number of square metres of collector used to derive these numbers. Unfortunately that's not stated, because the simulator cleverly eliminates it from the calculation by assuming that a 1Kwh peak array is being used (which will vary in size depending on the efficiency of the type and make of collector). Let's charitably use 20% effective efficiency, because it doesn't get too hot in Edinburgh and MacKay uses that too. We've thus got a 5m2 array.

    W/m2 (fixed) = 2450 / 5 / 24 = 20.4
    W/m2 (tracker) = 3100 / 5 / 24 = 25.8

    These numbers are within spitting distance of those used by MacKay. Certainly nothing there to indicate that his assumptions are way out.

    The tracker does indeed yield better results, by about 50% over the year. So I reckon you're right - if trackers are fitted to every south facing roof then we could expect 7.5 kWh per day per person (with 20% efficient collectors). Do you reckon this technology could be built out on mass scale like this? A link for more info, ideally with diagram, would be very interesting.

    Conclusion - it's so very easy to get confused by detail (can't see wood for trees). This is what makes MacKay's book so valuable. He cuts through the blizzard of numbers and complication and gets to the heart of the issue. The link you posted tallies pretty well with the book.
  8.  
    pmcc

    If you look at the graphs in this model for daily radiation for your example of Edingburgh in March you will notice a very large deviation in W/m2 between cloudy and bright days and between fixed and tracker. Based on 20% collector efficiency the model which best represents McKays book is the lowest fixed system curve. I think the more realistic figure we should be using is the mid tracker curve and the outputs that will generate.

    As regards mass scale I will let you know once we have the data from the 4000m2 array being installed but using evac tubes not pv panels for electric generation.
    •  
      CommentAuthordjh
    • CommentTimeJul 7th 2009
     
    pmcc, I expect you're aware that trackers gather more light in several ways.

    Firstly because their surface is always normal to the radiation, they experience fewer losses due to reflection and other causes in some cell constructions. That is definitely an inherent benefit of a tracker.

    Secondly, they gather more light by intercepting light that would fall on other ground. In the afternoon, they have a larger shadow to their east than an equivalent fixed panel, for example. For that reason, they have to be mounted further apart, or else part of their area is ineffective except at noon. To calculate yields over whole fields (or 100 km squares!) you have to allow for that extra 'dead' area when calculating the yield. I'm not sure whether your calculation did that?

    They can also increase system efficiency by reducing losses and/or costs but that doesn't affect light gathering per area.

    http://www.esolar.com/ is an interesting solar thermal generation company, BTW.
    • CommentAuthorpmcc
    • CommentTimeJul 7th 2009
     
    djh, thanks for the info. eSolar looks pretty useful, because it's modular, mass-produced and seems to be mostly based around well proven technology.

    My main concern about 2-d tracker technology is the large number of complicated moving parts. A balance must be struck between efficiency of collection and ease of installation, resilience and low maintenance. Although I'm sure elegant control systems can be built together with modular positioning gear, it would be interesting to look at mean time between failure even once they reach the mass-market. Roof-mounted kit is generally troublesome to maintain.

    1-d trackers are simpler and hence presumably a bit more reliable. The JRC site (John's link) indicates that a vertical tracker will yield only a small amount less than a 2-d tracker.

    Assuming the best trackers can uplift the collection rate by 50%, we then need to ask whether it's better to accept lower returns (or greater collection area) in return for ease of deployment and minimal maintenance. Given that MacKay is trying to deal with real rather than speculative solutions, he was probably right not to include trackers at this stage. Perhaps the next edition of the book could include it as a possible future option.
    • CommentAuthorpmcc
    • CommentTimeJul 7th 2009
     
    John,

    "If you look at the graphs in this model for daily radiation for your example of Edingburgh in March you will notice a very large deviation in W/m2 between cloudy and bright days and between fixed and tracker. Based on 20% collector efficiency the model which best represents McKays book is the lowest fixed system curve. I think the more realistic figure we should be using is the mid tracker curve and the outputs that will generate."

    Yes, our cloudy climate makes a huge difference to the available solar energy! It's one of the main reasons why people don't think of solar energy as being viable for this country.

    However, the whole point of MacKay's analysis (and my calculations above based on the JRC data which reach roughly the same conclusion) is that it averages out all conditions to give an overall energy yield. Of course on dull winter days we'll get a tiny fraction of what is available on sunny summer days. But to calculate the overall energy yield without getting bogged down in intricate calculations, a simple average gives a good approximation.

    "As regards mass scale I will let you know once we have the data from the 4000m2 array being installed but using evac tubes not pv panels for electric generation."

    That would be interesting. Have you a link for that? How does it compare to the eSolar technology which djh mentioned?

    Actually, by 'mass scale' I meant deploying on every south facing house roof in the UK, rather than very big solar farms. Do you reckon the tracker technology could realistically be deployed on such a large scale? Do you have metrics for mean time between failure of the moving parts? What about relative equipment, installation and maintenance costs compared to fixed panels?
    • CommentAuthorGBP-Keith
    • CommentTimeJul 7th 2009 edited
     
    Posted By: pmcc
    Assuming the best trackers can uplift the collection rate by 50%, we then need to ask whether it's better to accept lower returns (or greater collection area) in return for ease of deployment and minimal maintenance. Given that MacKay is trying to deal with real rather than speculative solutions, he was probably right not to include trackers at this stage. Perhaps the next edition of the book could include it as a possible future option.


    I am claiming that my system put out about 58% for the last year May to May on my twin axis tracker !
    http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=1672&page=2#Item_7

    I hope to prove this with the hard data which I'm just collating the last of now.
  9.  
    pmcc

    We have not gone down the esolar route as we need to reuse our existing roof spaces.

    Standard evac tubes using thermal oil instead of water but spaced further apart to allow for the rotating reflector based on a standard 24 hour central heating clock. (do they ever breakdown)

    Eventually we will scale down to domestic CHP scale but at present larger is easier.
    • CommentAuthorpmcc
    • CommentTimeJul 8th 2009
     
    Keith - great stuff! Look forward to seeing your data. Do you reckon the gain could also be available for roof-mounted collectors? Ground-mounted collectors are fine for those with decent sized well oriented gardens, but I suspect that roofs will be the best way to achieve greatest domestic coverage.
    • CommentAuthorpmcc
    • CommentTimeJul 8th 2009
     
    John - looks like you're in pre-publicity period, hence lack of link. I understand the general principle behind parabolic reflectors, and of course a clock-based tracking controller will be simple and very reliable. However, any moving parts will not be quite as simple and reliable, and will probably require maintenance. Any comment about my questions above (mean time between failure, additional cost & maintenance schedule)?

    For industrial pv, MacKay states that we could recover 50 kWh/day/person if we covered the land to the extent of 200m2 per person = around 5% of UK landmass. He assumes 10% efficient collectors. Energy return on energy invested is a relatively meagre 4, assuming 20 year collector life. Let's assume that tracking collectors could be cheaply mass-produced to yield net 25% efficiency. Then the same energy could be achieved using 80m2/person = 2% UK landmass - still a tall order, but at least worth discussing further.

    MacKay does talk about industrial solar thermal concentrators (p178). He reckons power output is around 15W/m2 when deployed in sunny countries. Do you agree with that estimate?

    I'm still not convinced that MacKay was wrong to exclude tracker technology from his estimates for domestic pv collectors. Keith's pv trials look promising, assuming the results could be replicated on the average roof. You're now hinting at improved solar thermal CHP technology without getting specific, and apparently it's currently on industrial rather than domestic scale. Fine, but I'm still no clearer what's wrong with the book.
    • CommentAuthorjon
    • CommentTimeJul 8th 2009 edited
     
    Posted By: pmccHowever, any moving parts will not be quite as simple and reliable, and will probably require maintenance.


    Sounds like a fresnel type array? (Credit: Acciona Solar & USDOE)
   
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