On Sat, Sep 24, 2005 at 10:41:28AM +0300, Peter wrote: > > Sorry for replying out of thread, I do not use a threaded mail reader > and I get digests in the evening normally. No worries. > Re: fuel heat value I used in calculation: I used gasoline which is > rated at 45 MJ/kg and priced as of yesterday in Texas (average about > $2.6/gal), according to the web. I assumed that the cost of other fuels > would be cheaper since gas is considered very expensive now. Weight of a gallon? Let me see if I can find it... 5.8 to 6.5 lbs. Let's take the average of 6.1 lbs. 2.2 lbs/kg so 6.1 lbs is 2.77 kg 2.77 kg *45 MJ/kg is 125 MJ rounded up. So gas is in the same ballpark as natural gas in terms of both MJ (125 MJ vs. 105.5 MJ) and cost ($2.60 USD/gallon vs $2.50 USD/therm) A "therm" worth of electricity is a shade over 29 kWh. And the burning efficiency of a kWh is virtually 100 percent, whereas the extremely high efficiency gas only approaches 90% (with the rest going up the flue). So it actually takes 11% more gas to produce the same amount of heat energy as an equivalent amount of electricity. So the corresponding "therm" for electricity is actually only 26.1 kWh as comparated to a high efficiency gas furnace. So compare the cost of a gallon of gas, a therm of natural gas, and a "therm" of electricity costwise: gas: $2.60 NG: $2.50 electricity: 26.1 kWh * $0.051/khW = $1.33 It's a no brainer! > Re: cost of heating $150/month: this is a figure I got from several > discussions wrt heating costs on the web, for the last year (2004), for > somewhere in the central US afair, using kerosene I think. It may be > double now. I was just trying to get some numbers. Probably triple from last year. So take that $150/month and make it $450/month and you're in the right ballpark. > Re: mylar mirror: The vacuum formed mirror is not really new but its > efficiency is only 0.8 or so afaik. I'm going to invoke James here: I'm willing to sacrifice some efficiency for low cost and ease of setup. > You can make a better mirror using > the spun cast method or by using a dish as a former for a fiberglass or > composite concrete copy. Cost? Complexity? > The mirror you make need not be round (pita to > handle), it can be a rectangle inscribed in the mold or a petal > (triangle as above). That way you lose some surface but the thing is > much easier to make, handle and array. No doubt. > The problem with the parabolic shape proposed is, you cannot make an > array from them (the focal point is way too close). So it turns out that > just taking plain glass mirrors (at $6 / sq. meter if what I read was > true) and warping them slightly in a frame (they will warp anyway) will > yield something closer to the 10 meter focal distance needed for an > array. Why 10m?! I actually wanted a much closer focal point. That simplifies tracking everything. Coordinating a mirror array and then a target that's 30 ft away is a daunting task. An ideal focal box would be 2-3m from the center of the array. > Then the secondary mirror can be on a pole, and IT can be > hyperbolic (using a parabolic dish as casting form and mirroring the > 'wrong' side), and the target on the ground and fixed, where you can get > at it. Again I was planning on tracking the target too, moving the whole assembly from east to west during the day. I agree with James that tracking individual mirror elements is way more complex than having a fixed mirror array that tracks as a single unit. > Imho the bad news with the array is, the mirrors need to move. Bingo! >The good news is that arrays scale. But the complexity scales too because the mirrors must track individually right? Or can each mirror be fixed in such a way that you can track an entire linear row with a single motor? Or am I missing something? > You can start with 2 mirrors and increase to > 50 later. That's why I wanted to go with the mylar. You build the whole assembly once as a single unit. >The bad news is that a moving linear array is subject to cos > phi/2 losses and that the mirrors shade each other beyond a relatively > small sun angle (say 45 degrees). But the time of useful sun is only > about 6-8 hours per day and at that angle cos phi/2 loss is <15%. Complexity is a killer here. A backyard setup has to be simple to be effective, even if that simplicity causes some losses. Tracking a single big mirror along with a target that tracks with the mirror is much less complex than tracking a wide array of individual mirrors. That's why Curnutt style furnances are a fixed array of mirrors with the whole assembly tracking the sun. The current Sunflower technology uses some type of gimbal arrangement where all 25 mirrors of the array are individually tracked using only a couple of motors. But it's complex. And complex leads to breakdowns. BAJ -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist