Solar Water Heating--Revised & Expanded Edition. Bob Ramlow
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Название: Solar Water Heating--Revised & Expanded Edition
Автор: Bob Ramlow
Издательство: Ingram
Жанр: Техническая литература
Серия: Mother Earth News Wiser Living Series
isbn: 9781550924497
isbn:
A solar water heating investment is different from most investments because the value of the free energy that a solar water heater harvests reduces a bill you would otherwise pay each month. If you heat your water with fossil fuel, you have a hot water bill each month that is part of your normal cash flow. When you install a solar water heater, your hot water bill is reduced. The savings gained from the solar water heater pays for the solar investment.
Figure 2.3 gives an example of a cash-flow analysis for a solar water heater that was used to offset an electric water heater. We used the same starting costs as in the life-cycle analysis but factored in some available rebates. Assume that you need to borrow the entire $5,000 to pay for the solar water heating system at 6 percent interest and make equal monthly payments for ten years. As you can see, the monthly loan payments initially exceed what is saved from not having to purchase electricity. However, over time the two columns level out, making much more comparable figures. Eventually the monthly loan payments are less than the monthly utility bill, meaning that you will actually have more money in your pocket from month to month. You would see a small negative impact on your cash flow for the first six years, and a positive cash flow impact thereafter. After ten years, when the loan has been completely paid off, your cash flow per month is greatly increased and will continue to increase as energy prices rise.
Figure 2.3: Cash flow analysis — solar vs. electric
Figure 2.4: Cash flow analysis — solar vs. natural gas
Figure 2.4 demonstrates a comparison between solar and natural gas. The cash-flow impact is slightly greater because natural gas tends to be less expensive than electricity. Nonetheless, the additional monthly cost diminishes over time, and after the loan is paid off in ten years all the savings contribute to increasing positive cash flows. Since the system is expected to last about 40 years, you can plan on seeing many years when you will have more money to spend on a monthly basis. When you finance the system, you are essentially locking in your monthly payments. You know what you will have to pay each month and will not be affected by the continually rising cost of energy. Some view this as a retirement investment. They pay off the system now when they have the cash flow to do so, and when they retire, their utility bills and monthly expenses will be greatly reduced.
The point of this analysis is to show you that you should not get hung up on the upfront cost of a solar heating system because the investment does not significantly impact your cash flow. You could have a solar water heater today for only an additional $20 a month.
At the start of this chapter we told you that we would demonstrate how you can get a solar water heater for free. The point of the life-cycle costing and cash-flow examples is to show that no matter how you look at it, a solar water heater will not cost you any more than its alternative. All you have to do is install the system and you can start saving today.
3
TYPES OF SOLAR COLLECTORS
FROM THIS POINT ON, this book will deal with the nuts and bolts of solar water heating and space heating systems. We think it is appropriate to first let you know our perspective. We both live in Wisconsin, where the weather can be severe, particularly in regard to cold temperatures and snow that stays for months. Our experiences with solar thermal systems have been heavily influenced by the weather here. It is interesting to note that approximately 90 percent of the population of the world lives in a warmer climate than we do. Green Bay, Wisconsin, just 50 miles away from our homes and at the same latitude, is the coldest city in the continental US with a population over 100,000. The average coldest temperature in Wisconsin is–44°F. (The record is–56°F.) In a cold climate like this we have seen everything go wrong with a solar thermal system than can possibly happen because of cold weather. We also get a lot of snow each winter, and this snow typically stays on the ground and on roofs for at least four months without melting. Also know that it gets plenty hot here during the summer, so we have seen that side of things as well. The bottom line is that there is no room for error around here in either system design or installation, and this has influenced our perspective significantly.
So we are picky when it comes to system designs. If you follow our suggestions that are based on “worst-case scenarios” you will have success, even if you are in a warmer or less severe climate than ours. The next question is, are we suggesting designs that are overkill? The answer is no, because it does not actually cost more to design and install a system properly whether you are in a hot, cold or in-between type of climate. Having lived through the oil embargo years and the ensuing solar boom, we have seen many systems installed that were not appropriate for the local climate or were sub-standard in other ways. These systems required significant repair costs that, when added to the original cost of the system, resulted in a more costly system than systems that were properly designed and installed in the first place.
Flat Plate Collectors
Flat plate collectors are the most widely used kind of collector in the world for domestic solar water heating and solar space heating applications. These collectors have an operating range from well below 0°F to about 180°F, which is precisely the operating range required for these applications. They are durable and effective. They are the standard to which all other kinds of collectors are compared.
Figure 3.1: Flat plate collector
Flat plate collectors are rectangular shallow boxes that typically are 4 feet wide, 8 or 10 feet long and 4 to 6 inches deep, but they also come in other sizes. A common metric size is 1 meter by 2 meters. These collectors are made with a strong frame, a glazed (glass) front, a solid back and insulation on the sides and back. An absorber plate lies just beneath the glazing. In most cases this absorber plate has manifolds that run across the top and bottom of the collector, just inside the frame. These manifolds are usually ¾-inch or 1-inch-diameter copper pipe and extend out both sides of the collector through large rubber grommets.
These collectors, called internally manifolded collectors, can be easily ganged together to make large arrays. Smaller riser tubes, typically ½-inch copper pipes, run vertically, usually brazed to the manifolds above and below, and are spaced 3 inches to 6 inches apart (the closer the better). Another type of absorber plate has a serpentine tube that meanders back and forth continuously from the bottom of the collector to the top of the collector. A flat copper or aluminum fin is then attached to each riser to complete the absorber plate. The fin must make intimate contact with the riser tube to facilitate effective heat transfer from the fin to the tube. Soldering, welding or roll bonding the fins to the tubes makes the best connection. The plates are also usually dimpled or corrugated to increase absorptivity. The absorber plates are not attached to the frame; they just sit inside it and can expand or contract as they are heated or cooled without being restricted by the frame.
Collector Frame
It is very important that a flat plate collector have СКАЧАТЬ