太阳能热水器分类及比较(英文).doc

http://en.wikipedia.org/wiki/Solar_water_heating
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Three types of solar collectors：
1、Flat-plate collector平板式
2、Integral collector-storage systems(ICS)不适合冰冻环境
3.、Evacuated-tube solar collectors真空管式

There are two types of active solar water heating systems：
1、Direct circulation systems泵循环式
2、Indirect circulation systems适合冰冻环境

Protect against freezing in colder climates. Some solar systems are appropriate for freezing climates, while others are not. As a general rule, if there is a good chance that temperatures will fall below 42°F, choose either a closed-loop system with antifreeze as the heat transfer fluid, or a system that automatically drains back when temperatures fall too low for comfort. Steer clear of batch systems if you live in a climate with distinct winters. Learn more about freeze protection

Estimate the capacity you'll need. Use the gallon capacity of your current water heater as a starting point. You can find your current water heater's capacity or volume on its nameplate or yellow Energy Guide sticker. This should help you discuss the sizing of your solar system with contractors. If you have recently added or are planning to add more occupants to your household or a new shower, faucet, dishwasher or washer that will be in frequent use, take that into account in your discussions.

Collector Types

Batch collectors, also called Integrated Collector-Storage (ICS) systems, heat water in dark tanks or tubes within an insulated box, storing water until drawn. Water can remain in the collector for long periods of time if household demand is low, making it very hot. A tempering valve is your protection from scalding at the tap. The tempering valve mixes in cold water to decrease the water's temperature before it's delivered to the tap.
Batch collectors are incompatible with closed-loop circulation systems. Thus, they are generally not recommended for cold climates.

Flat-plate collectors typically consist of copper tubes fitted to flat absorber plates. The most common configuration is a series of parallel tubes connected at each end by two pipes, the inlet and outlet manifolds. The flat plate assembly is contained within an insulated box, and covered with tempered glass.
Flat plate collectors are typically sized to contain 40 gallons of water. Two collectors provide roughly half of the hot water needed to serve a family of four.

Evacuated tube collectors are the most efficient collectors available. Each evacuated tube is similar to a thermos in principle. A glass or metal tube containing the water or heat transfer fluid is surrounded by a larger glass tube. The space between them is a vacuum, so very little heat is lost from the fluid.
These collectors can even work well in overcast conditions and operate in temperatures as low as -40°F. Individual tubes are replaced as needed. Evacuated tube collectors can cost twice as much per square foot as flat plate collectors.
Circulation Systems
Direct systems circulate water through solar collectors where it is heated by the sun. The heated water is then stored in a tank, sent to a tankless water heater, or used directly. These systems are preferable in climates where it rarely freezes. Freeze protection is necessary in cold climates.
Closed-loop, or indirect, systems use a non-freezing liquid to transfer heat from the sun to water in a storage tank. The sun's thermal energy heats the fluid in the solar collectors. Then, this fluid passes through a heat exchanger in the storage tank, transferring the heat to the water. The non-freezing fluid then cycles back to the collectors. These systems make sense in freezing climates.
Active, or forced-circulation, systems use electric pumps, valves and controllers to move water from the collectors to the storage tank. These are common in the U.S.
Passive systems require no pumps. Natural convection moves water from the collectors to the storage tank as it heats up.

Direct systems: (A) Passive CHS system with tank above collector. (B) Active system with pump and controller driven by a photovoltaic panel

Indirect active systems: (C) Indirect system with heat exchanger in tank; (D) Drainback system with drainback reservoir. In these schematics the controller and pump are driven by mains electricity

A typical programmable differential controller

Active systems with drainback
A drain-back system is an indirect active system where heat transfer fluid circulates through the collector, being driven by a pump. However the collector piping is not pressurised and includes an open drainback reservoir. If the pump is switched off, all the heat transfer fluid drains into the drainback reservoir and none remains in the collector. Consequently the collector cannot be damaged by freezing or overheating.[15] This makes this type of system well-suited to colder climates.

Active systems with a bubble pump

The bubble separator of a bubble-pump system

Freeze protection
Freeze protection measures prevent damage to the system due to the expansion of freezing transfer fluid. Some systems drain the transfer fluid from the system when the pump stops. In indirect systems (where the transfer fluid is separated from the heated water), this is called drainback and in direct systems (where the heated water is used as the transfer fluid) it is called draindown. Many indirect systems use anti-freeze (e.g. propylene glycol) in the heat transfer fluid. This approach is simpler and more reliable than drainback and is common in climates where freezing temperatures occur often.
In both direct and indirect systems, automatic recirculation may be used for freeze protection. When the water in the collector reaches a temperature near freezing, the controller turns the pump on for a few minutes to warm the collector with water from the tank.
In some direct systems, the collectors are manually drained when freezing is expected. This approach is common in climates where freezing temperatures do not occur often.
Other direct systems use freeze tolerant solar collectors. Here the water channels of the collector are made of flexible polymers such as silicone rubber. Being non-metal, these can freeze solid without cracking. One European solar collector is being produced to this specification under the Solar Keymark and EN 12975 standards.

Overheat protection
Particularly when no hot water has been used for some time, the water from the collector can reach very high temperatures in good sunshine, or if the pump fails to operate, such as during a power cut. Designs which may boil the hot water store usually allow for relief of pressure and excess heat through a heat dump. Almost all sealed and unvented solar circuits have pressure relief valves through which excessive water pressure or steam can be vented. Vented systems have a simpler safety feature already built in via the open vent, a simple and virtually fail-safe approach. Some active systems deliberately cool the water in the storage tank by heat export: circulating hot water through the collector at times when there is little sunlight or at night (when solar energy does not heat the collector). Heat export operates most effectively in systems which do not use basal heat exchangers to add heat to the water store (because cool water sinks below hot water).
11 possible types of overheat control in solar thermal have been identified in the International Energy Agency's Task Group 39 on Polymeric materials in solar heating and cooling.

A rough comparison of solar hot water systems
Comparison of SWH systems[19]
Characteristic
ICS (Batch)
Thermosyphon
Active direct
Active indirect
Drainback
Bubble Pump
Low profile-unobtrusive

Y
Y
Y
Y
Lightweight collector

Y
Y
Y
Y
Survives freezing weather

Y
Y
Y
Y
Low maintenance
Y
Y
Y

Y
Y
Simple: no ancillary control
Y
Y

Y
Retrofit potential to existing store

Y
Y
Y
Y
Space saving: no extra storage tank
Y
Y

Solar Water Heaters
Solar water heaters—also called solar domestic hot water systems—can be a cost-effective way to generate hot water for your home. They can be used in any climate, and the fuel they use—sunshine—is free.
How They Work
Solar water heating systems include storage tanks and solar collectors. There are two types of solar water heating systems: active, which have circulating pumps and controls, and passive, which don't.
Most solar water heaters require a well-insulated storage tank. Solar storage tanks have an additional outlet and inlet connected to and from the collector. In two-tank systems, the solar water heater preheats water before it enters the conventional water heater. In one-tank systems, the back-up heater is combined with the solar storage in one tank.
Three types of solar collectors are used for residential applications:
· Flat-plate collector
Glazed flat-plate collectors are insulated, weatherproofed boxes that contain a dark absorber plate under one or more glass or plastic (polymer) covers. Unglazed flat-plate collectors—typically used for solar pool heating—have a dark absorber plate, made of metal or polymer, without a cover or enclosure.
· Integral collector-storage systems
Also known as ICS or batch systems, they feature one or more black tanks or tubes in an insulated, glazed box. Cold water first passes through the solar collector, which preheats the water. The water then continues on to the conventional backup water heater, providing a reliable source of hot water. They should be installed only in mild-freeze climates because the outdoor pipes could freeze in severe, cold weather.
· Evacuated-tube solar collectors
They feature parallel rows of transparent glass tubes. Each tube contains a glass outer tube and metal absorber tube attached to a fin. The fin's coating absorbs solar energy but inhibits radiative heat loss. These collectors are used more frequently for U.S. commercial applications.
There are two types of active solar water heating systems:
· Direct circulation systems
Pumps circulate household water through the collectors and into the home. They work well in climates where it rarely freezes.
· Indirect circulation systems
Pumps circulate a non-freezing, heat-transfer fluid through the collectors and a heat exchanger. This heats the water that then flows into the home. They are popular in climates prone to freezing temperatures.

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Passive solar water heating systems are typically less expensive than active systems, but they're usually not as efficient. However, passive systems can be more reliable and may last longer. There are two basic types of passive systems:
· Integral collector-storage passive systems
These work best in areas where temperatures rarely fall below freezing. They also work well in households with significant daytime and evening hot-water needs.
· Thermosyphon systems
Water flows through the system when warm water rises as cooler water sinks. The collector must be installed below the storage tank so that warm water will rise into the tank. These systems are reliable, but contractors must pay careful attention to the roof design because of the heavy storage tank. They are usually more expensive than integral collector-storage passive systems.

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Solar water heating systems almost always require a backup system for cloudy days and times of increased demand. Conventional storage water heaters usually provide backup and may already be part of the solar system package. A backup system may also be part of the solar collector, such as rooftop tanks with thermosyphon systems. Since an integral-collector storage system already stores hot water in addition to collecting solar heat, it may be packaged with a demand (tankless or instantaneous) water heater for backup.
For more information about solar water heating system components, see the following information:
· Heat Exchangers for Solar Water Heating Systems
· Heat-Transfer Fluids for Solar Water Heating Systems
Selecting a Solar Water Heater
Before you purchase and install a solar water heating system, you want to do the following:
· Consider the economics of a solar water heating system
· Evaluate your site's solar resource
· Determine the correct system size
· Determine the system's energy efficiency
· Estimate and compare system costs
· Investigate local codes, covenants, and regulations.
For information about specific solar water heater models and systems, see the Product Information resources listed on the right side of this page (or below if you've printed the page).
Installing and Maintaining the System
The proper installation of solar water heaters depends on many factors. These factors include solar resource, climate, local building code requirements, and safety issues; therefore, it's best to have a qualified, solar thermal systems contractor install your system.
After installation, properly maintaining your system will keep it running smoothly. Passive systems don't require much maintenance. For active systems, discuss the maintenance requirements with your system provider, and consult the system's owner's manual. Plumbing and other conventional water heating components require the same maintenance as conventional systems. Glazing may need to be cleaned in dry climates where rainwater doesn't provide a natural rinse.
Regular maintenance on simple systems can be as infrequent as every 3–5 years, preferably by a solar contractor. Systems with electrical components usually require a replacement part or two after 10 years. For more information about system maintenance, see the following:
· Solar Water Heating System Maintenance and Repair
· Solar Water Heating System Freeze Protection
· Scaling and Corrosion in Solar Water Heating Systems
When screening potential contractors for installation and/or maintenance, ask the following questions:
· Does your company have experience installing and maintaining solar water heating systems?
Choose a company that has experience installing the type of system you want and servicing the applications you select.
· How many years of experience does your company have with solar heating installation and maintenance?
The more experience the better. Request a list of past customers who can provide references.
· Is your company licensed or certified?
Having a valid plumber's and/or solar contractor's license is required in some states. Contact your city and county for more information. Confirm licensing with your state's contractor licensing board. The licensing board can also tell you about any complaints against state-licensed contractors.
For contractor information, see the Professional Services resources listed on the right side of this page (or below if you'