Patent Publication Number: US-2011061843-A1

Title: Heat recovery method and system therefore

Description:
FIELD OF THE INVENTION 
     The invention relates to a heat recovery method and a system therefore. More particularly, the invention relates to a method and to recover heat from waste hot water and a system therefore. 
     BACKGROUND OF THE INVENTION 
     Saving energy not only means saving money but also preserving the environment. Today&#39;s main energy sources are fossil fuels. According to the Energy Information Administration, in 2006 primary sources of energy consisted of petroleum 36.8%, coal 26.6%, and natural gas 22.9%, amounting to an 86% share for fossil fuels in primary energy production in the world. Fossil fuels are formed by natural resources such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years. While fossil fuels are not renewable, world energy consumption grows about 2.3% per year. Fossil fuels will eventually exhaust. The burning of fossil fuels produces around 21.3 billion tonnes (21.3 gigatonnes) of carbon dioxide per year. Carbon dioxide is a greenhouse gas that contributes to global warming. Therefore, burning less fossile fuels not only makes economic sense but also slows globel warming. 
     One way to conserve energy is to recover energy that has been discarded. Many methods for recovering waste heat have been developed. For instance U.S. Pat. No. 7,569,194 discloses a method and apparatus for integrating a high temperature waste heat recovery system with a chemical or refining process which requires heat energy at a temperature less than the operational temperature of the waste heat recovery system. U.S. Pat. No. 4,875,436 discloses a boiler assembly including a heat recovery steam generator through which exhaust gas is flowed. However these methods and systems are complicated and suitable for industry use only. 
     Few efforts have been made for recovering waste heat from residential homes. If one household saves just five megajoules of energy, the magnitude of savings would multiply by the millions of homes in America and by hundreds of millions of homes in the world. This potential of energy savings has been overlooked. In conclusion, new methods and systems for recovering wasted heat are needed. Ideally the heat recovery method and system recover waste heat from everyday usage of hot water such as showers, baths, or heated pools. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention provides a method for recovering heat from household waste hot water which comes from showering, bathing, laundering, dishwashing, and any similar waste hot water sources. By “household,” I mean residential homes, hotels, motels, inns, fitness centers, and other similar facilities which use hot water. The waste hot water has a temperature of T 1 . It transfers heat through a heat exchanger to clean cold water which has a temperature T 2  to produce clean warm water which has a temperature of T 3 . T 1  is greater than T 3 , and T 3  is greater than T 2 . The warm clean water can be used for household and other uses. 
     The invention also provides a system for recovering heat from waste hot water. The system comprises a waste hot water supply, a heat exchanger, and a clean cold water supply. The waste hot water supply has a temperature of T 1 , and it transfers heat through the heat exchanger to the clean cold water supply which has a temperature of T 2  and produces a clean warm water supply which has a temperature of T 3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Water enters residences in the US at about 10° C. (50° F.) (which varies with latitude and season). It is heated for domestic uses such as showering and dishwashing. Most water heaters are the tank type. They are also called storage water heaters. These heaters may use electricity, natural gas, propane, heating oil, solar, or other energy sources. Natural gas heaters are most popular. Storage water heaters have the advantage of using energy (gas or electricity) at a relatively slow rate, storing the heat for later use. Larger vessels/containers tend to provide hot water with less temperature fluctuation at moderate flow rates. 
     Hot water temperatures of 40-50° C. (105-120° F.) are preferred for dishwashing, laundry and showering. They require the water temperature to be raised about 30° C. (54° F.) or more from the entrance temperature of housewater. It takes a great deal of energy to heat water. For instance, to raise a 40-gallon tank of water from 55° F. to 105° F. requires approximately 20 cubic feet of natural gas. 
     When the waste hot water exits from domestic uses, it often has a temperature of 10° F. below the initial hot water temperature. When the waste hot water goes to the drain, it takes a lot of heat with it. Up to 80% of heat energy thereof is thus wasted. 
     The invention provides a method to recover heat from the waste hot water. The method of the invention comprises passing the waste hot water through a heat exchanger before it goes to the drain. The waste hot water has a temperature of T 1 . The heat exchanger transfers heat from the waste hot water to clean cold water. The clean cold water has a temperature of T 2 . When T 1  is greater than T 2 , heat transfers from the waste hot water to the clean cold water. The clean cold water thus rises in temperature to T 3 . T 3  is greater than T 2  but usually less than T 1 . The closer T 3  is to T 1 , the more heat is recovered. Preferably, T 1  is at least 10° C. greater than T 2 . More preferably, T 1  is at least 30° C. greater than T 2 . Most preferably T 1  is at least 50° C. greater than T 2 . 
     The clean cold water preferably is house water from a public water supply or from a private well. Currently, the house water is usually piped into the water heater to produce hot water. According to this method, the house water passes through the heat exchanger and receives heat thereby from the waste hot water before it goes into the water heater. Therefore, the heat energy to produce the hot water having a temperature of T is reduced because the water entering the water heater has a temperature of T 3  rather than T 2 . The greater the difference between T 3  and T 2 , the more energy is recovered. Preferably, T 3  is at least 5° C. greater than T 2 . More preferably, T 3  is at least 10° C. greater than T 2 . 
     The waste hot water is transferred to the heat exchanger through pipes. Preferably the pipes are well insulated so that heat will not be lost through the pipes to the air. The pipes can be made of metals such as copper and steal, plastics such as polyvinyl chloride and polypropylene, the like, and mixtures thereof. The pipes can be insulated by, for example, polyurethane foams, polystyrene foams, glass fibers, cottons, the like, and mixtures thereof. 
     Many heat exchangers suitable for liquid to liquid heat transfer can be used for this invention. Preferably the heat exchanger is a shell-tube heat exchanger. A shell-tube heat exchanger consists of a shell with a bundle of tubes inside it. In one embodiment, the hot waste water runs through the tubes, and the cold clean water flows over the tubes (i.e., through the shell) to transfer heat between the two. In another embodiment, the cold clean water runs through the tubes and the hot waste water flows through the shell. The tubes can be u-shaped or straight. If the waste hot water runs through the tubes, straight tubes are preferred because straight tubes are easier to clean than u-shaped tubes. The tube material should have good thermal conductivity and corrosion resistance. Suitable tube materials include copper alloy, stainless steel, carbon steel, nickel and titanium. Copper alloys and stainless steals are preferred because they are relatively inexpensive and have high conductivity and corrosion resistance. 
     Preferably, the waste hot water is filtrated before it enters the heat exchanger to remove solid materials such as hair and grease which may block water flowing though the heat exchanger. The filters can be simply metal or plastic nets and spongers, or more sophisticated filters such as carbon filters. If desirable, the waste hot water can be neutralized before it enters the heat exchanger to reduce corrosion of the heat exchanger. 
     While many factors should be considered in selecting or designing the heat exchanger, balance between cost and heat transfer efficiency is most important. Heat transfer efficiency can be measured by heat transfer coefficient U. U is given by U=Q/(A·ΔT). Q is heat transfer rate, A is the surface area of the heat exchanger, and ΔT is the temperature difference of the two liquids. In this invention, ΔT is the difference between T 1  and T 2 . High heat transfer coefficient means more heat is transferred in a given surface area A and given ΔT. Preferably the heat exchanger has a heat transfer coefficient greater than or equal to about 0.4, more preferably greater than or equal to 0.5, and most preferably greater than or equal to 0.6. 
     The clean warm water which exits from the heat exchanger outlet can be piped into the water heater. Alternatively, the clean water which exits from the heat exchanger outlet can be used for any other purposes. For instance, it can be used for dishwashing or laundering. 
     The invention includes a heat recovering system. The heat recovery system comprises a waste hot water supply, a clean cold water supply and a heat exchanger. It optionally comprises a water heater. Preferably the waste hot water supply is a shower, bath, laundry, dishwasher, hot pool, the like, and mixtures thereof. Preferably the cold clean water supply is a public water system or a private well. In one embodiment, the waste hot water supply and the clean cold water supply are connected to the heat exchanger where heat transfers from the waste hot water to the clean cold water to produce a clean warm water. In another embodiment, the clean warm water is connected to the water heater. 
     The following examples only illustrate the invention. Persons skilled in the art will understand many variations can be made according to the claims that follow. 
     EXAMPLE 1 
     A person takes a shower. Water enters the residence at 10° C. (50° F.). The hot water temperature is about 45° C. (113° F.). Shower flow rate is 2.5 gallons per minute (11 liters per minute). When it reaches the bottom of the shower it has a temperature of about 40° C. (104° F.). The person takes 25 minutes of shower and uses about 62.5 gallons (284 liters) of hot water. It takes about 3938 BTU (4415 KJ) of energy to heat 62.6 gallons of water from 10° C. to 45° C., but 3375 BTU (3558 KJ) of energy goes with the waste water down the drain. The efficiency of heat use for shower is only 15%. In other words, 85% of the heat energy for the shower is wasted. 
     According to the invention, the waste hot water (40° C.) transfers heat to the residence water (10° C.) via a heat exchanger. When the residence water exits the heat exchanger outlet, it is expected to have a temperature of 15° C. (59° F.). Thus 563 BTU (594 KJ) of energy can be recovered per person per day and 205,495 BTU per person per year. 
     According to the US Censor Bureau, the US population today (Sep. 13, 2009) is 307,437,131. Multiplying the US population by 205,495 BTU per person per year gives approximately 6.3×10 13  BTU per year of energy savings in the United States. 
     EXAMPLE 2 
     The general discussion of Example 1 continues. When the residence water exits the heat exchanger outlet, it is expected to have a temperature of 20° C. (68° F.). Thus 1127 BTU of energy can be recovered per person per day and 411,282 BTU per person per year. 
     According to the US Censor Bureau, the US population today (Sep. 13, 2009) is 307,437,131. Multiplying the US population of 307,437,131 by 411,282 BTU per person per year gives approximately 1.26×10 14  BTU per year of energy savings in the United States. 
     EXAMPLE 3 
     Natural gas in the U.S. is measured in CCF (100 cubic feet), which is converted to a standardized heat content unit called the therm which equals to 100,000 BTU. Therefore, in Example 2, 1.26 billions CCF of natural gas can be saved every year in the United States. This indicates that the invention does not only mean saving money and energy, but also mean reducing a large amount of greenhouse gas (CO 2 ) by burning less natural gas.