Methods and apparatus for transferring thermal energy   

Abstract: Municipal waste water systems can be a significant renewable alternative energy source for heating and cooling of buildings. Waste water flow rate and temperature data is collected over a period of time and locations within the waste water system that can act as a thermal energy source or sink are identified. Candidate buildings proximate the locations are identified. Thermal energy transfer apparatus can be used to transfer energy between an identified location and building.

This application claims benefit to Provisional Application No. 61/479,617, entitled METHODS AND APPARATUS FOR TRANSFERRING THERMAL ENERGY, filed Apr. 27, 2011.

The present disclosure relates to methods for transferring thermal energy with waste water systems and related apparatus.

It is well recognized that municipal wastewater systems consume significant energy during operation. Large initiatives have been addressed and funded regarding reduction of energy consumption in these systems.

SUMMARY

Methods for heat transfer and related apparatus are disclosed herein. In various aspects, the methods may include the step of identifying a thermal energy target location within a wastewater system using flow rate and water temperature, and may include the step of locating a thermal energy transfer apparatus at the identified thermal energy target location. The methods may include the step of transferring thermal energy between the thermal energy target location and a building using the thermal energy transfer apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates by photographic overlay a map of a waste water drainage system including surrounding geographic features.

FIG. 2 illustrates a plot of water temperature as a function of time at a location within the drainage system of FIG. 1.

FIG. 3 illustrates a plot of water temperature as a function of time at another location within the drainage system of FIG. 1.

FIG. 4 illustrates a plot of water temperature as a function of time at yet another location within the drainage system of FIG. 1.

FIG. 5 illustrates plots of temperature drop in ° F. as a function of flow rate in gallons per minute (gpm) for selected rates of thermal energy extraction.

FIG. 6 illustrates a plot of Average Flow (GPM) as a function of time of day at a location within the drainage system of FIG. 1.

FIG. 7 illustrates by frequency chart the number of occurrences of a flow rate for the data of FIG. 6.

FIG. 8 illustrates heating and cooling energy values available at a location on different dates.

FIG. 9 illustrates a system that uses heat extracted from waste water to provide a portion of the energy needed to heat a building.

DETAILED DESCRIPTION

Methods for transfer of thermal energy with a waste water drainage system are disclosed herein. In various aspects, the methods may include identifying a thermal energy target within a drainage system. The thermal energy target may be a location within the drainage system having a sufficient combination of water temperature and water quantity for thermal energy transfer. The methods, in various aspects, may include identifying a user of sources and sinks for thermal energy and may include identifying a user of thermal energy sources or sinks that is located generally proximate the thermal energy target. The methods may include locating thermal energy transfer apparatus proximate the thermal energy target. The methods, in various aspects, may include transferring thermal energy with the thermal energy target. Thermal energy may be transferred with the thermal energy target by extracting thermal energy from the thermal energy target or by ejecting thermal energy into the thermal energy target. The methods may include providing thermal energy extracted from the thermal energy target to the identified user. The methods may include ejecting thermal energy from the identified user into the thermal energy target.

“Waste water drainage system”, as used herein, includes storm sewers, sanitary sewers, and combined sewers, as, for example, may be found in an urban setting. The drainage system may be generally placed below grade and covered with earth so as to be buried, in some aspects. The earthen covering of the drainage system may insulate the drainage system thereby preventing loss of thermal energy from the drainage system to the external environment. The covering may create insulation or in other a heat sink and in others a heat source. The geology surrounding the sewer pipe will govern the rate of heat exchange between the material around the pipe and the sewage. In some aspects, the drainage system may be formed, at least in part, as a conduit that may be made of concrete, reinforced concrete, masonry, clay tile, cast iron, combinations thereof, and so forth. In some aspects, various pumps stations, weirs, spillways, valves, gates, and other facilities may be provided at various locations about the drainage system. Various data collection apparatus may be provided about the drainage system, for example, to measure the flow at various locations, in various aspects. Various control systems may be provided about the drainage system to regulate the drainage system, in various aspects, and the control systems may be operably connected to the pump stations, valves, gates, and so forth, in various aspects.

“Source”, as used herein, may encompass a source, a sink, or both source and sink. Source, as used herein, may include both sources and sinks of mass. Source, as used herein, may include both sources and sinks of thermal energy. Heat and mass transfer, as used herein, include both extraction of heat or mass and the ejection of heat or mass. The sewage may act as either an exothermic source or and endothermic heat sink. That is chemical reactions within the sewage may generate heat or absorb heat more than clean water would.

Thermal energy may be available within the drainage system and the thermal energy may be capturable. The thermal energy within the drainage system may equal the combination of the thermal energy input into the drainage system by flow sources, the thermal energy input into the drainage system by non-flow sources, less the flow of thermal energy from the drainage system as carried by the outflow of waste water from the drainage system less energy losses from the system due to thermal convection, thermal radiation, and thermal diffusion.

The drainage system may collect flow from various flow sources, which may be point sources or diffuse sources, and the flow may have varying amounts of thermal energy depending upon the temperature of the waste water, which, in turn, may depend upon the nature of the flow source. The thermal energy input into the drainage system by the flow source may equal the thermal energy of the water of the flow source, which is a function of the temperature of the water of the flow source, combined with the flow rate of the source. Examples of flow sources providing flow that may have a generally higher temperature and thus corresponding larger quantity of thermal energy include the discharge of heated water from a boiler, institutional kitchen, commercial or institutional laundry, industrial processes, various other commercial, residential and institutional sources, and so forth. Commercial sources may include office buildings, stores, light industrial facilities, warehouses, strip malls, shopping malls, and suchlike. Institutional sources may include schools, hospitals, nursing homes, government buildings, jails, and other institutional facilities. Flow sources providing flow having thermal energy may include residential discharges from residential kitchens, laundry, bathing facilities, and so forth, and the residential discharges may be concentrated, for example, by flowing from an apartment building or other group living facility. Residential discharges may include discharges from individual homes, condominiums, townhouses, apartment buildings, and so forth. Flow sources providing flow having thermal energy may include various other discharges from residential, commercial, institutional, and industrial sources, in various aspects.

Flow sources may include groundwater and surface water flows into the drainage system or from the drainage system into the groundwater. Groundwater flow may occur as a point source into the drainage system at a particular location such as through a break in the conduit of the drainage system. Groundwater flow may occur as a diffuse source such as through joints between sections of the conduit of the drainage system. The thermal energy of the groundwater flow into the drainage system depends upon the temperature of the groundwater. Groundwater flow into the drainage system that results from snowmelt or other such low temperature flows may have relatively low temperature and thus relatively low thermal energies. Groundwater seepage flows may be heated by geologic activity, in some aspects, or may be withdrawals of groundwater that accumulated during a warm season and thus at least a modest amount of thermal energy, and thus may have relatively larger thermal energies. Flow from the drainage system into the groundwater may be a sink for thermal energy as such flow may convey thermal energy from the drainage system into the groundwater.

Flow sources may include discharges from sump pumps, roof leaders, and so forth, and the thermal energy of such flow source is dependent upon the water temperature of the water provided into the drainage system by the flow source, in various aspects. Other flow sources having various temperatures, and, hence, various thermal energies may be located about the drainage system, in various aspects.

In various aspects, there may be non-flow point sources or diffuse sources of thermal energy disposed about the drainage system. For example, thermal energy from the surrounding earth may diffuse into the drainage system. The thermal energy may be, for example, residual thermal energy from the seasonal cycle—i.e. thermal energy that diffused into the earth the previous summer. The thermal energy may be, for example, from the geothermal gradient. Other exemplary non-flow sources of thermal energy may be the thermal energy that may result from biological activity such as the decay of biomass, or thermal energy that may result from the reaction of chemical constituents within the drainage system. The biological or chemical reaction activity may be concentrated about a generally discrete location within the drainage system so as to be generally a point source of thermal energy, the biological activity may be generally distributed throughout the drainage system so as to be a diffuse source of thermal energy, or both, in various aspects. Various containment structures such as reservoirs, pools, ponds, and so forth may be provided about the drainage system to promote the production of heat through biological or chemical processes. The containment structure may be insulated in various aspects.

Evaporation of water from the drainage system may reduce the thermal energy of the drainage system by the latent heat of evaporation in combination with the quantity of water evaporated, in various aspects. Evaporation of water may be generally a non-point sink for thermal energy, in various aspects.

Water outflows from the drainage system to be discharged into a river, lake, reservoir, sea or other natural or manmade water body. The flow of thermal energy from the drainage system is a function of the temperature of the outflow water and the flow rate of outflow, in various aspects.

Thermal energy may be lost from the drainage system due to thermal conduction, thermal convection, and thermal radiation, which may be generally non-point sources of thermal energy. Thermal conduction may diffuse heat from the drainage system through the conduit or other boundary of the drainage system into the surrounding earth. Thermal conduction may be a function of the water temperature, the earth temperature, and thermal conductivity of the boundary of the drainage system and the thermal conductivity of the earth. Thermal conduction may vary seasonally. Thermal conduction may conduct thermal energy into the drainage system or conduct thermal energy from the drainage system depending upon the season. Thermal radiation may occur at location of the drainage system under shallow earth cover or not covered at all or otherwise exposed to the external environment.

Thermal convection may transfer thermal energy between the water and air, if any, above the water within the drainage system. The air may have motion due to thermal gradient or forced convection, for example, driven by fans. Thermal convection may input thermal energy into the water, withdraw thermal energy from the water, or both, and the input of thermal energy to or from the water may occur diurnally, seasonally, or both, in various aspects.

In various aspects, the methods disclosed herein include identifying a thermal energy target within a drainage system. The thermal energy target may be a location within the drainage system having a sufficient combination of water temperature and water quantity to provide an extractable or ejectionable quantity of thermal energy. The existence of a thermal energy target may depend upon the various sources and sinks of flow and thermal energy upstream of the thermal energy target, and such sources and sinks may be variable seasonally, diurnally, cyclically, or have some other variability which may be random or may be in some pattern. The thermal energy available at the thermal energy target may be generally constant or the thermal energy available at the thermal energy target may vary seasonally, diurnally, cyclically, or otherwise variably. The thermal energy target may be present seasonally, diurnally, cyclically, or with other variability. The location of thermal energy targets within the drainage system may change seasonally, diurnally, or cyclically, or show other variability. Identifying the thermal energy target may include identifying variability of the various thermal energy sources upstream or at the target including seasonal, diurnal, cyclical, or other variability thereof that may be random or may assume some pattern. In various aspects, identifying the thermal energy target may include identifying the location of thermal energy sources within the drainage system at various time during the day, seasonally, or at other times or time intervals. In various aspects, identifying the thermal energy target may include mapping the drainage system, may include characterizing the geometric shape, structural characteristics, flow characteristics such as slope and roughness, and other attributes of the drainage system. In various aspects, identifying the thermal energy target may include identifying residential, commercial, or industrial facilities proximate the drainage system and sources of flows from these facilities into the drainage system.

A thermal energy target may, for example, result from large flows of heated water from an industrial source absent significant dilution by cooler groundwater or other cooler flow sources and absent thermal energy losses due to various sinks. Methods for identifying a thermal energy target may include monitoring the water flow within the drainage system at various locations within the drainage system. Methods for identifying a thermal energy target may include monitoring the water temperature of the water within the drainage system at various locations within the drainage system, and the locations at which the flow is monitored may be coincident with the locations at which the water temperature is monitored, in various aspects. The flow, the water temperature, or both flow and water temperature may be monitored over various time periods and may be sampled at various time intervals.

The air within or about the drainage system may exchange thermal energy with the water in the drainage system; there may be sources or sinks of thermal energy that transfer thermal energy to or from the air, respectively, in various implementations. The air within or about the drainage system may form at least a portion of the thermal energy target, in various aspects, and thermal energy may be exchanged with the air proximate the thermal energy target. The air temperature at various locations may be monitored over various time periods in various implementations.

In various implementations, the step of identifying the thermal energy target may include identifying sources and sinks of thermal energy within the drainage system. In various implementations, the step of identifying the thermal energy target may include indentifying sources and sinks of thermal energy related to the water flow within the drainage system. In various implementations, the step of identifying the thermal energy target may include indentifying sources and sinks of thermal energy related to the air within or about the drainage system. In various implementations, the step of identifying the thermal energy target may include identifying thermal energy exchanges between the air within or about the drainage system and the water flowing within the drainage system.

The methods, in various aspects, may include identifying a user of the thermal energy transfer and may include identifying a user of the thermal energy that is located generally proximate the thermal energy target. The user may be an institutional, commercial, residential, or industrial facility, or combinations thereof, in various aspects. The user may be generally situated proximate the thermal energy target so that thermal energy may be utilized by the thermal energy target to the user.

Various thermal energy transfer apparatus may be utilized to extract the thermal energy from the thermal energy target or to eject thermal energy into the thermal energy target. The methods may include identifying the thermal energy transfer apparatus to be utilized to extract the thermal energy from the thermal energy target.

The thermal energy transfer apparatus may include heat exchangers, heat pumps, and suchlike for the transfer including ejection or extraction of thermal energy. For example, heat exchangers may be used to transfer thermal energy from the flow at the thermal energy target through an exchange of heat between the flow at the thermal energy target and a heat exchanger fluid within the heat exchanger. In various aspects, ethylene glycol may be the heat exchanger fluid within the heat exchanger. The heat exchanger fluid may then transport heat to another heat exchanger located about the user where the thermal energy may be transferred, for example, into boiler feed water. In various aspects, the heat exchanger may extract thermal energy from the water in the drainage system, from air within or about the drainage system, or both. In various aspects, the heat exchanger fluid may transport thermal energy from the user for ejection into the thermal energy target. For example, the heat exchanger fluid may transport heat ejected by a user\'s cooling system, and the heat may then be ejected into the thermal energy target for dissipation thereof.

Heat pumps may be utilized to transfer thermal energy with the thermal energy target. A heat source of the heat pump may be deployed within the water of the drainage system, within the air within or about the drainage system, or both within the water and the air to transfer thermal energy with the thermal energy target. In various aspects, the heat pump may extract thermal energy from the thermal energy target, or the heat pump may eject heat into the thermal energy target.

In various aspects, the water within the drainage system may be the thermal energy target, the air within or about the drainage system may be the thermal energy target, the ambient atmosphere may be the thermal energy target, combinations thereof, and the thermal energy targets may be sources of thermal energy, sinks of thermal energy, or combinations thereof. For example, thermal energy may be extracted from the water in the drainage system and ejected into the ambient atmosphere, in certain applications. An example, in various aspects, may be transferring heat from the Minneapolis sewer system to the University of Minnesota football stadium by circulating sewer water within the concrete structure beneath the seats in the stadium or melting ice on sidewalks and roadways in a similar manner.

Municipal waste water streams can actually be a significant renewable energy source. A research project was initiated to quantify the available energy in an operating wastewater process, define the methods of extraction, and predict the amount of energy captured and recaptured from this renewable source. The knowledge gained through engineering design criteria and formulating business models resulting from this energy capture and recapture research project can be implemented and replicated nationwide.

There is an ever-increasing need for alternatives to traditional fossil fuel energy sources. One potential way to address that need is a method for capturing and recapturing waste heat from their wastewater system. The wasted heat also represents a potential saleable energy source that could be provided to city facilities and eventually all customers of the municipal utility.

The research project was conducted in the city of Brainerd, Minn., which is a community of about 15,000 located in north central Minnesota. The project measured and established feasibility plans on how to extract thermal energy from the Brainerd wastewater collection system and transfer that energy through existing heat pump technology into Brainerd school facilities to replace carbon fuel systems.

Specific objectives of the research project were as follows:

Determine the specific amount of heat available from the wastewater system during seasonal and diurnal fluxes.

Determine the amount of heat available for selected locations within the community such as schools and other buildings within the City of Brainerd.

Determine the estimated cost of extracting the energy including the preliminary development of a system to extract and deliver the heat to the selected facilities.

Determine the ownership of the thermal energy within the wastewater system and develop a costing mechanism for the access to and sale of the thermal energy from provider to benefactor stakeholders.

Determine the new labor force that will be needed by the stakeholders for installation and maintenance of the extraction system, sales and billing of the renewable energy source.

Determine the payback period for the stakeholders and the selected facilities.

The project resulted in generation of a thermal energy map of the city waste water system. All the other objectives were based on this map. The thermal energy map is constantly changing. The thermal energy available is a function of both mass flow rate and temperature. Both parameters will change with the daily changes in personal living habits and industrial processes. In addition, seasonal temperature changes are highly influential to the temperature. Therefore, a full year was studied in order to capture a clearer picture as to whether thermal energy was seasonally available as needed.

The first task was a review of the wastewater collection system. Sewershed maps were verified and digitized. Each section of the sewershed terminates at a lift pump station. Each lift pump station was designated as a temperature and flow monitoring point. Sixteen lift stations were selected for the installation of automated temperature data logging equipment. One location within the system was selected for the installation of an automated flow monitoring device. The selected locations were inspected and photographed and a final sampling strategy was presented for approval. Downloading procedures were created for all monitoring systems and trained staff members in the proper collection and recording of data points. All installation sites were mapped on a GIS system for easy location by sampling staff and for reporting purposes.

The second task involved the installation of the sampling equipment at the sites selected in task one. Data logging thermal sensors were installed by the research partners. The thermal sensors were programmed to collect temperature measurements every 5 minutes continuously for one year. The data logging flow sensor collected data at 15 minute intervals. A system control and data acquisition (SCADA) system recorded data at 15 minute intervals. All installation sites were photographed and recorded on maps for easy location by sampling staff and for reporting purposes.

After the sensors were installed at the selected sampling locations, data was continuously collected. Every quarter temperature sensors were checked and the data downloaded. Thermal sensor and flow data was entered into the appropriate spreadsheets and prepared for analysis. The runtime data collected from the lift monitoring stations were also recorded and converted into flow data (runtime×gallons per minute=flow). The thermal energy was calculated in BTU\'s for each thermal sensor site. For each monitored segment of the wastewater system a mass balance of heat was calculated as well as for the system as a whole. From this information the most efficient heat producing sites were located.

On completion of all data analysis, a report was prepared that included the specific amount of heat available in the wastewater system and the amount available to selected facilities within the City of Brained. Cost estimates are developed for extracting the energy as well as estimates for the installation, system maintenance and sale of the new energy. The hardware needed to capture, extract and integrate the heat into selected buildings and the Brained High School heating systems was identified. The payback period for these sites was also calculated.

The flow rates and temperatures were measured independently at each location. All flow measuring points had a corresponding temperature measurement but not all temperature measurements had a corresponding flow measurement. In addition, some individual temperature measurements were made at locations identified through the iterative project review meeting process. All the temperature measurements were made with the same type of sensor/recorder.

The monitoring methodology for flow at the lift stations utilized information from the SCADA system. Two types of pump controls were used at the lift stations resulting in two types of data reported.

On/Off float switch control is the most common. This type of control alternates between two pumps and meters the time each pump is on. When the run time reaches 0.1 hour it is reported through the SCADA system. The 0.1 hour increments are recorded with a time stamp at 15 minute intervals.

Variable frequency drives are used for the highest flow locations. Usually a single pump operates at one time. As pumps accumulate run time they are sequentially operated in order to equalize operating hours. More than one pump can operate if high flow conditions warrant. The SCADA system reports operating RPM with a time stamp at 15 minute intervals.

The installed flow meter was an Isco 4150 Flow Logger. The flow meter measures the average velocity of the channel and the liquid level to calculate open channel flow. The velocity is measured using Doppler technology.

The data was downloaded using Flowlink 3 software, which shows the data in tabular form and is recorded at 15 minute intervals and later downloaded.

All temperature data was recorded with a HOBO Pro v2 model U22-001 manufactured by Onset. The recording time interval was 5 minutes thus providing high resolution relative to the time cycles of interest. High resolution was justified by the potential need to better understand the rate of temperature change. The sensors were secured to a nylon strap with a metal weight to hold them below the surface of the water.

All the data collected was available for individual analysis. However, neither the temperature or flow data individually were a good predictor of available thermal energy for this study. For example, the highest temperatures recorded were from a location near an industrial rug cleaner. The temperature cycles were highly repeatable due to the repeatable nature of work patterns but the flows were some of the lowest in the system. Thus the available thermal energy was low.

Thermal energy available is expressed in British Thermal Units (BTU\'s). A BTU is defined as the amount of energy needed to raise one pound of water one degree Fahrenheit.

BTU=(change in temperature, degrees Fahrenheit)(change in mass, Pounds)

Where One gallon of waste water=One gallon of clean water One gallon of water=8.35 Pounds

The defining limit for heat extraction is the lowest assumed temperature post energy extraction. For example, a heat extraction system may be capable of removing 10 degrees of heat from a waste stream. If the waste stream is flowing at 40 degrees then the theoretical post-extraction temperature would be 30 degrees. This would not be considered feasible for this project since the resulting temperature would be below the freezing point of water, 32 degrees F.

Conversely, the same logic applies to calculating the amount of thermal energy available for cooling. When cooling, the post energy extraction temperature will be higher than the pre extraction temperature.

In order to calculate comparative values between locations, a common upper and lower post-extraction value was used for all locations regardless of the actual temperature recorded. This method allows for comparing seasonal and diurnal cycles. For heating, the lower limit was 40 degrees. For cooling, the higher temperature limit was 75 degrees. BTU charts showed varying amounts of energy available. If the mass flow rate were unchanged, the total energy available would not change and the ratio of energy available for heating and/or cooling would simply be a corollary function to waste water temperature change. But, since the height of the bars change, this infers the total energy available was increasing or decreasing with the change in flow.

FIG. 1-7 illustrate a sample of some of the results of the research project.

FIG. 1 illustrates the drainage system that services Brainerd, Minn., a community of approximately 15,000 people. The flow in the drainage system of FIG. 1 is primarily sanitary sewage, although the flow may in include some snowmelt, groundwater flow, and storm runoff included in this drainage system at various locations at various times. Drainage is generally toward point A and outflows through a sewage treatment plant generally at point A into a river, as shown.

FIG. 2 illustrates the water temperature as a function of time at a location the drainage system of FIG. 1 proximate a light industrial area. The water temperature, as illustrated in FIG. 2, oscillates periodically as a function of time, which may indicate an industrial discharge of heated water into the drainage system by an industrial user. The water temperature data in FIG. 2 is recorded generally during winter months and the mean water temperature of the water temperature oscillations declines over time. This may be due to conductive thermal energy exchange between the drainage system and the external environment through the earth. This water temperature may indicate that this location is a thermal energy target. The thermal energy available at this thermal energy target may be related to both the water temperature of the water and the flow rate at this location.

FIG. 3 illustrates water temperature as a function of time at another location within the drainage system of FIG. 1. The water temperature, as illustrated, oscillates about a mean value of 63° F. with spikes in excess of 70° F. This water temperature may indicate that this location is a thermal energy target. The thermal energy available at this thermal energy target may be related to both the temperature of the water and the flow rate.

FIG. 4 illustrates water temperature as a function of time at yet another location within the drainage system of FIG. 1. The water temperature, as illustrated, steadily declines during the winter months from about 50° F. to about 44° F. This water temperature may be too low so that this location is not a thermal energy target.

FIG. 5 illustrates a plot of temperature drop in ° F. as a function of flow rate in gallons per minute (gpm) for selected rates of thermal energy extraction. Curve 131 illustrates the temperature drop as a function of flow rate for thermal energy extracted at a rate of about 1,200,000 BTU/Hr. As indicated by curve 131, thermal energy extracted from a flow of about 238 gpm at a rate of about 1,200,000 BTU/Hr would lower the water temperature by about 10° F. As indicated by curve 131, thermal energy extracted from a flow of about 688 gpm at a rate of about 1,200,000 BTU/Hr would lower the water temperature by about 3.5° F. Thermal energy extracted at a rate of about 1,200,000 BTU/Hr may heat about 50 homes, in various aspects.

Curve 133 in FIG. 5 corresponds to a thermal energy extraction rate of about 3,439,000 BTU/Hr, which may heat about 143 homes in various aspects. As indicated by curve 133, extraction of thermal energy at a rate of about 3,439,000 BTU/Hr lowers the water temperature by about 10.0° F. at a flow rate of about 688 gpm.

As illustrated in FIG. 5, the ability to extract thermal energy from a thermal energy target is limited by the temperature of the target and the quantity of flow at the target to an available thermal energy. The rate of thermal energy exchange may be limited according to the water temperature of the target and the flow at the target to avoid phase changes and so forth. For example, thermal energy extraction at a rate of about 1,200,000 BTU/Hr would lower the water temperature by about 10° F. at a flow rate of about 238 gpm, the water temperature should be a minimum of about 44° F. For water temperatures less than about 44° F. the thermal extraction rate would be less than about 1,200,000 BTU/Hr and for water temperatures greater than about 44° F. the thermal energy extraction rate may be increased above about 1,200,000 BTU/Hr. Various systems and apparatus may be provided that, for example, calculate the thermal energy available at the thermal energy target and that then limit the extraction of thermal energy from the target to less than or equal to the available thermal energy at the thermal energy target.

FIG. 6 illustrates sample data of the average flow rate (gpm) as a function of time of day (24 hour clock) at a selected location within the drainage system of FIG. 1. As illustrated in FIG. 6, the average flow rate is about 587 gpm with a minimum average flow rate of about 350 gpm at 06:30 and a maximum average flow of 735 gpm at 15:00. Data illustrated in FIG. 6 was collected over a period of about 43 days.

FIG. 7 illustrates the frequency of flow rate occurrences in 25 gpm increments for the flow data of FIG. 6. As illustrated in FIG. 7, the average flow was about 587 gpm. The frequency of flow rate peaked at a flow rate between about 376 gpm and about 400 gpm about 222 times. The frequency of flow rate peaked at a flow rate between about 676 gpm and about 700 gpm about 399 times.

As illustrated by FIGS. 6 and 7, the flow rate at a particular location may be statistically distributed in various ways according to a flow probability distribution. A flow probability distribution may be selected and the selected flow probability distribution may be fit to flow rates as measured at the particular location.

As illustrated by FIGS. 2, 3, and 4, the temperature at the particular location may be statistically distributed in various ways according to a temperature probability distribution. A temperature probability distribution may be selected, and the selected temperature probability distribution may be fit to water temperatures as measured at the particular location. Combining the flow probability distribution with the temperature probability distribution may result in a thermal energy probability distribution that may indicate that a quantity of thermal energy is available at the particular location at a particular time with a corresponding probability. The thermal energy probability distribution may be used to identify a thermal energy target, in various implementations. It may be desirable, in various implementations, that certain quantities of thermal energy are available with certain corresponding probabilities and thermal energy targets may be identified as locations that meet availability of the certain quantity of thermal energy with the certain corresponding probability.

FIG. 8 shows energy available for heating and for cooling at one of the drainage system locations on 13 monthly dates over a one year period.

FIG. 9 illustrates heating system 200 in which thermal energy from waste water in waste water drainage system 202 provides some of the energy needed to heat building 204. System 200 includes gas fired boiler 206, building radiators 208, preheater 210, heat exchanger 212, waste water supply line 214, waste water return line 216, and natural gas supply line 218.

A heat transfer fluid, such as ethylene glycol, is circulated from boiler 206 to radiators 208 to preheater 210 and then back to boiler 206. At preheater 210, heat from heat exchanger 212 is used to preheat the fluid, which reduces the amount of heat that must be provided by boiler 206.

Heat exchanger 212 receives waste water from drainage system 202 through supply line 214. Heat is extracted from the waste water by heat exchanger 212 and supplied to preheater 210. Waste water is returned from heat exchanger 212 to drainage system 202 through return line 216.

The heat extraction by heat exchanger 212 can provide significant cost savings in heating building 204. Consider an example in which 24,000 gallons per day (GPD) of waste water at 55° F. flows to heat exchanger 212 and is returned to drainage system 202 at 49° F. Approximately 50,000 BTU/hr is available for extraction by heat exchanger 212 to supply heat to preheater 210. That equates to a value of potential energy savings of $15,425/year using as a measure of energy cost a price for residential electric power of $0.12 per kilowatt hour.

While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.