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What happens to a solar system when it snows?

01 Mar 10 | Solar Electric, Solar Hot Water, solar thermal

I have good customers, they ask good questions.  One such question asked of me lately has been “what happens to my solar system when it snows?”  Since I have both a solar thermal system and a photovoltaic system on my house, I can tell them.  Enough sunlight gets through the snow that the panels begin to heat up.  This, in turn, causes the snow to slide off.  Here is a picture of a ground mounted system after receiving over two feet of snow:

It helps that the panels are tilted to 40 degrees, roof mounted systems likely will not shed snow like this.  Still, on a roof mounted system, the snow will melt off, it might take a little longer.  The only system I would be careful of in this climate would be an evacuated tube collector.  Because the tubes have a vacuum, no heat is transfered to the glass envelope, which is really good for collecting heat, but not so good for melting accumulated snow off the collector.

Tags: PV, SDHW, weather

Hot Water tank stratification

03 Jun 09 | Solar Hot Water

There are many considerations to ensure that a solar domestic hot water system will perform at it’s optimum. The collectors should be facing south, tilted to latitude, unshaded,  etc.  One consideration that is usually not thought about or understood is the storage tank.  Like any energy storage system, there are some physics that accompany a hot water storage tank.

Stratification simply means to divide into layers.  Heated water rises because it is less dense than cold water.  The warmest water will be found in the layer right at the top of the tank, hence, most tanks have their hot water outlet at the very top of the tank.

When pumping water out of a solar storage tank, through a heat exchanger and back again, it is very important not to completely mix the water in the tank.  In most SDHW systems, the temperature sensor for the storage tank is at the very bottom of the unit.  If the tanks gets mixed, chances are the collector temperature and the tank temperature will reach equilibrium and the system will shut off.

If the solar storage tank water is pumped slowly, so that the tank stays stratified, the system will net much more heat.  This works especially well in a two tank system where tank number one is the solar tank which pre-heats the water going into tank number two, which is the back up heating system.  If done correctly, both tanks will  have a thermocline about 1/3 up from the bottom of the tank.

There are two good ways to accomplish water side heat exchanger pumping without breaking the solar tank stratification.

1. Use a small ac pump, such as a TACO 003B and throttle the output side of the pump with a ball valve.  This pump uses very little electricity (rated for 42 watts, 115 VAC) and therefore is pretty efficient.  Restricting the flow slightly with a ball valve will not hurt it.  The water going into the heat exchanger from the solar tank should be about 5 – 10 degrees (Δt = 5-10° F) cooler than the water coming out.
2. Use a PV powered DC pump.  There are two DC pumps that run directly from a 12 volt PV panel, the Liang D5 series and El Sid.  These can also be throttled on the output side for temperature rise of 10 degrees from input to output.    The advantage of this system is that the pump speed will adjust to the available sunlight (thus available heat) making the system more efficient.  The disadvantage is it is more expensive.

Experience shows that a good rule of thumb is 0.0125 gallons per minute per gallon of storage.  Therefore, for an 80 gallon storage tank, optimum flow rate on the storage tank side of the heat exchanger would be 80 gallons x 0.0125 = 1 GPM.  For a 120 gallon tank, 1.5 GPM and for a 240 gallon tank, 3 GPM.  This will generally give a 10 degree temperature difference between the top and bottom of a vertical tank.

Tank stratification is an important design factor that is often not thought of when a dual pumped internal or external heat exchanger system is installed.

Tags: physics, SDHW, solar thermal

Can plastic piping be used in a solar hot water system?

12 May 09 | Solar Hot Water, Training, solar thermal

Short answer: Don’t do it.

Plastic piping such as PEX, PEX AL PEX, PVC, ABS, etc. can be safely used with hot water systems, radiant floor heating and so forth.  It is much cheaper and usually easier to work with than copper or stainless steel.  That being said, it is not appropriate for use in any solar thermal application.

Solar thermal systems have much less control over high temperatures than conventional fossil fuel based systems.  Summer time collector stagnation temperatures can easily reach 300° F.  At these temperatures any plastic piping will melt.  This will cause the Heat Transfer Fluid (HTF) to leak creating a big mess and likely an insurance claim.  The only type if piping that should be used in a collector loop is copper or stainless steel.

Even copper fittings with rubber gaskets (AKA Pro-Press or Viega fittings) are only rated for 250° F.  They should not be used in a solar loop either.

It is worth the extra time, effort and expense to solder copper piping and or purchase stainless steel tubing for use in the solar loop.  This will ensure that the system works well for years to come with no leaks and no call backs.

Tags: installation tip, piping, SDHW

Calculating energy needed to heat water

05 May 09 | Solar Hot Water, solar thermal

In order to properly size a Solar Domestic Hot Water (SDHW) system, a few pieces of information are needed:

1. Current and future occupants of the house or average hot water use.
2. Water supply temperature
3. Desired hot water temperature
4. Stand by loss of heating unit

We know that in this area (Mid Hudson Valley) ground water temperature averages 53 degrees.  I know this because I have personally measured the well water temperature at all of our SDHW installations.  This is a good starting point.

Most people desire their hot water temperature to be between 110 to 120 degrees.  There are some applications where hotter water (laundry, dish washers, etc) is desired.  For general purposes 115 degrees is a good ending point.

We also can base average hot water useage on the number of occupants of any house.  The rule of thumb is 20 gallons per person for the first two people, 15 gallons per person for any additional people.  This means that the average family of four uses 70 gallons of hot water per day (20+20+15+15 = 70).

Standby losses for water heaters generally range from 5-10% for electric and oil fired systems and 40% for natural gas or propane water tanks.

For the purposes of Solar Hot Water, an appropriate unit of energy would be the BTU.  If we were using SI units (metric) it would be the Mega Joule (MJ).  Since most HVAC contractors understand things in terms of BTUs, it is easiest to use this unit.

A BTU is defined as amount of heat required to raise the temperature of one pound of liquid water by one degree Fahrenheit.   That is close enough for our purposes.

Therefore, the formula to calculate energy use is:

BTUneeded= 8.34 x Gallons x (desired°F-supply°F) x Standby

Where:

• BTUneeded = BTUs needed to heat the water for one day
• 8.34 = Weight in pounds of one gallon of water
• Gallons = Gallons of hot water used in one day
• desired°F= Desired temperature of the hot water
• supply°F= Cold water supply temperature
• Standby= Standby loss of the heating appliance

A typical family of four heating their hot water with electric or oil would expect to use:

BTUneeded = 8.34 x 80 x (115°F-53°F) x 1.10 = 45,503 BTU/day

A typical family of four heating their hot water with gas or propane would expect to use

BTUneeded = 8.34 x 80 x (115°F-53°F) x 1.40 = 57,913 BTU/day

To get an idea of cost, BTUs need to be converted to energy units that are used for electricity, oil, and gas.

• Electricity has 3412 BTU per kWh.  Therefore 45,503 ÷ 3412 = 13.3 kWh.  Going rate per kWh is about $0.16.  13.3 kWh x $0.16 = $2.13 per day or $778.83 per year
• Heating oil has 138,700 BTU per gallon.  Therefore 45,403 ÷ 138,700 = 0.33 gallons.  Going rate per gallon $2.459.  0.33 gallons  x $2.459 = $0.81 per day or $269.19 per year.
• Propane has 93,000 BTU per gallon.  Therefore 57,913 ÷ 93,000 = 0.62 gallons.  Going rate per gallon $2.428.  0.62 gallons  x $2.428 = $1.51 per day or $549.46 per year.
• Natural gas has 102,000 BTU per CCF.  Therefore 57,913 ÷ 102,000 = 0.56 CCF.  Going rate per CCF is $1.633.  0.56 CCF x $1.633 = $0.93 per day or $338.42 per year.

Tags: energy costs, SDHW, Solar Hot Water

Solar Hot Water System components

27 Mar 09 | Solar Hot Water, Training

Solar Domestic Hot Water systems are a great way to save money, cut down on the use of fossil fuels and do a big favor for the environment.  We have install many of these systems over the last two years and they work very well, even in the middle of winter.

I decided to install drain back systems because I like their simplicity and their easy maintenance.  The average home owner can very easily keep track of the water in the sight glass and add water if needed.  They perform well and when properly installed are pretty much bullet proof.  I like that.

This is an 80 square foot 80 gallon storage tank system.  Enough to provide 80% annually of the hot water for an average family of four.

The system components consist of Flat Plate collectors:

These are Alternative Energy Technology AE-40 collectors.  They are elevated slightly from the roof pitch to facilitate snow removal and better drain back performance.  They are also tilted to the left so that the water drains out of the bottom of the collectors when the pump is off.  This is a very important detail to avoid freeze damage.

The piping is 3/4 L copper tubing insulated with closed cell (AKA Rubatex or Insultube) R-5 foam insulation.  Where ever possible, the insulation is slid over the ends of the pipe instead of cut lengthwise and placed over the pipe.  The ends and any slit pieces are glued together with special glue called R-420.  The exterior runs are covered with PVC jacket to protect the insulation from UV damage and improve the system appearance.

The drain back tank is mounted on a shelf attached to the basement wall.  This is a 10 gallon stainless steel drain back tank with an internal heat exchanger.  It has a sight glass which is marked with the proper fluid levels for when the system is running and when it is off.

The solar loop pump is a TACO 009BF5.  I use bronze pumps in the solar loop of a drain back system because the water gets sloshed around quite a bit and becomes oxygenated.  A cast iron pump will rust and foul the site glass.  It also keeps the solar loop a “potable water system” and thus avoids and questions about the single wall heat exchanger in the drain back tank.  The pump is mounted below the lowest fluid level in the drain back tank.  At the very bottom of the solar loop is the drain valve.

The storage tank loop is a TACO 006B4.  This is a larger pump that normal because the storage tank is located about 15 feet away in another room.  This configuration is slightly unusual, however, it was the only way to fit the solar system in a crowded basement.

In the storage tank loop there is an air vent at the highest point in the loop to bleed out any air that may become trapped in that loop.  Trapped air can cause pump cavitation and or reduce the flow in the loop storage tank loop.  For maximum efficiency, the loop needs to move about 4-6 gallons per minute from the bottom of the storage tank through the heat exchanger and back to the top of the storage tank.

The storage tank is an 80 gallon off the shelf unit with a 12 year tank warranty.  It has electric back up elements which are not connected because the home owner has an indirect oil fired tank connected to their home heating system.

The system controller is a DTC-2 (AKA Eagle 2) by IMC.  I really like these controllers because they have temperature reading for the storage tank and the collectors.  They also have variable set points for the high limit and temperature on differential.

Finally, the output to the backup heating tank has a Watts 1170 tempering valve.  This is very important because the solar storage tank temperatures can get very high durring the summer months.  With out a tempering valve scalding water can be sent to the showers and sinks in the house.

Every time I commision one of these systems, I think to myself  “There is less oil.”

Tags: SDHW, Solar Hot Water, solar thermal, system components

Formulas for Solar Hot Water Systems

09 Apr 08 | Solar Hot Water

Something that I get asked quite often is “How do you know this will make enough hot water?” That is a very good question and there are several rules of thumb regarding the size of Solar Domestic Hot Water (SDHW) systems. A properly sized SDHW system will provide between 70-80% of the annual hot water. While it would be nice to provide 100 percent, this is not a realistic goal in the Northeast because of the limited daylight hours during December and January.

General rules of thumb are:

1. Allow 20 gallons per day of use for the first two people
2. Allow 15 gallons per day of use for each additional person

So the basic house with four people would need 70 gallons of hot water per day. The closest conventional water heater sized to that use is 80 gallons, therefore an 80 gallon SDHW system would be appropriate for that household.

Solar Collector surface area is based on the size of the solar storage tank. Again, these are rules of thumb that have been tried and tested since the 1970’s for SDHW systems:

• Northern New England: 1Ft²/0.75 gallons
• Northeast, New England, Mid Atlantic and Northwest: 1ft²/1.0 gallons
• Midwest and Mountain States: 1Ft²/1.25 to 1.5 gallons
• Southeast, Sunbelt and Hawaii: 1Ft²/1.5 to 2 gallons
• Sunbelt desert areas: 1Ft²/1.75 to 2.25 gallons

Calculating Energy to heat water

If you are not interested in rules of thumb, here is how to calculate the actual energy required for any SDHW system. First, use the Hot Water Formulas and Calculations to determine how much hot water will be used. That use needs to be converted to a unit of energy. In the US, we use BTU, while the rest of the world uses Joule as a measurement of energy.

The basic formula is:

Energy (BTU)= V_gal x 8.345 x (T_expected – T_in) x eff

Where:
V_gal is the volume of water in gallons
T_expected is the expected hot water temperature
T_in is the temperature of the cold water supply
Eff is the system losses

The temperature for both the cold water supply and the expected hot water need to be know to calculate (T_expected – T_in). This is called the Δ T (delta T), or change in temperature. For example, the incoming water supply from a well is 45 degrees F. The expected hot water temperature is 125 degrees, this leads to a Δ T of 125 degrees – 45 degrees = 80 degrees.

From the above rules of thumb, or the Hot Water Formulas and Calculations, the example household is using 70 gallons of hot water per day.

A gallon of water weighs 8.345 pounds. It takes 1 BTU to warm 1 pound of water 1 degree Fahrenheit.

Therefore, 70 gallons of water x 8.345 pounds is 584.15 pounds of water. To 584.15 pounds x 80 degrees F = 46,732 BTU, plus efficiency losses and system losses. Efficiency losses in a SDHW system are in the solar collector glazing transmissivity, heat exchangers, pumps, etc. Generally they run around 10 to 15 percent. System losses are stand by tank loss, piping loss, etc. Generally they run about 10 percent.

Therefore, the entire solar collector array will need to collect 125 percent of the required BTUs noted above, or 46,732 BTU x 1.25 = 58,415 BTU per day.

Below are comparisons of how much conventional fuel would be used to heat the water in the example household:

1. Electricity has 3,413 BTU per kWh. Electric hot water systems are 100 percent efficient, but have standby losses. Therefore (46,732 BTU x 1.10)/3,413 = 15 kWh per day. At $0.175 per kWh utility company rates, that equals $2.625 per day
2. Propane has 91,600 BTU per gallon. Propane hot water systems are about 65 percent efficient and have stand by losses. Therefore (46,732 BTU x 1.55)/91,600 = 0.80 gallons of propane per day. At $3.05 per gallon, that equals $2.41 per day
3. Natural gas has 100,000 BTU per Therm (Therm is 1 CCF or 100 cubic feet). Natural gas hot water systems are about 65 percent efficient and have stand by losses. Therefore (46,732 BTU x 1.55)/ 100,000 = 0.73 CCF per day. At 1.60 per Therm, that is $1.15 per day
4. Heating oil (#2 distillate) has 140,000 BTU per gallon. Oil fired hot water systems are generally 80 to 85 percent efficient and may or may not have standby losses. Therefore (46,732 BTU x 1.2)/ 140,000 = 0.40 gallons per day. At $3.97 per gallon, that is $1.59 per day.

Calculating size of solar array based on energy needed

Here is where things get a bit complex. Every location has a different amount of Insolation which is the amount of solar radiation received on a given surface. The NREL (National Renewable Energy Lab) has a program called PVWATTS which can give very specific data on a month by month basis. This is important for sizing of solar thermal space heating systems. Usually this data is given in units of kWh/Meter² per day. That is acceptable because that can be converted to BTU/Ft² per day by multiplying kWh/M² by 317. Each kWh equals 3,413 BTU, A M² equals 10.76391 Ft². Therefore 3,413 BTU/10.76391 = 317.

I like to pick a moderate month, such as April or September, and size the SDHW system to meet 100 percent of the load in that month. I often find that this is the best compromise for the New York region as it will give more hot water than needed during the summer months, and less during the winter.

The example household requires 58,415 BTU per day from the solar system. According to the PVWATTS program, a solar collector tilted at latitude for the month of April will receive 4.63 kWh/M² per day. Convert to BTU per Ft², 4.63 kWh/M² x 317 = 1,579 BTU/Ft², therefore 58,415 BTU/1,579 BTU/Ft²= 37 Ft².

That would be a perfect world theoretical solar collector and it is a good median figure. Like many things, there are other considerations:

1. The efficiency of the collector absorber plate coating
2. The efficiency of the Heat Transfer Fluid (HTF)
3. The incident angle of the sun on the surface of the collector in both the horizontal and vertical axis
4. The ambient temperature of the collector
5. The internal temperature of the collector and the HTF

At this point, the equation becomes a calculus problem and a somewhat complex one at that. The general idea is to increase size to overcome the collector losses. Field work indicates that in the Northeast, doubling the theoretical size works well. This equates to about a 50% loss over the theoretical model noted above. This is how the above noted rules of thumb on collector size vs. storage tank size are formed.

1. Absorber plate coating also called “Black Chrome.” Solar selective coating has come a long way since the 1970’s. Basically, it is a type of paint that accepts and converts more energy, in the form of visible, infrared and UV light, from the sun and converts it to heat without radiating it back into the atmosphere. These are highly specialized products and are not normally available to the general public.

2. Heat Transfer Fluid or HTF. HTF removes the heat from the absorber plate and transfers it to a heat storage tank. It can be water, antifreeze, oil, etc. Water is the best HTF as far as efficiency is concerned, but can present freezing problems.

3. Sun incident angles on the collector surfaces. A solar collector is at it’s optimum when the sun is 90 degrees from the surface. The further away the sun is from perpendicular, the less dense the energy is that is striking the surface.

4. The solar collectors have operating categories based on the ambient temperature (Ta) that the collector is in vs the water temperature within the collector (Ti). This is known as the Ti-Ta. In the summer time when the Ti-Ta is 36 or less degrees Fahrenheit, the collector is operating as a Category C unit. In the winter time when the Ti-Ta can be 90, 100 degrees or even greater, the collector is operating as a Category D unit.

Tags: SDHW, Solar Hot Water

Solar Thermal Systems

21 Feb 08 | General

When you say solar, most people assume that you are talking about photovoltaics. Solar thermal systems have been around for longer than photovoltaics and have a proven track record of working well and paying back there costs many times over.

I install both types, and lately I have been receiving quit a few calls regarding solar thermal (i.e. solar hot water, or radiant floor heating) systems. I think this will continue as the price of energy goes up.

A solar thermal system move fluid through solar collectors, which collect heat.  The fluid is then stored or used in the building. In reality, a solar thermal system is about plumbing.  A Solar Domestic Hot Water (SDHW) system has three unique parts that other hot water systems or heating systems don’t have. The first is the solar collectors, the second is some type of heat exchanger and the third is some type of controller. As regarding the solar collectors, I believe that SDHW systems work best with flat plate collectors.

The flat plat collector design has been around for many years. Newer solar selective coatings have been created that increase the system efficiency. In addition to that, better insulation and better high transmisity glass have all improved on the flat plate collector design. Evacuated tubes run at higher temperatures and have problems with the seals between the glass tube and the copper pipe on the inputs and outputs of each tube.

The next unique thing in a solar system is the heat exchanger. The heat exchanger takes the hot fluid from the solar collector and cools it with the fluid from the solar hot water tank. This can be implemented in a number of ways. Some heat exchangers are part of the solar storage tank, some are a part of a drainback tank, and some are external.  All heat exchangers are made of metal (stainless steel or copper) and use counter-flow properties to move the heat from one fluid reservoir to another.

Finally, the system controller, which measures the temperature of the collector outputs and the solar storage tank. If there is enough energy in the collectors to transfer to the storage tank, the controller turns the system on, which begins collecting energy.

Beyond that, a solar system is copper piping, valves, drains, hot water tanks, pumps, and other miscellaneous hardware which is all available at the local plumbing supply house.

Solar thermal systems that are designed for space heating are very similar to SDHW, only they are usually quit a bit larger with more storage.

In most cases, all solar thermal systems should have some way of operating in backup mode in case there is a long period of inclement weather. These back up systems entail some type of conventional heating system installed in parallel with the solar system. For example a SDHW system may have an electric tank or electric element in the solar tank designed to turn on if the water temperature gets too cool. A radiant floor heating system may have a small oil or gas fired backup furnace in standby duty.

A well designed solar system should be designed to produce about 80 to 85 percent of the energy needs. More than that and the system design will be too large, causing it to over produce and over heat under normal operating conditions. Less than that and you are leaving a good deal of money on the table, to be taken by the gas/oil companies.

Tags: radiant floor heat, SDHW, Solar Hot Water, solar thermal

Does solar hot water work in cold climates?

03 Jan 08 | Solar Hot Water

I was at a lecture this past September when the speaker stated “Solar thermal just doesn’t work in this climate (the Northeast), it is not a consideration…” I wanted to stand up, raise my hand and say “Um, excuse me, but you are wrong.” There is a persistent misconception that solar thermal only works in temperate climates. The truth is that is works in cold climates as well. As evidence I submit the following:

It is January, the temperature outside is 17 degrees (that was the high temperature of the day) with a foot of snow on the ground.

The water temperature coming back from the solar collectors on my roof is 150 degrees.

Furthermore, last night it was 0 degrees and I suffered no freeze damage to the collectors or piping, even though I use distilled water as my heat transfer fluid.

This is no accident, of course. A properly designed and installed solar hot water (solar thermal) system can work in almost any climate. The snow surrounding the collector reflects sunlight unto the collector itself, making it more efficient. I have a drain back system, so when the sun goes down and the temperature drops, all the water drains out of the collectors into a reservoir inside the house. The collectors and piping all are sloped so that the water drains out properly thus providing excellent freeze protection without the use of expensive anti freeze.

Naturally, I didn’t say anything to the photovoltaic guy who made the above statement. After all, we are playing on the same team.

Tags: SDHW, Solar Hot Water, solar thermal

Gravity Film Heat Exchangers or GFX

29 Dec 07 | Conservation, Environment, Solar Hot Water

Update: Sun Volt Solar is now an authorized dealer of ECO-GFX gravity film heat exhangers!  For more information, check out our GFX web page.

I am always looking for ways to improve efficiency, especially in energy use. I came across something called a Gravity Film Heat Exchanger or GFX system. These units recover heat from waste water and return it to the hot water tank. Since 80-90 percent of household hot water heat goes down the drain, literally, what a great way to recover some of that energy and reuse it.

Here is how they work: Hot water from a shower or sink runs down the drain and out into the sewer. If there is a vertical run of pipe, the water, because of adhesion, runs down the wall of the pipe (and not the center). If the pipe is metal, heat is transfered from the water to the metal pipe wall until the pipe is the same temperature as the water. All of this happens in any sewer line. If the vertical section of pipe has a heat exchanger attached to the outside of it, the heat from the metal pipe is conducted away to be reused. Most often cold water feed into the hot water tank is run through the heat exchanger. The system efficiency depends on the difference in temperature between the waste water and the incoming cold water. The greater the difference, the higher the efficiency.

This system only works where there is simultaneous hot water use and immediate drainage, say a show or sink. Something like a bath or laundry would not work well because the hot water is drawn off, used, then drained away.

It appears that the system is around 45-50 percent efficient. Therefore, if you use 70 percent of your hot water in showers or sink use, you would recover 30-35 percent of your total hot water energy. Not too bad.

These systems would work very well with solar hot water systems. I am contemplating installing one at my house to see if they work as advertised. The only requirement is that they be installed vertically. In fact, in order to work properly, great care should be taken to make sure the unit is perfectly vertical. For more information, see the NREL website on waste water heat recovery or this .pdf file called Heat Recovery from waste water using Gravity Film Heat Exchangers.

Tags: Environment, GFX, SDHW, Solar Hot Water

Hot Water Formulas and Calculations

09 Dec 07 | Solar Hot Water

The general use calculation for hot water heating system design is about 80 gallons of hot water for a family of four. That is a very rough estimate, however, it holds up well in most cases. As far as solar hot water system design, I (and others) recommend installing systems this size on most residential housing units. It is not all that much more expensive than a 60 gallon system, which is the next size down.

Even if less than four people live in that particular house, solar domestic hot water (SDHW) systems have service lives of 25-35 years or more. Chances are good that the property will transfer ownership in that time, possibly to a larger family.

Several years ago, the US Department of energy put a great deal of effort into defining an equation which would more precisely calculate hot water use. Their work is below:

Daily Hot Water Use

Use the following equation to estimate average daily hot water use in gallons per day.

vol =

(

-1.78 + 0.9744 x per + 6.3933 x age1 + 10.5178 x age2 + 15.3052 x (age3+age4) -0.1277 x thermostat + 0.1437 x tanksz – 0.1794 x wtmp + 0.5155 x atmp + 10.2191 x athome – (1-dw) x dw_use – (1-cw) x cw_use

)

x IF(senior_only,0.379,1) x IF(NOT(pay),1.3625,1)

variable	units (or equation)	description
age1		number of people aged 0-5 yrs
age2		number of people aged 6-13 yrs
age3		number of people aged 14-64 yrs
age4		number of people aged 65- yrs
per	= sum(age1,age2,age3,age4)	total number of people in household
tstat	°F	water heater thermostat setpoint
tanksz	gals	rated volume of water heater
wtmp	°F	inlet water temperature
atmp	°F	average annual outdoor air temperature
athome	= 1 if TRUE, 0 if FALSE	adult at home during day
pay	= 1 if TRUE, 0 if FALSE	residents pay for energy to make hot water
senior_only	= 1 if TRUE, 0 if FALSE	only seniors live in household and it is a multifamily residence<
dw	= 1 if TRUE, 0 if FALSE	dishwasher present in home
cw	= 1 if TRUE, 0 if FALSE	clothes washer present in home
dw_use	=0.692^per+1.335^per gal/day	hot water use by household dishwasher
cw_use	=1.1688^per+4.7737^per gal/day	hot water use by household clothes washer (if use hot or warm water for clothes washing)

Daily Water Heater Energy Use

To estimate average daily hot water energy consumption in BTUs per day, use the following equation.

variable	units (or constants)	description
vol	gals/day	average daily hot water use.
RE		recovery efficiency of water heater
UA	BTU/hr-°F	standy heat loss coefficient of water heater (if not available see section below)
Pon	BTU/hr	rated input power of water heater
Ttank	°F	tank thermostat set point
Tinlet	°F	inlet water temperature
Tamb	°F	air temperature around water heater
dens	8.293752 lb/gal	density of water
Cp	1.000743 BTU/lb-°F	specific heat of water

Annual Water Heater Energy Use

To estimate average annual hot water energy consumption by type of fuel, use the following equations.

fuel type	equation (with units)	description
electricity	= 365 * Qin / 3412.76 kWh/yr	annual water heater electricity use
natural gas	= 365 * Qin / 100,000 therm/yr	annual water heater natural gas use
propane	= 365 * Qin / 91,500 gal/yr	annual water heater propane use
fuel oil	= 365 * Qin / 138,690 gal/yr	annual water heater fuel oil use
variable	units	description

Standby Heat Loss Coefficient

To calculate UA, if necessary, use the following equation.
UA has units of BTU/hr-°F. EF, RE and Pon come from water heater product data.
This calculation is based on the DOE Energy Factor test procedure for water heaters.

variable	units	description
EF =		Energy Factor
RE =		Recovery Efficiency
Pon =	BTU/hr	rated input power
Qout =	41093.7 BTU/day	Energy content of water drawn from water heater during 24 hour test.

Source:
1996 Lutz, J. D., Liu, X., McMahon, J. E., Dunham, C., & McGrue, Q. T. “Modeling Residential Hot Water Use Patterns” No. LBL-37805. Lawrence Berkeley National Laboratory. November 1996
About Hot Water Heating Module

Tags: design, SDHW, Solar Hot Water, solar thermal

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