Shortage of Solar Hot Water Collectors
21 Aug 08 | General Business, Solar Hot WaterThis was almost inevitable. It seems that there is a growing shortage of solar hot water collectors. One manufacture that I spoke to is having difficulty getting glass for its larger sized collectors. Others are struggling with higher product demand and fixed manufacturing assets. Solar thermal manufactures may be leery of making large investments in facilities because congress still has not approved the renewable energy tax incentives past December 31, 2008 (which is fast approaching).
On the surface, this would seem to be a good thing. The solar business is growing, more and more people are aware of solar, not just Photovoltaics, but solar hot water too. More and more people want these systems installed on their homes and businesses to off set energy use and save money. Those are the positive aspects.
However for a solar installer, it is difficult to get business if you cannot give the potential customer an installation schedule. I am right now, waiting on several collectors to show up so I can finish two jobs. I am also leary of Congress and the lack of progress on the renewable energy tax credits. As I have said before, unless they pass, a great majority of home owners will not be able to afford solar thermal systems. I do not want to take on a large inventory of flat plate collectors that I will not be able to sell in six months.
And so we wait.
I curse incentives and subsides. Too much tinkering around with the market forces if you ask me.
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Tags: Solar Hot Water, solar thermal
How long do Solar Systems last?
26 Jul 08 | Solar Electric, Solar Hot WaterThat is a very common question. The answer is, it depends. Solar systems, like all other mechanical systems require some maintenance. Last week, I came across a solar hot water drain back system that was 28 years old. The great thing is, it was still working just like the day it was installed. The only problem the home owner had encountered was a bad circulator pump, which the plumber replaced.
Properly installed drain back solar hot water systems using distilled water could, in theory, last almost indefinitely. Solar Hot water systems that use antifreeze will likely last only 30 years or so. Still, that is a great payback. For either system, over the course of its operational life, it should easily pay for it self 4 to 5 times over.
Photovoltaics are said to last 25 years however, their output slowly declines over time. After 25 years, most current photovoltaic panels will be producing about 80% of their rated power. Still, that is not bad, and a well designed photovoltaic system should pay for itself at least two to three times over its operational life (with current incentives).
The advantages of renewable energy systems, for those that are in it for the long haul, are:
- Stabilizes energy prices at or below their current levels, gives the property owner more control over expenses.
- Increases the property value of the residence or building they are installed on.
- Reduces emissions and environmental pollutants from nearby electrical plants.
- Reduces overall electrical load on grid, thus reducing the need for more power plants and high tension distribution lines.
- Spreads out electrical generation capacity, thus making it more difficult for any one catastrophic event to cause a regional blackout (distributed generation).
- Reduces the use of fossil fuels and thus dependence on other countries to provide energy for us.
As you can see, there are many advantages to a solar thermal, photovoltaic, wind, or microhydro system
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Tags: distributed generation, Environment, photovoltaics, Solar Hot Water
The price of Solar Energy
16 Apr 08 | Sales, Solar Hot WaterThe prices of home energy in New York State, week of April 14, 2008:
- Heating oil: $3.974/gallon
- Propane: $2.953/gallon
- Natural Gas: $1.413/Therm
- Electricity: $0.175/kWh
Source: NYSERDA
The New York State Energy Research and Development Authority (NYSERDA) tracks energy prices with in the state and posts them to the Energy Prices and Supplies page of their website.
This is a great resource for the renewable energy dealer/installer as it allows you to make direct comparisons to the cost of solar energy. The logical way to do this would be to divide the systems projected lifetime production by the net capital cost.
Example:
A solar hot water system is purchased in Ulster County, NY. It consists of flat plate collectors totaling 80 Ft2, a tank and some miscellaneous pumps, piping, valves etc. The expected lifetime of that system is 30 years ±5 years.
A check of the NREL insolation maps indicates that Ulster County receives 4.4 kWh/M2 per day. Convert 80 Ft2 to M2 (80 Ft2 x 0.09290304=7.432243 M2) The above solar array should expect to produce 4.4 kWh/M2 x 7.432 M2 = 32.701 KWh per day, less efficiency and losses, which total around 60%. Therefore, the above solar system should be expected to produce 32.701 kWh x 0.40 = 13.08 kWh per day, without shading.
Yearly that adds up to 13.08 kWh x 365 days = 4774 kWh/year. 4774 kWh x $0.175 = $835.51 per year. Thus, this home owner can expect to save $835.51 per year in electricity costs.
The net system cost is the installed cost minus the tax incentives, or $7,500.00 - ($7,500 x 0.3 (≤$2,000)) - ($7,500 x 0.25 (≤$5,000)) = $7,500 - $2,000 - $1,875 = $3,625.00
If this system were paid for with 60 month loan:
Principal amount: $3,625.00 Payment amount: $71.61 Interest rate: 6.900% Interest compounding: Monthly Total payments: $4,296.51 Total interest: $671.51 Total system cost $4,296.51
Simple System payback $4,296.51 ÷ 835.51 = 5.1 years.
Simple system savings, without utility rate increases, 25 years x $835.51 = $20,887.75 (this is likely low by about $5K).
Therefore, the cost of heating your water with solar is the amount of energy saved, multiplied by the life of the system, divided by the cost of the system, or: 4774 kWh x 25 years = 119,350 kWh. $4296.51÷119,350 kWh = $0.036/kWh.
The cost of solar hot water vs electric hot water is
3.6 cents vs 17.5 cents per kWh.
The average rate of CO2 emissions during electricity production is 1.34 pounds/kWh (source, US Department of Energy). Therefore, the above system will save 119,350 kWh x 1.34 = 159,929 pounds of CO2 emissions, or about 80 tons.
To give you an idea of how much that is, it equates to about
- 130,000 vehicle (average car) miles
- 101,000 vehicle (large SUV) miles
- 347,000 train (AMTRAK, light rail or subway) miles
- 266,000 air miles
- 61,500 tractor trailer (heavy truck) miles (loaded)
- burning 7,175 gallons on heating oil
- burning 40 tons of coal
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Tags: marketing, sales model, Solar Hot Water
Formulas for Solar Hot Water Systems
09 Apr 08 | Solar Hot WaterSomething 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:
- Allow 20 gallons per day of use for the first two people
- 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)= Vgal x 8.345 x (Texpected - Tin) x eff
Where:
Vgal is the volume of water in gallons
Texpected is the expected hot water temperature
Tin 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 (Texpected - Tin). 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:
- 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
- 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
- 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
- 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:
- The efficiency of the collector absorber plate coating
- The efficiency of the Heat Transfer Fluid (HTF)
- The incident angle of the sun on the surface of the collector in both the horizontal and vertical axis
- The ambient temperature of the collector
- 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.
Related posts
Tags: SDHW, Solar Hot Water
Solar Thermal Systems
21 Feb 08 | GeneralWhen 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.