NESHW system design
Overview
Our belief is that the more information we can disseminate about solar hot water, the more comfortable consumers will be with the technology. The technology is not new and evolving like photo voltaics, but rather it is proven and simple. However, the small size of the solar hot water industry and the 'boom and bust' cycle of the early 80's have created uncertainty. For this reason, a very large portion of our customers are engineers who have taken the time to understand the technology and are comfortable with the performance and reliability. These individuals have often researched solar hot water for months and even years before contacting us. For this reason we are happy to dispense some technical details (even at the risk of enabling competitors or new entrants)- the more information consumers have, the higher the comfort level, the better the industry, economy, and environment.
For a basic overview of solar hot water follow this link.
Control
All solar hot water systems rely on a differential control. A small processor compares the heat at the collector (T1) to the water temperature at the bottom of the storage tank (T2). If T1 exceeds T2 by some amount, then it is wort
hwhile to start the solar loop pump to begin heat transfer to the storage tank. When the 'delta T' value falls to a lower number, the pump stops. Typical values for the 'on' and 'off' differential values are 15 and 5 degrees. These values are usually programmable and larger values may be required if the distance from the collectors to the storage tank is very long. In this situation, the pump may start, but the large amount of cold glycol in the plumbing lines will 'confuse' the sensors causing the pumps to 'toggle' on and off. T3 is the temperature at the top of the tank, and is not usually used for pump starting and stopping decisions. T3 is sometimes used as a secondary parameter for starting the pumps, and is often used for reading the maximum storage tank temperature for high limiting the system or closing a separate relay for a secondary heat source or a heat dump.
System Size
System sizing is inexact, and our philosophy is that when in doubt, size down. In this way, the system is as close to 100% utilized as possible and the best return on investment is obtained since expensive, excess capacity is not sitting idle. One can find 3,4 and even 5 flat plate collectors on a raised ranch home. This is almost always unnecessary and is a result of the huge tax incentives from the 80's. A 70% tax credit meant that installers could sell very over sized and expensive systems and the purchaser's 'out of 'pocket cost would still be small. This mind set prevails today unfortunately.
Generally, an occupant of a home uses 20 gallons of hot water a day. For 4 or more occupants, this per person allocation decreases as 6 people do not require twice as many dishwasher and laundry loads as 3 people, and someone is usually waiting to shower so the shower usage is less etc. So, 6 people might use 80 gallons of hot water a day. From the usage, we size the solar storage tank, and here standard tank sizing comes into play. Tanks generally come in 40, 50, 80, 100 and 119 gallon sizes. So for 4 to about 6 people we use an 80 gallon tank. From the storage size, we calculate our collector size. Domestic hot water storage can use from 1.2 to 2 times the collector area in gallons of storage. An example is a 30 tube collector or 2 flat panels being mated to an 80 gallon tank.
Collector Orientation and Shade
The broad range of ratios for collector area to storage volume is a result of the need to compensate for shade and collector ‘compass’ alignment (azimuth). For example, significant shading will result in a design with more collector area as will a purely Eastern or Western facing roof pitch. Perfect, unshaded, southern exposure at the collector may dictate a 30 tube/ 80 gallon system while 60% solar exposure will result in a 40 tube/ 80 gallon system. As a practical matter, systems are viable down to 55 or 60% of the available sun. Shaded systems such as these will often perform surprisingly well in the winter as the trees are bare. With the purely eastern or western facing collectors, one cannot oversize as much as expected. This is due to the fact that the longer summer exposure duration means that the system will operate at nearly 90% of its ‘pure south’ capacity in the summer. So the collector area can only be moderately oversized to prevent overheating in the summer, while its winter performance will be less than an equivalent ‘southern’ system. The very large DHW system pictured above is a ‘pure’ west system and is 60 tubes and 100 gallon of storage. If it were a south facing roof, it would have been only slightly bigger at 119 gallons.
Shade and orientation can be measured in a number of ways. To the right is a sample NESHW ‘Solar Pathfinder’ shade report that tells us what proportion of the available sun is available at different times of the year. We will often use the Solar Pathfinder at many locations on a roof to decide where to site the collectors. Often, factors such as an annual summer trip or kids away for college during much of the year will affect the collector location decision as the monthly solar exposure will vary considerably as a function of the time of year at a given home.
Basic solar hot water system design
Most solar hot water systems are ‘preheater’ designs. This means that the solar heated water is fed from a solar storage tank to a conventional (usually pre existing) hot water heater. The cold street or well water is fed to the ‘cold in’ on the solar storage tank. The ‘hot out’ goes to the ‘cold in’ of the conventional hot water heater. When there is no solar heated water, cold water flows from the solar tank and the back-up HWH ‘calls for heat’ just as though solar had not been installed. This design means that a household is usually using ‘yesterday’s solar hot water’. There are also ‘single’ tank designs that have a coil type heat exchanger in the bottom of the tank and either a boiler loop coil or an electric element in the top. While saving space, these expensive tanks have some inherent limitations;
The back-up hot water supply is essentially limited to the volume of the top half of the tank that is heated by the boiler or with an element. During periods of no solar heat gain, this is the only hot water available.
A severe efficiency problem is created as described in the following scenario;
- Heavy hot water usage in the morning will deplete the tank’s hot water which is then replaced by cold water from the street.
- The tank thermostat will then ‘call for heat’ from the boiler and the top half of the tank is then quickly heated by the boiler without any solar input.
- The sun comes out a few hours later but much less thermal energy is collected since most of the tank is already hot.
NESHW system designs
‘Coil in tank’ heat exchange
Most residential solar hot water systems are ‘closed loop glycol’. Ninety percent of these systems use a ‘coil in the tank’ style heat exchanger to extract heat from the solar heated glycol and heat the potable water in the tank. This type of heat exchanger relies on the large surface area and simple convection. The glycol warms the metal of the coil, the potable water that is in immediate contact with the coil is warmed and slowly rises away from the heat exchanger and towards the top of the tank. This simple mechanism can be considered the ‘brute force’ technique of heat exchange- simple and effective, but not as efficient other techniques. NESHW has installed many of these systems and one is pictured here. It’s main features are;
- The excellent Stiebel Eltron glass lined ‘indirect style’ solar storage tank
- The ‘Flowstar’ pump station with a Wilo AC pump
External, flat plate heat exchanger
A flat plate system works on a different principle than a coil in a tank. A flat plate heat exchanger is a small component comprised of flat metal plates brazed together at their edges to form many very thin chambers through which fluid flows. The chambers are assembled such that every other chamber is plumbed together so alternating layers of glycol and potable water can be pumped past each other, separated by the thin metal walls of the chambers.
In the photo to the right, the four ports are for glycol in and out and potable water in and out. This heat exchanger creates ‘active counter flow’ of the two fluids. This means that rather than relying on convection to slowly move the heated potable water away from the surface of the heat exchange coil, the fluids are actively pumped past each other in a turbulent manner. This creates much more efficient heat transfer and a more efficient solar hot water system. More BTU’s are gathered and the storage tank will be significantly hotter than with a ‘coil in tank’ design using exactly the same solar collector. Besides greater efficiency at the actual heat exchanger, the flat plate heat exchanger allows the entire system to have the following benefits;
- There is less total glycol in the system since there is not 50’ or more of tubing in a coil heat exchanger that needs to be filled. Before your solar hot water system can heat your potable water, the mass of the glycol must first be heated- less glycol, more BTU’s delivered to your storage tank.
- Since the system’s heat exchange happens outside the tank, the solar storage tank is now just for storage and has no coil. This means that one can often upgrade to a superior quality Marathon or Superstor tank for the same cost as a quality glass lined tank with a coil. If a glass tank is used, then this design makes it much easier to replace if it fails in 15 or 20 years. There are no glycol connections- just potable water in and out.
- If bad water chemistry calcifies your heat exchanger (scaling), the external heat exchanger is easily isolated and cleaned. If a ‘coil in the tank’ heat exchanger scales, you must remove and replace the entire tank.
Generally, this flat plate design is more difficult and expensive to install. It requires additional plumbing and components since there is an additional ‘potable’ circulating loop required to and from the storage tank to the flat plate collector. This is fundamentally a superior design though, and NESHW is still able to provide these systems for a lesser installed cost than competitors’ ‘in the tank’ heat exchanger based systems.
The flat plate design is common in large, commercial scale, solar hot water systems. It is now economical in residential scale systems because Purist Energy in Portland, Maine has scaled and packaged this technology in a single component. Along with the increased efficiency cited above, the system also uses DC pumps that last longer and use much less power. The bronze Laing pumps, use a magnet/ ceramic bearing design, and have no shaft and no shaft seals. They are rated for 50,000 hours which is about 30 years of solar duty. This is much, much longer than the AC ‘circulator’ type pumps used with almost all other designs. The entire system uses less than 50 watts when running.
Data logging and control software
The Purist unit allows serial port connection to your computer. A simple software application then allows you to view data about the operation of your NESHW system in real time. The sensor data is shown on a graph with a variable time scale where you can observe the effects of a passing cloud on the T1 sensor, or the effect of running a load of laundry on your T2 tank temperature. You can see how much heat your tank losses at night. The NESHW system based on the Purist pump station is the only system that allows you to monitor your system in this way. You will not have this functionality from any other system design unless you pay for very expensive ($1,000 plus) ‘stand alone’ data logging components and software. Below is a sample screen shot that shows a small 20 tube system operating on one of the coldest days of the year. NESHW can set this system up on your computer during the system installation.
