1.  Solar Hot Water Basics  

SHW industry it is around for quite a bit. First batch of Solar Water Heater was manufactured in California in 1890’s and they been sold at that time for 25$. From the very beginning it had been claimed that ROI (Return of Investment) was great, saving 9$ a year (from coal). In 1920’s the system had change hands and start to be manufactured in Florida. Improvements and modification have been brought; first thermo siphon system had leveraged the batch system and in 1950’s half of the residential in Miami, Florida had such a system installed on their roof. Carter era and the start of the Golf War have brought the SWH industry back. For the first time we witness a substantial push for subsidies (1970, 40% federal tax rebate). Once subsides have drained out the industry got almost busted.

Today, this type of industry it is in high demand. It is a much regulated industry and some good institutional laboratories are giving professional credibility: SRCC, FSEC, IREC and on the Canadian site RETT.
Solar water heating has applicability for living space heating; water pool heating and using solar water heating concentrated tracking systems for thermo power station empowering turbines and generators and replacing coal type of power stations. 

All SWH systems are based on the same types of principles: we have to collect the solar energy and transform that into thermal energy, we have to stock the thermal energy and to distribute the thermal energy into existing dwelling system.  

An open loop pressurised indirect solar hot water system.

 The hot solar agent it is collected at the top of the solar panels, at that time a controller activate the circuit flow and hot solar agent from the solar tank is transferred to collector; cold solar agent circulate making use of the pump. To benefit of this system for a longer period in the day an external boiler will condition the hot portable water temperature to the level required before it is send to taps. For excessive hot days because we can not afford to have temperature stagnations an expansion pressure vessel will relax the system. Such a system uses glycol as solar agent for the solar collector. Solar collector can be a pressurized evacuated tube collector. In this case the working temperatures can reach 200F.

Most of the time the SWH kits are sold with all the connection we need, plus a half inch outlet port for the dwelling system. PEX or copper will make the physical connection to the existing half inch in house distribution system. Personal we recommend PEX, first it is more environmental friendly, second it can be colour coded as red for hot water, blue for cold water and plain white for return, it is easier to maintain and offers lot more flexibility in the installation phase. The connection with the existing system can be made using a PEX type of manifold; in this way the transition to existing copper pipes is seamless. Clean energy brands offers PEX and PEX accessories and tools.

For the collector panel record to the system components we do not recommend the use of PEX even if it is an oxygen barrier type in case of evacuated heat tubes. Based on the technical specification of PEX; at 100psi the max service temperature is 170F whereas the solar panel in most of the cases exceeds this temperature working in vicinity of 200F. That will make the PEX to become soft and the connection sealing to permanently alter. In such a situation the use of copper is mandatory.

The wick point in our chain is the solar tank or the place where we stock the thermal energy. This is affected by: stand by losses, slow recovery times and low life expectancy.  Another important factor affecting our system efficiency is the microclimate condition. In some geographical areas we may need to install a solar water tank together with a second tank which in most of the cases is the home heating boiler system. The preconditioned solar heated water will reduce the local energy bill with 35% to 50% over a year time period.

As an alternative to SWH in areas with hot temperature year round, an integral heat pump water heater storage tank is an efficient solution. They are working based on pooling the heat from the warm environment and transferring as thermal energy to the water inside the tank.   In this way the preconditioned water is brought to the desired temperature by an electrical resistive heater located in integral boiler type. The general use for the electrical heater it is a short time period, reducing the electric energy consumption up to 50%. They last long, 20 years of service, working on a reverse Carrie cycle they cool the environment nearby the boiler, but they come with an initial cost investment. The heat pump water heater as an alternative to SWH is sometime more complex to install and in case we want to use it as recovery heat exchanger from an intensive thermo process like an AC unit they need an auxiliary duct to capture the heat and bring it up to the heat pump.

Another solution to consider is a tankless heater as on demand electrical water heater which can be connected to the solar tank and bring the temperature by demand up to 120F. In this way we reduce the stand by lost of a bigger household water tank which suppose to keep 120 gallons water at 120F

CleanEnergyBrands offers parallel to hot water solar panel kits by demand electrical water heaters and heat pump water heaters.

Contact us at tecsupport@cleanenergybrands.com

1.  Economics of a Solar Hot Water System

It is worth the dollars to install a solar hot water system? And what are the economical cycles we look at to asses the pay back in such a situation?

The practice bevies, that from the first day of functioning such a system is investment free. Not only based on a very low maintenance costs, some may estimate around 2$ a month, but through a 10% average increase in  conventional energy costs on a 20 years period it becomes a source of investment with a pay back ratio of 10% to 20% to the basic investment value. Additional we gain in equity house investment and that by raising the house energetic potential and becoming a lower energy consumer registered with the utility company at that address.

A SWH system has a cycle of 4 to 5 years recovery costs. After that period of time its operational costs per year are levelled to zero.

Even in a scenario where the conventional energy costs are becoming insignificant such a system retains its value, making the house hold energetic free and reducing the carbon footprint in the microclimate where it is located.

In general solar hot water systems costs are varying depending of the geographical area where are installed. We have to take in consideration that in cold areas SWH systems are more complex and they are more labour intensive. In this cases most of the time a SHW System installation has to be performed from a licence installer. In cold area such systems are more in demand almost heavy duty type compared to the Sunbelt states, where very lean systems, almost inexpensive can be fixed as a do it yourself type of job. Before we undergo any type of solar heating or cooling, which in turn are the harder on the invested dollar to recover, negawatt measures (energy conservation measures) have to be considered; insulation, waterproofing, thermo windows. These are contributing to a reduction up to 2 cents per kWh in energy costs.

1.  Type of Solar Hot Water Systems

Solar Hot Water systems are either active type of systems with an open loop or a close loop, where the water pump moves the thermal energy towards the dwelling;
 or passive type where based on gravity and water density the thermo energy transfer it is made possible.

The passive type is the most known type and in the same type the cheapest once, it can be found in tropical and mild type of climate. Its schematic is quite simplistic and is very easy to install and maintain.

The Batch or ICS (Integrated Collector System) are around for more than 1oo years. In such a system the collector and the solar storage tank are in the same box located on the roof.

The agent it is portable water. This constructive type placed a considerable load on the roof, over 500 lbs when is full.
that makes the system on the roof very stable.

It is not the best solution for areas with hurricanes, mostly the coastal areas. For that type of places a thermosiphon system it is more adequate. Other places to be used are recreational facilities during the summer time and cottage industry where they are used only on a temp base. The way they are manufactured make them total inappropriate to cold type of climate. They will freeze at temperature of 29F. Adding glycol and using the batch collector in a heat transfer system will deteriorate the copper inside the tank. This type of solar system can be used together with PEX.
In such a system the collector is mounted south in the northern hemisphere. The best operating time is between 12 am and 8 pm.

 The schematic is very simple. In most cases we have a solar storage tank as a back up tank; the back up water storage tank has to be at least equal to the collector capacity. It is usually around 40 gallons; we assure that the batch collector can be totally drained for periods we do not have hot water consumption, avoiding overheating and scalding. In the same time in areas where ambient temperature becomes lower as the ground water temperature during the night time or for couple of good month in a year, we have to drain the batch collector and avoid freezing and permanent deteriorations.  A set of isolation ball valves are used to drain the collector and a tampering valve it is used to mix hot and cold before the water is pushed to collector. In this way we precondition the water to reach a better temperature value during cloudy days.

Most recent ICS kits will come equipped with antifreeze valve and a pressure relief valve. In this way we extend the operational limits of this type of system and we offer a protection against scalding or freezing.

Thermosiphon Systems

It is by far the most spread systems and very common at tropics and hot geographical microclimate. They work as a passive system in a direct or indirect mode. 

Comparing to the ICS, the thermosiphon system takes apart the collector from the solar storage tank. That improves the collector efficiency; the solar irradiance do not apply to a massive body of water instead will transfer thermo energy faster within collector. Another big advantage is the heated water it is well isolated into the solar water tank above the collector having a much smaller heat transfer comparing to ICS. Another important gain is a more reduce amount of copper used and a way lighter footprint on the roof. This type of system in most of the cases works using antifreeze valves placed at the lower and high end of the collector. The Dole drip valves FP 45 starts to drip at 44F. We can use such a system in a mild microclimate with the same schematic of an ICS. The collector becomes empty for the cold night and the water tank keeps the freezing point out. A vent valve placed on top at the tank outlet port will clear air blockage inside the collector. We have to replicate the vent valve similar on the inlet cold water system before collector.

A better way to deal with temperate type of climate is to use indirect thermosiphon systems. That helps by improving the collector response to freezing temperatures. The solar fluid is a 50/50 propylene glycol water mixture. No freeze protection is provided for the collector (we have to conserve the mixture) in the same time the vent valve on the collector inlet port is no more necessary. A solar tank with good thermo isolation will keep the system safe from over chill at an external temperature of 38F. During the day we can reach a functional service temperature for the potable hot water between 12 am and 8 pm. Same principles we apply for extensive cold period of time or for protection against scalding:
we drain the solar tank into the back up tank. We definitively use oxide barrier PEX, temperature out of the collector do reach values exceeding 170F, PEX max service temperature (potable hot water comfort temperature is 135F).

Active Direct Systems

All active systems are running based on active parts in actions, a pump or circulator to transfer water from the storage tank to collector. In the same time we have a system to regulate the pump a controller. Always a check valve will prevent collector thermosiphoning. In this way during the night time the hot water from the storage tank it is prevented to ascend to the collector and be cooled down. If that happens we loose thermo energy and or in the same time we may jeopardise the antifreeze protection.         

Active Open Loop Systems

  We have two types of active open loop systems: automatic drain down and manual drain down. In both cases it is recommended they are installed in areas with 0.8 freezes in a year time, that it will translate to areas where temperature very rare reaches 48F at the lower range
In order to prevent freezing and scaling collector will never be mounted in landscape mode usually in portrait mode with a 20 tilt. A problem with the check valve, it has to be properly seated in case of spring loaded check valve otherwise the thermosiphoning will affect the proper functioning of the freeze valve and controller sensors.   This type of system requires periodic decaling because of poor quality of water in the coastal areas. Therefore potable water has to be filtered and conditioned before enters the collector.

For decaling it is recommended a solution based on muriatic acid.
In such a system we can use an existed electrical tank and retrofit this to become an open loop solar tank. If we consider the standard 80 gallons electric boiler present in most of the households and enough to supply potable hot water to a average family, we have to do few adjustments: feed in the cold water using a tee into existing drain, disconnect the actual electrical element and fetch a new one at 2/3 form the tank capacity. The collector feed has to be lowered 10 inches bellow the active electrical element and that using the existing cold feed in pipe. The hot out it has to be totally sealed. Opposite to this is a new solar tank of 80 or 120 gallons available on the market.

In both cases the differential controller reads information form the bottom of the solar tank and near the hot water port of the solar collector. The controllers, depending of the manufacture type, have an on/off range between 8F to 24F which in turn can be adjusted. That it means the pump is on when the differtial temp tank collector is around 24F and is off when the differential temperature is around 8F. All the differential controllers have a selectable minimum starting temp and a cut off temp. The minimum starting temp is between 80F to 90F and the cut off is between 110F and 180F. The upper limits it is used for systems with glycol fluids when we do not afford to have temperature stagnation thus will ruin the pressure relief valves and the air vents. Most of the differential controllers have a setting for antifreeze whereas at 44F collector sensor temperature the pump starts to work and the motorized check valves opens. That is an automatic antifreeze drain back. Before the automatic antifreeze drain back to starts to fire the freeze valves will start dripping at 48F as first in line to protect the system. As a freeze protection measure a vacuum breaker valve has to be mounted near the hot water port of the collector. In this way when we drain the system they will open removing potential vacuum from the collector. That will help to reduce temperature stagnation on the supply column after the system is rolled back on. In case of a manual drain back type of system a vacuum breaker valve is mandatory.

The turn on set temperature for controller has to be at 80F at least. In addition a timer will power the controller when usually the level of irradiation it is enough to generate hot portable water.

A time interval range between 9 am and 6 pm or between 8 am and 5 pm will help keep the system generating income. A much better solution is to use a 10W solar panel powering a 10W low flow DC pump. In this way the pump will start running when the level of irradiation is optimal to heat the water inside the collector.

Drain Back System

Drain back system it is the most used system in cold climate, almost freeze free, and very suitable for poor quality portable water supply. A drain back system produces 10% more BTU than an anti freeze closed loop system with identical collector area.
In order to improve anti freeze performance the collector HTF (High Temperature Fluid) uses a mix of 25 to 33% propylene glycol in water. Downsize is the presence of a heat exchanger inside the system and the mix glycol water reduce system efficiency.

The way it works with a drain back system: when the differential temperature between the collector and the water tank is less as 20F the pump mounted on the inlet side of collector stops working and the collector will empty back into the drain back reservoir. Therefore no control valves are present comparing to a drain down anti thermosiphoning system. This is an unvented active closed loop system. The pump is the work horse and most of the time we have to use AC pumps with high head to build up pressure at start and push the HTF fluid out of the drain back reservoir into collector. The gain is the collector is empty and we do not need to wait until a cold water body equivalent to a gallon in case of one collector system is heated. The HTF fluid we push through the system it is already thermo conditioned. The speed of such a pump is a determinate factor for the system thermo efficiency. Speed it is measured in gpm or gallons. The system by itself it is not pressurized.   

 For installations where the pump on the collector side request a higher head is a good chance the pump will cavitate at start up. In order to avoid pump cavitation and burning a flow control ball valve on the return of the collector loop will help to build up pressure in system. Therefore in sight distance to this valve a 0 to 60 psi control pressure gauge it is a good instrument to have.   

For systems with one or two solar collectors it is recommended to use in the collector loop ¾” copper or PEX.  PEX is a very good solution based on the fact the working temperature will stays under the PEX service temperature (170F).

Coming back to the pump selection for such a system on the collector side we have to fix high head AC pumps with adequate flow. A variable speed pump is the best solution. As a good number for a single collector panel a speed of 1GPM with a head lost of 1,4 feet and 3GPM with a max head lost of 10,7 feet for a double collector panel or up to 128 sq feet collector area are reference values to follow and pretty much what we need for a residential type of SWH system using drain back systems.

The pump head is the second variable to look at. Usually we consider the static head as the height between the upper level of the collector and the fluid sit level in the drain back reservoir.

As rule of thumb the minimum physical distance is around 12 feet. The normal height is at 15 feet. This is the static lift for the pump at start up. When the static head is over 30 feet two pumps in series are mounted on the collector loop. The second pump kicks in with a delay between 30 to 300 seconds regulated from an interval delay timer relay. That it is translated in psi or system pressure. The pressure in the collector loop is generated by the pump work and has to add to 15 psi atmospheric pressure and 1 psi for each 2 feet of static lift. As a rule of thumb a minimum 30 psi in system can be consider a good number and can reach up to 65 psi for two collector panels. A psi gauge meter has to be mounted on the pump side of the collector loop on top of the flow meter.

The pump speed can be adjusted from the ball valve on the collector return mounted before the drain back reservoir and it can be read from a flow meter (.5 to 8.0 GMP) mounted on top of the pressure gauge. A higher speed reduce the thermo exchange efficiency and the system becomes excessive noisy.

For the collector loop we look for AC pumps in range of 140W to 215W with a low speed up to 8 GPM and working pressure up tp 130 psi (double the max psi required in order to extend the pump longevity) 

   In some cases for pipe runs to the solar water tank from the drain back reservoir we may need to mount a pump on that loop. That pump has to be low flow and work a bit over the atmospheric pressure not to destratificate the water tank. A 1/100 hp or smaller AC pump mounted to the same differential controller it is required.

The drain back reservoir it has to be 10 gallons for one single panel collector and 15 gallons for a two panel collector that including average pump runs fill up fluid.

 Very important when this system it is mounted are rules to obey in order to achieve the drain back gravity based effect. Panel have to be vertical positioned with a slop of 20 degrees and pipes have to have a slop of couple of inches from the upper level of the panel to the reservoir in order to guaranty the drain back happens. The piping system has to be tied every 4 inches to prevent sagging.

Close Loop Freeze - Protection Systems

1.  Collector Panels used in Solar Water Heating (SWH) systems

ICS type of Solar Collector Panel

The CopperHeart is a product of SunEarth and is a batch collector designed to meet the international demand for a simple, durable and inexpensive domestic water heating system.  The CopperHeart typically serves as a solar preheater to an existing electric or gas water heater, but may be used as the primary water heater in certain climates.  ICS units are excellent choices for residential new construction in the U.S. Sunbelt or other mild climates that do not experience hard freeze conditions.

Components of such a system are a top protection based on Low Iron Tempered Glass with absorber coating, Seamless Copper Storage Cylinders mounted in an Extruded Anodized Aluminium Casing and Capstrip with stainless steel fasteners having the back on the box an aluminium backsheet. The copper cylinders are isolated with hard foam 1 ½ inches thick to the box and the glassing has two layers of sealing a Primary EPDM Glazing Seal and a Secondary Silicone Glazing Seal. Glassing is 1/8 inch thickness, and has a minimum transmissivity of 91%.

Filled with portable water is weight goes close 600 lbs for a 40 gallons capacity, even empty is quite heavy, dry weight for the same 40 gallons is 264 lbs. it is easy to install but to bring on the roof it needs more work force as a single installer can handle. The price of such a unit is in range of USD 3600 $.

Packaged Thermosiphon System

The collector by itself it is placed into an Extruded Anodized Aluminium Casing and Capstrip with an Aluminium Backsheet. The active elements are made from copper absorber plates soldered to the ¾ copper fluid carrier pipes and are corrugated. Two manifolds are closing the internal fluid loop and are made from copper too. Low iron pre temperate glass with absorber coating in order to gain more as 91% absorption ratio it is used at the top. An 80 gallons capacity Marine Grade Stainless Steel Tank it is placed at the top and connected over a heat exchange chamber to the solar collector system. Extra thick foam and a heat exchange metal jacket are enclosing the tank and give the final look for that product.

This system comes with a tank capacity of 48, 80 and 116 gallons on the market and different panels sq feet size. 65 sq feet collector and 116 gallons tank are the choice for a household bigger as a 4 members. The price tag of such a system is USD 4000.

High Efficiency Evacuated Tube Collectors

The tubes are mounted, with the condenser bulbs up, into a heat exchanger (manifold). The manifold is a shaped copper pipe that wraps around both sides of each condenser bulb. Potable water from the circulation loop flows through the manifold and picks up heat from the condenser bulbs.

The maximum operating temperature of the heat pipe is the critical temperature of the dual-phase fluid, since no evaporation or condensation above the critical temperature is possible. The heat pipe also provides the system with a thermal diode function, so that when the sun is not shining, heat loss from the potable water is kept to a minimum. This occurs because heat is lost only from the header, not from the absorber surface of the array. The header is insulated with polyurethane foam. Within each condenser bulb, the maximum working temperature is controlled by means of memory-metal snap discs to a level below the critical temperature.

The memory metal is programmed to change its shape at a preset temperature. This allows the condenser fluid to be retained inside the condenser. When the programmed temperature is reached, the memory-metal spring expands and pushes a plug against the neck of the heat pipe, blocking the return of the condensed fluid and stopping latent heat transfer. At temperatures below the maximum programmed limit, the spring contracts, allowing the condensed fluid to return to the lower section of the heat pipe. The solar heat from the absorber plate then causes the condensate to evaporate, transferring thermal energy to the condenser. The flexible neck system absorbs both thermal and mechanical shocks. Such a system it is a very efficient system offering very hot portable water even in cloudy days. It is not so efficient where we have lot of snow because tubes are relative cold at exterior, we can not scrap the ice down and snow does not slide easy.

 As price tag they are around $1300 USD for a 30 tube collector and they require additional expensive components to match the working temperature. Copper is the single way to connect such a system to the solar water tank. As pay back to investment the average daily temperature using such a system is 120F. We reach very easy on a cloudy day 200F.

1. Installing and Commissioning a Solar Hot Water System Basics

Plumbing permits and possibly electrical permits are required for all solar thermal hot water installations. If structural work is needed to support the system, a building permit will be needed as well. For all projects requiring Design Review we need to provide sufficient information about the solar thermal installation to evaluate the proposed building design and eventual alteration.

Permitting documents

 When we perform a commissioning check on a new system first we have to check that all
 the permitting documents are present and signed. We have to check against the component   list that the system document folder is complete and all the pluming diagrams and the  system elevations are present and signed.

design

Check that the design correspond to the lifestyle of the home owner eventual through a visual inspection of the main consumer components.

Visual inspection

Visual inspection will confirm that the system it is mounted conform present documents.

Pressure system testing

For that in most of the cases a pressure kit is necessary to perform this test. The pressure has to be raised inside the system at a value corresponding to maximum value the system can support plus the safety factor.

Systems flush check and valves check

 Test pumps and control elements

 Cleaning and temperature and pressure probe checks

Related links form our Knowledge Base resources

Buying a Solar Hot Water system

BOS components for Hot Water Systems.

           Pool hot water systems