NEW GENERATION SOLAR HYBRID HOT WATER SOLUTIONS Ranjith M MEP Consultant based in Kochi. He is also NEC Member, IPA Kochi Chapter. Integrated Building Consultants 54/2477A,Subash Chandrabose Road,Kadavanthara P.O, Kochi-682020 P: 0484-4010774, 2205774 M; +919895070797 E: ibcconsultants@rediffmail.com Conventional methods for generating hot water usually require direct or indirect burning of fossil fuels, example oil or gas, for the generation of electricity to heat water. These methods not only have high operating costs for the consumer but also result in the emission of harmful pollutants in the environment. The use of free solar energy to generate hot water via solar collectors can offset these energy requirements, with conventional solar systems being able to provide typical energy savings or a Solar Contribution Factor SCF of 50% to 80% of daily water heating requirements. The remaining energy requirement, especially on days with poor solar contribution due to cloudy weather etc., is normally provided from traditional fuel burning methods or electric resistive heating elements. A Heat Pump is an efficient device that moves heat from one location called the 'source' at a lower temperature to another location called the 'sink' at a higher temperature. This process is commonly seen with reverse-cycle airconditioners. Heat pump technology can be used as a very efficient method of generating hot water. Energy in the form of ambient heat is first extracted from the surrounding air and is used to convert the refrigerant within the sealed system into a gas. The gas is then compressed to generate even more heat, which is used to heat the water in the tank. Temperatures of up to 60 C are readily achievable. In ambient temperatures of 20 C and above, heat pumps typically have a Coefficient of Performance (COP) of 4-4.5 or higher; which means that for every 1 kw of input electrical energy, 4-4.5 kw of heating to the water is obtained. This means 3-3.5 kw is gained free which is from the ambient air. The COP of conventional electric water heaters is 1, which means that only 1 kw of heating is obtained from every 1 kw of input electrical energy. Summary: By combining Heat Pump technology in conjunction with solar collectors (a hybrid solution) some powerful synergies can be gained. Example: Let s consider an example, a typical scenario of a customer with a daily water heating requirement of 10,000 litres. To heat this volume of water from 20 o C to 60 o C requires approximately 480 kwh of energy per day. If we heat this volume of water with conventional electric resistive heating (COP of 1), we would require on average 480 kw of electrical energy input per day. 14
If we were to utilize solar energy instead, with enough solar collectors installed to provide an average of 75% solar contribution factor, with the remaining energy requirement being supplied from electric resistive heating, we would require on average approx. 120 kwh of electrical energy input per day. Alternatively, if we were to halve the quantity of solar collectors installed, such that a solar contribution of 37.5% was now expected, and utilize a heat pump for the remaining energy requirement of 62.5%, as the heat pump has a typical COP of 4, the required electrical input would be 62.5% / 4 = 15.6%, which equates in our example to only 75 kwh of electrical energy per day. The heat pump is providing approximately 46.9% (225 kwh) of the heating requirement for free (62.5% - 15.6%), when combined with the 37.5% of free energy from the sun, means that 84.4% (405 kwh) of our average daily water heating energy is provided at no cost! Hence the energy savings for the hybrid solution are much greater than for either a conventional solar solution or a solution based on the installation of heat pumps alone. There is a lot of talk of going green, saving energy, green buildings, solar energy, heat pumps and solar hybrid systems. It has to be noted that one has to be careful and also clear in choosing a technology for a particular requirement on hand. This article will guide one to choose the right technology and system that will perform effectively and also has a low life cycle cost. The hot water system should be adequately protected from both internal and external factors that affect the durability, reliability and performance directly and most importantly the safety. One of the critical factors to consider before a purchase of a system is whether to go with direct heating technology or indirect heating technology. In case of direct heating, one allows water to flow through the collectors or any other heat exchanger such as the PHE or the coaxial heat exchanger inside the heat pump to exchange heat. This can be done provided we have very good water quality. What we mean by this is to use water with very low impurities. The impurities in water are of two types - suspended and dissolved. The impurity that affects system performance is the dissolved impurities as in water with high TDS and hardness. This is the quality of water that is generally available for use. Water quality is generally not good in many parts of the country. The hardness in water causes scaling even on a very smooth surface such as glass and at ambient temperatures. March 2018 / 15
Scaling is highest at higher temperatures. One can imagine the kind of scaling that can happen inside the collectors and also in the heat exchanger inside the heat pump if such water is allowed to flow or used. Scaling causes reduction in efficiency, performance, increase in running cost and failure of the collectors and heat exchanger inside the heat pump. There is also high maintenance cost and down time involved in descaling the system. The descaling is done with acid that corrodes the material and in some time the heat exchanger as well as the collector fails. Indirect type of heating does not allow the water to pass through the solar collectors or through the heat exchanger inside the heat pump. Usually a close circuit liquid that is anticorrosive and anti-freeze in nature is used to transfer the heat thus eliminating the complications as explained above resulting in effective performance of the system with high efficiency, durability and reliability with a very long life, no or low maintenance and very low life cycle cost. Summary: An indirect type of heating system is better, more suitable for the conditions prevailing; performance is better, lower maintenance costs and has a long life and hence has a very low life cycle cost. Once the decision is taken about whether the system has to be direct or indirect then the components that go into the system like storage tanks, collectors, heat pumps, insulation, etc. have to be chosen Storage Tank: One of the most common problems with conventional hot water system is the storage tank. It is quite common that most of the local and ordinary hot water systems use stainless steel to fabricate the hot water tank. It is also the most common part to fail or needs constant repair and maintenance due to leakage due to corrosion. Corrosion is a chemical change that occurs on the surface of the metal and rusting is the most common example. Few metals escape corrosion in water regardless of their quality including stainless steel. Gold or platinum do not corrode but their cost and properties make them unsuitable for storage water heaters. The damage to a stainless steel tank is even greater and more rapid should it be pressurized. Storage tanks manufactured Corrosion begins its and tested under DIN 4753 Standards. destructive process as a result of small flaws on metal surfaces, at weld points or from stress. A common assumption is that water is not corrosive. This is not the case and some water supplies are more corrosive than others. Hot water storage systems are not immune from corrosion and the most effective way of dealing with the problem is to fuse the inside of a strong steel storage tank with a ceramic lining to isolate the tank structure from potential corrosion. Thus ceramic lined or enamelled tanks are best suited for hot water storage. What is enamel coating process? The enamel coating on a metal is basically a glass like material. Enamel is fused with steel due to an electro chemical reaction. The coating acts as an electric insulator; it completely avoids galvanic corrosion and also results in unity, smooth and anti-adhesive surface. The enamel coating provides maximum protection for the base metal. The coating does not transmit any odour to water and it does not affect any flavour. Enamel does not give out any toxic or allergic substances. The enamel coating is available in two colours blue and white, but both having same properties. The physical strength of enamel is determined by the compression strength that it can handle. Similarly the enamel is chemically acid resistant, alkali resistant, resistant to attack from detergents, hot water, steam and steam condensate. Unique features: The enamel has a symptomatic behaviour which means that the corrosion decreases as the enamel coating become older. It is absolutely hygienic since its closed surface does not allow bacteria to accumulate Enamel coating does not allow any solid, liquid and gas to react or come into contact with the steel base 16
Glass lined steel is mandatory wherever the service conditions of the process is particularly difficult Why is enamel coating so reliable? The enamel composition is inorganic in nature; there is a chemical bond between the enamel layer and the metal surface after the fusion of the glassy substance at over 800 C in the oven during the baking process. The steel contributes to the mechanical strength, while glass enamel guarantees the chemical resistance with rust protection because it does not absorb water or conduct ions. The enamel provides the steel tank with 99.9% rustproof protection, as there may be the occasional uncoated points though this happens very rarely. The remaining 0.1% is guaranteed by inserting magnesium sacrificial anodes. Once these have been installed inside the steel tank, they control the action of the agents that may otherwise eventually cause rusting. Protection against corrosion: It is wise and advisable to take corrosion protection even further by the installation of a simple replaceable component that takes the brunt of any corrosive potential the water may have. By using a bare metal component (known as an anode ) which is a more readily corrodible metal, it protects the tank. This approach to corrosion protection is employed all over the world where metal objects are immersed in water. Literally billions of dollars are invested in ships and oil rigs worldwide - they are all protected by replaceable anodes which withstand the harshest of climates and water conditions. Over time, the anode dissolves, releasing a completely harmless magnesium salt into the water. The life of the anode varies depending on the quality of the water typically between 5 and 15 years. Bare steel is never exposed to water in the system. An added advantage with a protective anode is that it can be easily checked during regular services and replaced, as necessary, to ensure lasting protection of your tank and extend the life of your water heating system. Summary of Advantages of Enamelled Tanks with Anode protection: Protective coating: 3/10 mm (0.3mm) ceramic lining fused to steel at 850 C. Extent of pin-holes and crevices in protective coating: Pin-holes limited to 0.01% of surface area of coating. Design avoids crevices. Galvanic corrosion mechanism: Magnesium anode corrodes in preference to steel to protect the steel at pin-holes present in the enamel coating. Pit and crevice corrosion mechanism: Magnesium anode releases ions that attract chloride (Cl) ions, dissolved magnesium chloride (MgCl 2 ) gets flushed away. Stress corrosion mechanism and fatigue corrosion mechanism: Thick steel tank walls (2.0 to 2.5mm) are lightly stressed and less susceptible to fatigue. Mechanical fatigue: All pressurized water heater storage tanks are subject to pressure cycles that lead to metal fatigue of the tank walls. If corrosion failure does not occur first, fatigue failure will eventuate. Thick wall sections are less susceptible to fatigue failure than thin sections. Flange for mounting element: The storage tank should have a suitable flange for mounting the auxiliary heating element of the required capacity around the centre of the tank. The auxiliary boost should heat only top half of the tank. Flange mounting at bottom or top: A larger flange needs to be provided at the bottom of the tank thereby periodic cleaning and maintenance work can be carried out. The anode tends to corrode and settle as fine powder at the bottom of the tank which should be periodically cleaned. Temperature and pressure relief valves: Close circuit indirect systems are preferred to open vented direct systems as explained earlier. The close circuit and the storage tank are well protected with the installation of temperature and pressure (T&P) relief valves. In case of overheating or excess pressure, the valve opens automatically and releases the excess pressure or dumps water and reduces the excess temperature. These valves have to be of the specifications required for both temperature and pressure and have to be calibrated. Valves of reputed makes only should be used. Temperature gauge: It is recommended to fix a silicon filled thermometer to give a steady reading of the stored water. Temperature Sensors: Only sensors and probes of high quality and reliability should be used in the designated place on the tank and the heat pump for proper feedback to the controllers and also to maintain right temperature in the system. Collectors: There are several types of collectors available in the market at various price levels. Each has its own advantages as well as disadvantages. Evacuated tube collectors are higher in efficiency in the beginning but in the long run when the tube loses the vacuum due to sealing issues between two materials having different 18
coefficient of expansion, the efficiency drops. ETCs are prone to breakages, handling and maintenance issues. The most common fin and tube collectors are almost obsolete and outdated in the developed world due to poor construction and very low efficiency. Constant research and testing at various site conditions has proved that flat plate collectors with electronically sputtered surface finish with a suitable selective surface coating are the best in terms of efficiency, constant performance, durability, reliability and most importantly, value for money. The best coating available today is titanium oxide and it goes with various brand names like Bluetech, blue coated selective surface coating. Collectors should be tested andcertified as per EN-12975-2 under BIS &TÜV SÜD America Standards type with the heat pump taking a major load. There are two main types of heat pumps available air to water and water to water heat pumps. The air to water heat pump uses the ambient air as the heat source as it absorbs the heat from the ambient air. The by-product of air to water heat pump is cold air. The air expelled out is about 6-7 C lower than the ambient air. In a water to water heat pump water is the source. Heat exchange takes place with the water medium. The by-product of water to water heat pump is cold water which is lower by 5-6 C than the inlet water temperature. There are many manufacturers in the world who offer heat pumps. The critical components in a heat pump are the compressor, evaporator coil, heat exchanger, expansion valve and heat pump controller. The company that offers the best combination of all of these, has reliable electronics with PLC, offers test certificates and only declares the COP after testing in an isothermal laboratory under varying conditions will be the heat pump to opt for. The compressor is the heart of the equipment and has to be of a reputed make and type. The most efficient compressors that are available today are scroll type from major manufacturers like Copeland, Panasonic, Mitsubishi, etc. The refrigerant used should have distinct product behaviour and thermodynamic characteristics and be the one suitable for a compressor to work on a heat pump. There are several refrigerants available but the better and more suitable refrigerants are 407 C, 410 A and 134 A. They are green and environmentally friendly and are also available in plenty in India. Summary: A flat plate collector with electronically sputtered surface finish with titanium oxide (Bluetech) coating is the best suited for our conditions. Heat Pump: The heat pump is an integral part of the hot water system especially if the system installed is of hybrid The evaporator coil plays a very important role in the system. The COP of the system depends on the efficiency of this coil. The coil being metal is exposed to vagaries of the environment especially if installed in corrosive climatic conditions and also near the sea coast. The coil should have a good protective coating against corrosion. The good coatings available today are the Kirby Coating which is blue in colour and Gold coating that is gold in colour. March 2018 / 19
Heat Exchanger Heat exchanger is the vital component and as the name suggests it is where the main heat exchange function happens. There are many types of heat exchangers that are available. The differences are based on type of construction like plate type, shell in tube and coaxial type and on materials like copper/pvc, PVC/titanium, titanium/titanium etc. One has to carefully choose the type of heat exchanger based on the type of heat pump and the application. The gap in the walls of the heat exchanger is narrow and limited by design. The heat exchange surface reaches high temperatures. If the quality of water flowing across the heat exchanger is not good, with hardness and TDS, then this will cause scaling on the walls of the heat exchanger gradually reducing the efficiency and finally choking the tube causing failure of the heat pump. Expansion valve throttles the refrigerant from high pressure to lower pressure to be ready for the next cycle. Thus precise functioning of the valve is required to fully utilize the characteristics of the chosen refrigerant. Heat pump controller is the brain and controls the entire functioning of the heat pump. It also acts to protect the heat pump from over load, low flow, overheating; apart from making the heat pump work under the set parameters. The controller has to be most reliable, easy to operate and least complicated as this governs the working of the heat pump. It is very important to choose the right heat pump in terms of technology, capacity, refrigerant used, coating on the coil, body material and type of heat exchanger based on application. But above all, it is very important to run the heat pump on close circuit fluid as indirect heating system than circulate water directly. Most heat pumps fail due to scaling caused by poor quality of the water that is supplied. Insulation: It is most common to see stainless steel water tanks being covered by mineral/rock wool as insulation and an aluminium cladding wrapped around it. It is important to note that this is a very old and primitive form of insulation where the thermal loss is quite high. Apart from this, both mineral and rock wool are hazardous material, carcinogenic, not child safe and is banned in the developed world. It is important to have a good insulation and polyurethane (PU) is the most suitable material available. Both soft and hard forms of polyurethane foam can be used in full when injected around a jacket or in sections to wrap around the hot water storage tank. Manufacturing as per Chapter-6 USEC-India under IPA-2017 Standards. Summary: Polyurethane insulation around the tank is best suited for our climatic conditions and also for long life and better performance of the system. Installation & Commissioning: The best designed equipment, comprising high quality components, will perform well and as per the design requirement only if it is installed and commissioned in the right or proper way. The entire equipment must be housed in a suitable place. The tanks should not be directly exposed to the weather and all the electrical and electronic controls must be protected from water and rain. The heat pumps should have proper ventilation. Plumbing: Once the equipment sizing is done to suit the requirement, the plumbing should also be designed 20
to connect all the subsystems that comprise the equipment for optimum performance of the system. The plumbing design and execution must be done by plumbing and hot water experts and only those who are well trained in installation work and safety. The plumbing material recommended is a metal pipe either of copper or SS for the solar circuit as the temperature can be high. The high temperature will determine the choice of material for the other plumbing lines for the heat pump, supply or return line. All the lines must be completely, properly and neatly insulated to avoid any loss of heat or energy. The insulation should be UV stabilized for exposed areas and properly covered if required. ( Chapter 8 USEC-India under IPA-2017 Standards) Commissioning: Once the plumbing is installed, the commissioning should be in sequence and as per individual subsystem requirement. The close circuit fluid must be filled in the right quantity and dilution rate along with DM or distilled water, or treated water if available at site, pressurized and tested. The circulation can be switched on only if there is no pressure drop or any leakage detected in the system. Thereafter the controllers and PLC are programmed as per the design and hot water requirement. The hot water circulation must be switched on after a couple of hours and the cut in and cut off time must be correctly set on the timer/temperature based control panel. The system must be allowed time to stabilize before there is a draw off. Running and Maintenance: After the system is correctly installed and commissioned it should be completely on auto mode. There is no requirement for constant interference to manage the system. However it is mandatory to check the working parameters on a daily basis to ensure smooth running of the system. Please ensure the collectors are clean, pipe lines are clear, the circulating pumps are working smooth, the hot water circulation pump is working on timer as per the setting, the drain pipes from heat pumps are not clogged and ensure that there is no leakage in the close circuit or the supply and return lines. Annual Maintenance contract: On the completion of the defects and liabilities period it is advisable to award AMC to a competent agency preferably to the company that has installed and commissioned the system. Conclusion As we conclude this article we have to emphasize the importance of equipment design: it is very important to design the equipment to best suit the site requirements. The requirement has two sides: one is the actual hot water demand and the other is to suit the equipment as per the site conditions. Sizing: A detailed study is required to estimate the hot water demand. The daily demand depends on the number of persons using hot water, other areas of hot water consumption such as spa, kitchen, restaurants, back of the house and laundry. The type of shower fittings and equipment in use such as regular shower, shower panel, rain shower, bath tub and jacuzzi determines the quantity of hot water required per person as each fixture has different flow rates. Site Conditions: Each site is different in terms of location and available area for the equipment. It is best to check the available space on the terrace and in the plant room before we select the equipment including number of collectors, storage tanks, heat pumps, expansion tanks and controllers. It is very important to ensure proper ventilation for the heat pump so that it can draw fresh ambient air and be able to transfer cold air away from the equipment. ipt March 2018 / 21