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Many water heater options are available for sustainable buildings. The water heater selected for a specific facility depends on the supply energy demand. The plumbing engineer should know the facility’s energy goals before deciding which option will work best.
Two water heating options for reducing the energy consumed for a building are instantaneous systems and storage-type water heaters. But, the plumbing engineer must be aware of the energy demand of a building with both of these options.
Peak energy demand can be a problem when designing energy efficient buildings. Most utility suppliers have few problems supplying energy on ordinary days. But, supplying energy during peak demand times is expensive. During peak electrical load demand times, utility suppliers might add capacity with more expensive generation methods and fuels. For example, a utility may use coal as a fuel for the base electrical load. When demand climbs, natural gas-type generators may be used, which costs more to operate. Utilities can also purchase power from other utility providers at a higher energy cost. Other additional costs can include the power infrastructure, including wiring, transformers, switching, and other equipment that is expensive when it is sized for a high peak load.
This demand problem was once thought to be a concern only in hot and humid regions with high air-conditioning energy usage. During extreme cooling days, electrical utilities had problems keeping up with demand. However, the extremely cold winter of 2013–2014 demonstrated that in some regions natural gas utilities also can have problems providing gas to users, including peak demand flow. A report from the U.S. Department of Energy on April 15 stated:
“A regional energy crisis was narrowly averted in part because ISO New England (a regional transmission organization) took unconventional, aggressive, and preemptive steps to ensure energy supplies in advance of peak demand during cold winter months.”
Instantaneous-type water heaters
Instantaneous heaters work well in single-family or small facilities, even during peak demand. When they are combined with other “on-demand” type energy users, they may be problematic. From a utility grid scale system perspective, providing the potential power for hundreds, and sometime thousands, of instantaneous systems to energize in a short amount of time can be expensive. The end result is that an individual user may use less power with instantaneous water heaters; however, the grid uses more energy to help that individual use that instantaneous power.
Another energy problem is electrical spikes. When equipment starts, it can create a spike in electrical demand. When more than one piece of equipment starts at the same time, the spike is compounded. For example, in a residential setting the air-conditioner, refrigerator, dishwasher, and water heater all activating at the same time can result in a peak demand spike in the electrical service to the building. To reduce this spike, a smart meter can be used to monitor power usage and delay the timing of equipment startup. The goal is to reduce electrical spikes and peak energy demands.
Some utilities provide incentives for using smart meters. A storage-type water heater may be used in a smart grid application because delaying the start of a storage-tank type water heater by a minute or two is not usually a problem. In this case, the plumbing engineer may want to compare the amount of energy that is lost in a storage-type water heater when compared to the reduced energy rates for a smart grid. On the other hand, delaying the start time of an instantaneous heater can result in no hot water when it is needed, or it can cause the wasting of water while someone waits for hot water to enter a tap. The demand in a single residence may not be an issue. However, a condominium building with several hundred instantaneous heaters may result in a large electrical distribution system that could make the installation of the instantaneous heaters too expensive.
Some utilities offer reduced rates for using energy at night. From a grid perspective, it is more efficient to generate power overnight, even if it is not used and wasted. They do this so they can have generation capacity in the morning when the demand rises. As a result, using power at night does not increase generation capacities or greenhouse gases. Some storage-type water heaters can be used for this application. The temperature of the water in the tank can be above the temperature of the water for the system. A mixing valve can be used to mix the temperature down to a workable system temperature. A timer or smart grid type system should be used to time the operation of the heater for nighttime operation.
Using stored energy to pre-heat domestic hot water
Some facilities are adding on-site energy-generation systems to decrease water-heating costs. The plumbing engineer can provide direction in the type of technology that can be used in on-site power generation. For example, using photovoltaic panels to generate electricity and then using the electricity to heat water is not as efficient as using a hydronic solar system to heat water, which can reduce upfront and operational costs.
These systems generate power, however, the peak demand in the building could be different than the peak energy generation time. For example, peak solar generation most likely is midday, while the peak building demand could be late at night.
To offset the demand of the building and the available power, the building will need a way to store energy. This can be done with chemical batteries or flywheels, which can be expensive to purchase and operate. A more economical method is to use the domestic water heater system as a method to store energy. The excess energy from the on-site energy generator system can be used to preheat the supply water to the domestic water heaters. As its name indicates, in a domestic hot water preheat system, a coil in the domestic water preheat tank circulates the condensing water through the tank, so the water entering the water heater is already preheated.
Here are some important points that the plumbing engineer will need to take into account when designing a domestic hot water preheat system.
• Legionella can be a concern when storing water in low heat systems. So, minimize the amount of time the water will be in the storage tank, or use a method to treat or heat the water to prevent the risk of Legionella.
• Provide a bypass around the tank so it can be serviced.
• Provide a way to ensure that the maximum temperature does not exceed the domestic water maximum temperature.
• Some regulatory agencies may require double-wall piping on the heat exchanger.
• The tank should be insulated.
The plumbing engineer may have to run an analysis on the amount of power that is generated during the peak power generation times of day. This is compared to the times of day for the hot water demand. For example, a restaurant that serves three meals a day will have a dishwasher hot water demand that starts after breakfast has begun. The hot water demand could be constant all day and continue late in the day after the last meal is served. If a solar hydronic system is used, the peak generation time would be midday, which could be a good match for the building load. As a result, the preheat system could be small because the generation and demand are close to the same time.
This is also an option with wind generation systems. There are times when a wind system will generate more power than is needed. As a result, storing the energy is an efficient way to reduce grid power usage. For a large-scale example, in April 2012 the Portland Oregon Business Journal reported that the Bonneville Power Administration had to temporarily shut down the wind farms in its system for a few hours on a Sunday morning and again on Monday morning because of low electricity demand. In all, 10,100 megawatt hours of wind energy was curtailed. A wind farm for a smaller on-site system could generate extra power that can be used and stored in the domestic hot water system.
Another type of water storage preheat system that could be used is a heat recovery system, which removes excess energy from equipment. Equipment can include load-sharing generators, chillers, exhaust systems, and food service refrigeration systems. The extra energy can be used to pre-heat domestic hot water.
Plumbing engineers have several options to reduce the energy used in water heating systems. Instantaneous-type or storage-type heaters can be used, as can preheat systems. To make the best selection, the plumbing engineer will need to know the energy issues with the building and the energy issues with the utility.
Winston Huff, CPD, LEED AP BD+C, is a senior project manager, plumbing fire protection designer, and sustainable coordinator with TRC Worldwide Engineering, Inc. He serves as an ASPE representative on the ICC Green Construction, Energy, and Water Code Development Committee and is on the U.S. Green Building Council’s Water Efficiency Technical Advisory Group. He was the founding editor of Life Support and Biosphere Science and has served as its editor-in-chief. He is also the editor of Me Green You Green, a LEED credit databank at www.megreenyougreen.com.