Hospitals & HVAC
December 1, 2007
By Laurier Nichols, ing. DESSAU
At a time when provinces like Quebec have elected to construct very large buildings of up to 2,000,000 sq. ft. in order to consolidate services from other institutions, it is especially important to c...
At a time when provinces like Quebec have elected to construct very large buildings of up to 2,000,000 sq. ft. in order to consolidate services from other institutions, it is especially important to consider what are appropriate approaches to the design of their mechanical and electrical systems.
The design of mechanical and electrical systems for today’s hospitals represents a significant challenge because of the numerous services and systems these buildings must accommodate. Hospitals are equipped with new analytical and diagnostic devices, increasingly specialized laboratories, increasingly complex treatment systems, and so on. All these improved services require more mechanical and electrical systems than their predecessors. The result is that mechanical and electrical systems now represent over 45% of the total construction budget for a new hospital.
Most basic systems — plumbing (water supply and sewers), fire protection, medical gas supply and electrical supply — have relatively little impact on the energy efficiency of the building. Such systems must be designed to current standards and conform to the usual principles of proper installation.
By contrast, the selection of approaches to HVAC systems can have a considerable impact on a building’s performance. In some cases, energy consumption in a standard hospital building can be double that of one designed to be energy efficient.
The wrong approach
During an exploratory meeting to study the possibility of using a central cooling and heating plant for a megahospital, one of the potential P3 partners asked whether air conditioning using an absorption chiller could yield significant savings. The fact that the question came up in this way demonstrates that some project proponents have a limited understanding of the field. An absorption chiller is absolutely not appropriate for a cooling system in Quebec’s energy context. A standard absorption chiller consumes 10 times the energy consumed by an electrical cooling unit.
Certain project proponents have also suggested the construction of a centralized steam plant. This too indicates a lack of understanding of a modern hospital’s real needs. Too much inspiration is being drawn from the approach to hospital construction used in the 1920s and 30s. At that time, energy costs were very low. For example, in 1970 heavy fuel oil cost $0.03 per Imperial gallon.
Several hospitals built over 50 years ago have attempted to retrofit facilities that were not originally designed for efficiency. Energy consumption statistics for some of these buildings before their retrofits reveal consumption levels exceeding 3.5 to 4.0 GJ/sq.m./year (nearly 100 kWh/sq. ft./year). That is a very high level of energy consumption. On closer examination, what is particularly surprising is the high residual consumption in summertime.
In older buildings, maintaining a steam distribution network pressurized to approximately 1,440 kPa (125 psi) can result in an hourly consumption of greater than 2,350 kWh (8,000 lbs. of steam per hour). That level of consumption can result in the burning of over 175,000 cu.m of natural gas per month during the entire summer.
Marginal steam use in today’s hospitals
We should therefore try to limit the use of steam distribution in the design of today’s hospitals.
In modern hospitals, the need for steam consumption is marginal. For one thing, they generally do not have laundries because this service is contracted out. In former times, laundry needs drove a hospital’s basic steam consumption. Once this steam requirement is removed, only three primary uses remain: sterilizers, steam cookers and warming tables in kitchens and cafeterias, and humidification. We assume that the building is heated using a hot-water system.
Most sterilizers, which are scattered throughout different hospital departments, use steam for the sterilization cycle. Steam is difficult to replace for these needs, but we need to ask if hot-air sterilization is viable. If steam is finally chosen, it is necessary to use decentralized steam production units located in proximity to needs. Otherwise, it is necessary to use a distribution network shielded by high-performance thermal insulation.
Steam for kitchen requirements can be very effectively replaced by electric or direct gas heat. Electric heating is markedly more efficient than steam.
Humidification needs can be served by decentralized vaporizers. These devices are only used in winter and can be shut off in summer. There are also other, still more efficient, humidification methods (water spray) that could be considered for some parts of a hospital.
Harnessing waste heat from the ventilation system
Ventilating a hospital is very expensive, since ventilation standards require two changes of fresh outdoor air per hour. This ventilation standard represents an intake of fresh air at a rate of 1.7 L/s/sq.m. Unlike in a commercial building, this rate must be maintained 24/7/365. For instance, a building with 100,000 sq.m of usable floor space, with fresh air heated to 23C, will require an energy use of 32,000,000 kWh per year. That represents a cost of $2,875,000 if the cost is $.09/kWh (high unit cost of electricity for heating). The same heating done by natural gas would cost approximately $2.4 million in Quebec.
The most economical way of heating most of the incoming fresh air is to harness the heat from vented air to heat incoming fresh air. There are commercially available devices that can reduce fresh-air heating needs by 70% to 90%. We therefore need to question the quality of proposed designs that include no provisions for reclaiming heat from ventilation networks. Considering the large amounts of fresh outside air that must be drawn into a hospital at all times, this reclamation of heat from ventilation must be a priority. In the case of laboratory exhaust, there are regenerators that eliminate all contamination risks: closed exchangers are used to heat an intermediate fluid. For general needs without a contamination risk, reclamation can be done with conventional equipment.
It is true that it would be difficult to reclaim heat from certain types of laboratory exhaust and other potentially contaminated exhaust. In general, such exhaust systems represent a very small portion of overall ventilation. An economic assessment of the methods to be used generally makes it possible to determine whether it is viable to reclaim heat from difficult situations.
Harnessing internally produced heat
A typical hospital department is equipped with numerous heat-generating devices. For example, computers and image processing equipment are increasingly commonplace. These devices perform data processing operations involving electronic components that generate appreciable amounts of heat. In fact, every device that uses electricity will transform some of its consumption into heat. In most cases, this heat is dissipated into the immediate surroundings, while in others the equipment is cooled by a dedicated system. All activities inside a hospital generate heat (from equipment, lighting, people, etc.). This heat can be harnessed for heating needs with a heat pump, a type of device used successfully in numerous buildings.
The reclamation of heat from inside spaces is possible with specialized equipment such as heat pumps. However, these devices in turn require the use of a low-temperature heating system (43C). At that temperature, heating with natural convection is no longer possible. It is thus necessary to use a forced convection heating method. With a 43C heating network, it is necessary to use a fan to push cool air into an exchanger with a slightly larger exchange surface. Several newer buildings successfully use HVAC systems with low-temperature heating systems. It is interesting to note that high-efficiency boilers (hot-water only, not steam) must be used with low-tempera
ture heating systems. These boilers can reach efficiency levels ranging from 92% to 98%.
It is easy to envision a scenario in which a heat pump-type unit making use of internally generated heat can meet the heating needs of a building even when the outside temperature is as low as -5C. In such a case, internal production is in balance with heating needs. When the temperature falls below -5C, additional heat can then be generated with a high-efficiency hot water heater. This approach permits the use of smaller equipment that adapts well to the output variations required by fluctuating heating needs.
Although a hospital can generate significant heat internally, and despite the fact that it is possible to reduce heating needs by harnessing heat from ventilation systems, in certain locations of the hospital the demand for heating can quickly exceed the heat available from internal sources and there is a need to provide supplemental heat. In these situations, before resorting to short-term boiler systems, even high-efficiency ones, it may be beneficial to use geothermal energy.
This approach involves using a heat pump to draw the energy required for the building from an underground exchanger. It is the same principle as that applied to harnessing internally generated heat. In this case, instead of harnessing heat within the building, the heat pump draws heat from the ground. In an urban area, the ideal location for the heat pump is under the building, which involves drilling a geothermal well field or using the building’s support piles. This method of harnessing geothermal energy is very popular in Europe.
It is important to note that the use of a low-temperature heating system makes it possible to use a geothermal heat pump. Geothermal energy can reduce the need for short-term heating by over 80%. Geothermal systems are generally not designed to meet 100% of heating needs. During short periods of extreme cold, it is usually more economical to use a high-efficiency boiler on an ad hoc basis.
The designer must be capable of selecting the appropriate equipment for every element of the design. He or she must assess the building’s needs based on variations in outdoor temperature, sunshine, building use, internal heat generation over time, etc.
Such a large number of variables must be considered simultaneously, so it soon becomes apparent that the load calculations must be performed with simulation software. Simulation involves a set of very powerful tools that experienced designers use to justify the different elements of a design. The tools generally make it possible to identify quickly the project elements that will have a significant impact on energy efficiency.
Often, however, during the integrated design session, much conventional wisdom is put forward but it becomes abundantly clear that that few designers are in a position to identify the most relevant energy efficiency improvements.
Designers of a building’s mechanical and electrical systems have very powerful analytical tools at their disposal. To demonstrate the advantages of a more efficient design, all it takes is a willingness to learn how to use them. It is a fact that a more efficient building requires more expensive equipment. However, a well-documented case can demonstrate the economic viability of more efficient designs. It is important to avoid an easy approach that only serves to perpetuate an outdated 1950s-era model.
A hospital designed to follow the principles outlined above should have an energy consumption of approximately 1.65 GJ/cu.m/year (41.2 kWh/sq.ft./year).
Laurier Nichols, Ing. is vice-president, special projects and manager of energy efficiency expertise with DESSAU, Montreal. He is a Fellow of the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) and has earned several awards for the energy efficiency of buildings he designed. He is also an editorial advisor to Canadian Consulting Engineer magazine.