Keywords

heating; ventilation; air conditioning; efficiency; cycle

The Heating, Ventilation, and Air Conditioning Industry

The energy used by Americans for heating, ventilation, and air conditioning (HVAC) is second only to the energy used for transportation. HVAC applications consume about the same amount of energy per year as the 130 billion gallons of gasoline consumed annually in the United States for transportation (see Table 6.1). From a greenhouse gas emission perspective, about 8.8% of carbon dioxide emissions are from space heating furnaces compared to 33.9% from electrical power production. A large fraction of the electrical power is used for HVAC.

Consumers use electricity, fuel oil, liquid petroleum gas, kerosene, and natural gas for heating. Table 6.2 shows that natural gas and fuel oil provide most of the space heating needs.

The competition created by alternative technologies and fuel choices provide relatively stable heating/cooling costs, just as with electrical power production. In the HVAC industry, energy savings result from improved building construction with better insulation and double or triple pane windows. The consumer can choose between investing in energy-efficient buildings with lower yearly energy costs or they can install less expensive HVAC equipment with higher annual energy expenses. Diversity in energy sources and competition among equipment manufacturers has made the HVAC market one of the success stories in American free enterprise, but the story does not end there.

HVAC technology can provide the flexibility needed to achieve additional reductions in greenhouse gas emissions and improve the overall efficiency of electrical power generation networks. The way to achieve these two objectives is best explained by the following examples:

Potential Impact of Thermal Energy Storage for Peak Load Shifting with Heat Pumps or Air Conditioning

The peak demand of electricity relative to base load electrical power varies during the year. Peak demand in April is typically about one and a half times the base load. In July, the peak demand can become twice the base load. It is possible to make the demand for electricity nearly constant at a higher baseline load level by using a “peak load shifting strategy.” For example, a higher base load can justify the higher cost of installing a 50% efficient combined-cycle natural gas power plant operating continuously instead of the 28% gas turbine units used to supply peak power just part of the day.

During the night hours operating at a base power level, more electricity is generated than can be sold. This excess power could be “stored” (storing electricity—electrons—is not easy. It would have to be a huge “battery?”—maybe). During the hot daytime, when the demand for power exceeds the generating capacity, power would be added using the “battery.”

For every 1.0 kWh that is shifted from peak demand to base load there is 2.0 kWh that benefits from the improved efficiency; 1 kWh that was shifted and used with the 1 kWh during the peak demand period. The base load has been increased to produce power for the shifted load.

Peak load shifting saves fuel and reduces costs for essentially every application. For those applications where savings are passed to consumers by the local electrical provider, the consumers can realize quick paybacks for investments in energy storage devices (see insert).

Example of Value of Peak Load Shifting

(Data from http://www.xcelenergy.com/EnergyPrices/RatesTariffsMN.asp, Rate Codes A01 and A04)

Peak demand electricity is more expensive and less efficient to generate than base load. The highest peak loads occur during the 4 months of high air conditioner use. The peak loads increase to a maximum at mid-afternoon and then decrease each day during the 4 months of summer. These peak load times represent about 14% of the full year. The generators producing this peak power operate at about 10% of annual capacity. Investment costs to build these facilities are recovered by increasing the rates for peak demand electricity or by increasing the rates for all the power produced. In addition, economics dictates that capital equipment costs be minimized and these generating units are less efficient and often use more expensive fuels.

The Northern States Power Company has programs that provide customers with incentives to reduce peak demand consumption. Their nuclear power infrastructure provides inexpensive base load availability. Two of their residential rate programs are listed below.

Standard Rate Code A01:

 June–September 7.35 cents/kWh

 October–May 6.35 cents/kWh

This standard rate code provides easy bookkeeping, but it does not reflect the true cost and availability of electricity. Other rate codes are used to create markets for excess winter capacity and give a reduced rate of $0.0519 during the winter for electric space heating. The time of day service option (Code A04, below) reflects the difference in costs for providing peak load versus baseline load and allows the homeowner to adjust and use pattern to reduce costs.

Standard Rate Code A04:

 On-Peak June–September 13.95 cents/kWh

 On-Peak October–May 11.29 cents/kWh

 Off-Peak 3.27 cents/kWh

When the price of electricity reflects the cost of providing peak power versus off-peak power, the peak electricity costs 4 times the off-peak power. The definition of peak demand for this plan is 9:00 AM until 9:00 PM. This plan gives a real incentive to program use of electricity to off-peak hours.

EnergyGuide Labels

(from http://www.eren.doe.gov/buildings/consumer_information/energyguide.html)

The US government established a mandatory compliance program in the 1970s requiring that certain types of new appliances bear a label to help consumers compare the energy efficiency among similar products. In 1980, the Federal Trade Commission’s Appliance Labeling Rule became effective and requireed that EnergyGuide labels be placed on all new refrigerators, freezers, water heaters, dishwashers, clothes washers, room air conditioners, heat pumps, furnaces, and boilers. These labels are bright yellow with black lettering identifying energy consumption characteristics of household appliances.

Although the labels will not tell you which appliance is the most efficient, they will tell you the annual energy consumption and operating cost for each appliance so you can compare them yourself.

EnergyGuide labels, such as illustrated by Figure 6.1, show the estimated yearly electricity consumption to operate the product along with a scale for comparison among similar products. The comparison scale shows the least and most energy used by comparable models. An arrow points to the position on that scale for the model the label is attached to. This allows consumers to compare the labeled model with other similar models. The consumption figure is printed on EnergyGuide labels, in kWh, assuming average usage. Your actual energy consumption may vary depending on the appliance usage.

EnergyGuide labels are not required on kitchen ranges, microwave ovens, clothes dryers, on-demand water heaters, portable space heaters, and lights.

Air Conditioning

Ventilation (open windows and fans), evaporative coolers, and effective building design with proper landscaping often provide air conditioning. Cooling and humidity control go hand in hand during much of the air conditioning season. The cool temperature created by an evaporative cooler is produced as water passes over a grid. Dry warm air passes through the grid evaporating the water, lowering the temperature, and cooling and humidifying the air. Modern central air conditioning systems condense water on the cooling coils out of view and it flows to a drain. It can be seen as a puddle of water under a car parked after the air conditioner is used. The energy consumed condensing the water from air can be about the same as the energy used to reduce the air temperature.

Ventilation is the least expensive of all the nonarchitectural options to keep a house cool. Evaporative coolers that cool air by evaporating water into dry, warm air are a good option in hot dry climates. Vapor-compression air conditioners are the most common air conditioners used by homeowners to cool and dehumidify the summer air. They are responsible for most of the energy consumed to cool buildings. These units use the compression and condensation of refrigerants to pump heat (see insert).

Summaries in Table 6.2 show that the energy consumed by air conditioning is about one-tenth the energy used for heating. Air conditioners use less energy because they “pump heat” out of the house. By contrast, furnaces directly convert the chemical energy in fuels to heat or they use electrical resistance heaters to convert electrical energy into heat.

The Seasonal Energy Efficiency Ratio (SEER) is defined as the BTUs of cooling provided per Watt-hour of electricity consumed. The SEER Federal efficiency standards require that heat pumps (air conditioners that provide both heating and cooling) have a SEER rating of at least 10.0 with some units providing SEER values above 14 [2]. Dividing the SEER rating by 3.41 gives the cooling and electrical consumption in the same units. Modern air conditioners remove 3–4 times more heat from the house than electrical energy consumed.

Compare the SEER heat-pumping capacity for air conditioners to the typical amount of heat added to a Midwest house during winter. Heating the house in winter takes three times as much heat as is removed in the summer. This factor of 3 seems right about considering the longer heating season (6 months heating compared to 3 months cooling) and that the outside temperatures will typically vary from about 70°F less (in winter) than the inside temperatures. In summer, the outside temperature is about 30°F higher than inside.

For commercial buildings in the Midwest, about twice as much energy is consumed for heating as for cooling. Commercial buildings are generally larger and have less outside wall area per square foot of floor space. The smaller outside surface areas provide better insulation. As a result, lighting, electrical office equipment, and heat given off by people will have a larger impact when compared to outside weather conditions. The heat from each heat-producing sources must be removed from the building during summer.

How Air Conditioners Work?

Air conditioners use the work produced by electrical motors to “pump heat” out of a house. This can be expressed mathematically as follows:

Three BTU heat pumped from house + 1 BTU of electrical work=4 BTU heat pumped outside.

This equation shows the conservation of energy in this process and gives air conditioner a SEER rating of (3 BTU ÷ 1 BTU) × 3.41 (BTU per Wh)=10.23.

As illustrated in Figure 6.2, a vapor compression air conditioner consists of four components: evaporator, compressor, condenser, and an expansion valve. Air inside the house circulates through the evaporator. At about 40°F, the evaporator is cooler than the inside air and cools the inside air from 80°F to about 50°F. For example, 3 BTU of heat is transferred from the house air into the refrigerant circulating in the air conditioner. The heat causes the refrigerant to boil and leave the evaporator as a vapor.

With the addition of about 1 BTU of work, the vapor from the evaporator is compressed to a higher temperature and pressure. The high pressure allows warm outside air to be used to condense the refrigerant vapor (high pressures increase boiling points and favor formation of liquids) in the condenser located outside the house. The 4 BTUs of heat is released from the condenser and makes the warm outside air even warmer. Liquid refrigerant leaves the condenser, proceeds through an expansion valve reducing the pressure, and causes some of the refrigerant to evaporate, which cools the liquid to 40°F in the evaporator where the cycle started.

Residential and commercial air conditioning uses 6–7%¹ of the electricity generated in the United States. While this number appears small, consider that this use occurs during the hottest one-third of the year. Summer is the warmest and the most humid in part of the United States in a year and most of the demands is during the daytime rather than night. This 6–7% rapidly increases to 50%² or more of the peak electrical power load during the warmest summer afternoons.

The peak load shifting of this air conditioning power demand provides an opportunity to justify more efficient base load electrical power generating capacity and for substantial reductions in greenhouse gas emissions and fossil fuel consumption.

Heating

Air conditioning arrived with the industrial revolution at the beginning of the twentieth century. The generation of heat is as old as civilization and often considered a trivial process. The combustion of fuels converts chemical energy into heat. The electrical resistance in wires will convert electrical energy to heat.

Furnaces have historically converted wood, coal, and even cow chips (dried manure) into useful heat. The reduced soot from kerosene, fuel oil, natural gas, and liquefied petroleum gas allowed these fuels to dominate the heating fuel market into the mid-twentieth century. The historical low cost of natural gas combined with pipeline distribution to commercial and residential buildings makes natural gas the popular furnace fuel in the United States.

In the warmer parts of the United States, heating is often necessary in the winter, but the low annual fuel consumption for heating does not justify the cost of installing natural gas pipelines. For these locations as well as other locations where natural gas is not available, heat pumps are now an available alternative.

Using the same vapor compression cycle as an air conditioner, a heat pump “pumps” heat from outside air into a building. As illustrated in Figure 6.3, the four functional components of an air conditioner can be configured to pump heat out of a house or pump heat into a house. This is done with the addition of two valves that reverse the flow of refrigerant (reversing the direction of the heat flow from outside to inside the house). These valves can be installed at small incremental cost when the air conditioner is manufactured, providing cooling in summer and heating in winter.

The HSPF rates a heat pump performance based on the BTUs of heat provided per Wh of electricity consumed. An HSPF rating of 6.8 or better is required by federal efficiency standards [3]. The HSPF rating is a function of outside temperatures and decreases rapidly as outside temperatures drop below 30°F. The compressor has to work harder to generate a higher pressure difference necessary to overcome higher temperature differences operating as a heat pump.

While air conditioners typically have to overcome about a 15°F temperature difference (75°F inside temperature vs a 90°F outside temperature), heat pumps often have to overcome temperature differences in excess of 40°F (75°F inside temperature vs a 35°F outside temperature). This explains why the HSPF ratings of heat pumps are lower than SEER ratings of air conditioners. As outside temperatures get lower the HSPF rating also gets lower. At temperatures lower than the freezing point of water (32°F, 0°C) the water on the evaporator coil can freeze and the heat flow slows or stops.

For an incremental increase in cost above that of a conventional air conditioner (a couple valves and minor equipment changes), the heat pump provides a significant performance advantage over electrical resistance heaters that convert electrical energy to heat. By tapping into the electrical power grid, the heat pump also uses the diversity of the electrical power infrastructure including the mix of fuels, using nuclear, wind, or hydroelectric energy. In addition, increased use of electrical power during off-peak seasons (winter) and off-peak times (night) can provide the incentive for building more efficient electrical power facilities. These improvements will become important as fossil fuel reserves are consumed and the reduction of carbon dioxide emissions becomes an important global priority.

At close to 10 billion gallons per year, liquid fuels used for residential space heating (heating oil, kerosene, and liquified petroleum gas) contribute about $10 billion of the $200 billion for annual imported crude oil (and products). These fuels along with liquid fuels used for commercial heating and hot water heaters are part of the problem and must be part of the solution for long-term energy security.

The stationary nature of space heating and hot water heating applications makes electricity particularly attractive for these applications. Heat pumps used in combination with resistance heating can provide the heating demands while moving point-source fuel combustion emissions from inside the city to power plants outside the city. If the electrical load created by these applications adds to the base load during the winter, this can lead to benefits. By installing new, efficient electricity generating plants, electricity costs and greenhouse gas emission can be reduced. This will provide efficient power for space heating (during the winter) and replace less-efficient electricity generation (peak load units) during the rest of the year.

For example, creating 100 additional days of base load demand for electric heating could justify a new nuclear power plant or wind turbine farm that would provide base load power those 100 days. This generating capacity is available the other 265 days of the year. Figure 6.4 is a graphical representation of this proposal. This would replace imported liquid heating fuels and potentially replace peaking electrical power units that consume natural gas or petroleum. These changes would generally be cost-effective when all factors are considered. However, the accounting mechanisms may not be in place for local electrical providers to pass the savings on to the consumers who control when energy is used.

Peak Load Shifting and Storing Heat

Peak load shifting refers to changing use of electricity from the middle of the day to night when the electricity is in low demand. Figure 6.5 illustrates how electrical demand can vary during the 24 hours of a day. This change in demand should not be a surprise since it is during daytime that air conditioners run when the outside temperature is high. It is during the day that the clothes dryer runs and hot water heaters recharge after the morning shower.

On a hot summer day, air conditioners can be responsible for over 50% of the electrical demand. Peak demand power generation is both inefficient and costly. The case study at Fort Jackson illustrates the typical costs associated with peak demand electricity and illustrates how this problem might become an asset.

For large commercial or military installations, electrical power providers offer a number of “rate plans” to pass savings to these consumers as a reward for working with the provider to reduce the cost for supplying electricity. These rate plans are typically based on the principal of reducing peak demand and purchasing predictable amounts of base load electricity. Paying premium prices for all electricity purchased above the base demand is an example of such a rate plan. For the profile of Figure 6.5 at Fort Jackson, electricity consumed beyond 19 MW is electricity at the premium price.

In 1996, the Fort Jackson Army installation paid a $5.3 million electrical bill with 51% of this ($2.7 million) as “demand” electricity. The nondemand portion of the electrical bill is referred to as the energy charge because it is intended to reflect the cost of all electricity consumed at base-load prices. During summer months, it was normal for 50% of the electrical bill to be demand charges. For most Army installations this demand bill exceeds one-third of the total electrical bill.

To reduce the cost of demand electricity, a chilled water storage tank was installed at Fort Jackson. During off-peak hours, air conditioners (chillers) ran to cool water to about 42°F and stored it. The chilled water is used to provide cooling during the day. The dashed line in Figure 6.5 illustrates how demand for electricity is typically reduced when the chillers are turned off at mid-day and the stored chilled water is used to provide cooling. An additional advantage of this storage system is it allows the most efficient chiller to be used at full capacity during off-peak hours while minimizing the use of less efficient units. Also, the chiller is operated more during the cooler hours of the day when the chiller operates more efficiently. A disadvantage of chilled water storage is that about 10% of the cooling energy is lost to “heat added” to the tank from the hot summer air and mixing with the circulated chilled water returned to the tank as warm water.

Ice storage is also used for peak load shifting of electricity. Ice takes in considerable energy when “thawing” back to water. Ice storage units are smaller than water storage units. Special materials, such as waxes, eutectic salts, and fat/oil derivatives have been developed to freeze at temperatures close to room temperature. These materials are referred to as phase-change materials. Figure 6.6 illustrates a configuration developed in New Zealand that uses a wax phase-change material.

In this configuration, a phase-change material (PCM) is encapsulated in a spherical nodule about the size of a golf ball. The encapsulation keeps insects and air away from the PCM and prevents the PCM from mixing with circulated air. These spherical nodules are then placed in a tank about twice the size of the building’s hot water heater. Air flows through the tank and then through the house to provide cooling. Air is circulated between the air conditioner and nodule tank to freeze the PCM at night.

When used in Los Angeles, the peak load shifting reduced monthly electrical bills from $19,941–14,943. These savings were made possible because of a rate plan charging $0.11 per kWh for peak electricity and $0.061 per kWh for off-peak electricity. Using PCMs, air conditioning use went from 75% during peak demand times to 75% during off-peak times. When the tank storage is located in the building, these units can approach storage and recovery efficiencies of nearly 100%—far better than available methods for storing and recovering electrical energy.

For heating applications, the conversion of fuel to electrical power used during the winter increases base load demand at night. Shifting the electrical demand from peak load during the day to peak load during the night increases the annual base load. Even though the increase in base load may only be for a few months each summer to winter cycle, it can create the incentive to build more efficient power generation facilities by providing a higher annual base load (off-peak load) market.

Chilled water or ice storage systems are generally preferred for larger buildings or groups of buildings while PCM storage tanks are preferred for small buildings. The Ft. Jackson chilled water system was estimated to have a payback period of 5 years. A similar chilled water storage system at the Administration Center in Sonoma County, California, cut the electrical utility bills in half and saved an estimate $8000 per year due to reduced maintenance of the chillers that were sized at half capacity before installing chilled water storage [4]. Just as peak load shifting reduces the maximum peak loads for electrical power providers, peak load shifting can also reduce the peak chiller operation demands allowing use of less expensive units.

The US Department of Energy reports that federal government installations could save $50 million in electricity costs each year. Since ice or chilled water storage systems have been commercially available for over 50 years, several manufacturers and options are available.

For air conditioning, the greatest demands occur during the heat of a summer day when people are most active. The air conditioning load adds to and inflates the daytime peaking of electrical consumption that serves other activities. When heating during the winter, the coldest times are at night during off-peak hours. As a result, except for the storage of solar energy or when using ground source heat, storing thermal energy does not have the same benefits as storing chilled water.

Heat storage can eliminate fuel energy consumption when solar heat is stored. Usually, solar heating systems experience extended periods during the spring and fall when additional heat is not needed during the daylight hours, but heat may be necessary during the cool nighttime hours. For such systems, solar heating systems can be equipped with energy storage. Most solar heating systems do offer energy storage options.

The Role of Electrical Power in HVAC to Reduce Greenhouse Gas Emissions

Energy storage and HVAC energy consumption impact strategies to provide cheap, abundant, and environmentally acceptable energy. Well-planned government programs could create incentives necessary to build the next generation of higher efficient electrical power plants and substantially reduce the amount of fossil fuels consumed for heating.

The energy consumed in the United States for HVAC applications is about the same as that burned in gasoline engines; HVAC can play a role in reducing greenhouse gas emissions if this becomes a national priority. The 14 quadrillion BTUs of energy consumed for HVAC probably underestimates the impact HVAC can have on greenhouse gas emission reduction.

The impact of near-zero-greenhouse-gas technologies is increased when electricity demand is stabilized both on the 12-month cycle through increased use of electric-based heating and on the 24-hour cycle by energy storage. Increased annual base load electrical demand should serve as incentive for building the most efficient combined cycle natural gas power production facilities (operating at over 50% thermal efficiency) and can lead to a new generation of more efficient nuclear power plants.

These technologies can be used to reduce greenhouse gas emissions to 1990 levels and provide the transportation industry with time to develop new energy technologies to attain less than 1990 levels. These technologies and conversions are cost-effective when electrical providers and consumers share the cost benefits. When electrical power providers have mechanisms to pass their peak load reduction savings to consumers and the available technology is fully implemented, the transition can occur.

We hear messages from scientists that the dangers of climate change are real; little has been proposed as cost-effective solutions. For electrical power generation and HVAC, the quantity of energy involved is big. The technology is available and the knowledge and understanding to make the transition is known.

Example Calculations

The principle behind converting units is the mathematical axiom that any number multiplied by one is unchanged. Therefore, since the following is known to be true:

$1000\mathrm{g}=1\mathrm{kg}\mathrm{or}1000\mathrm{g}=1\mathrm{kg}$

Then

$1=\frac{1\mathrm{kg}}{1000\mathrm{g}}$

An example application of this conversion is as follows:

$140\mathrm{g}=140\mathrm{g}\times \frac{1\mathrm{kg}}{1000\mathrm{g}}$

The conversion is complete by recognizing that units, g, cancel.

$140\mathrm{g}\times \frac{1\mathrm{kg}}{1000\mathrm{g}}=0.140\mathrm{k}\mathrm{g}$

Table 6.4 summarizes commonly used conversions used in energy calculations.

Table 6.5 summarizes commonly used physical properties and abbreviations. The physical properties of gasoline and diesel fuel will vary based on the source of the petroleum, refining practices, and seasonal-specific formulations.