BSP-031: Designs that Work: Hot-Humid Climate (Lake Charles, LA)
Section 3: Advanced Technologies for Hot-Humid Climate
Base energy reductions strategies are for the most part easy to incorporate into residential production building. The technologies are very similar to many traditional construction practices, so training construction crews to adopt slight variations to normal techniques, while not always easy, is at least feasible. Usually a short learning curve is required at the beginning, however, once the techniques are adopted, savings can sometimes be made from less material handling and installation time. These base techniques are also more easily justifiable from a cost analysis point of view.
As we push for more and more energy efficient homes, the limits of some of the base strategies begin to be stressed. Further increases in insulation levels become less practical, achieving increased air tightness becomes more difficult, and efficiencies of equipment begin to reach the limit of current technology.
At this point, additional energy saving strategies should be examined. Some of the more advanced strategies that are currently gaining in popularity are the use of Geothermal Heating and Cooling Systems, Solar Hot Water Systems, and Photovoltaic Technologies.
Energy Analysis Overview
The case study house was modeled with the following additional energy consumption reduction and energy generation strategies. The strategies were modeled individually to demonstrate the relative impact of each. The final row highlights the total whole house energy consumption reduction if all of the strategies are applied together.
The case study model design achieved a whole house 64.2% energy reduction when all the advanced strategies were employed at the same time compared to the Building America Benchmark.
On the basis of BTU/sf/yr of site energy, the above calculations yield the following:
Geothermal Heating and Cooling
Geothermal (Ground Source) Heat Pumps work similar to air source heat pumps, except the energy is transferred to the ground instead of to the atmosphere. The higher efficiency that can be achieved from a geothermal heat pump is due to the relatively stable ground temperature during the heating and cooling seasons compared to the variable air temperature of air source heat pumps. While these systems are more efficient the standard air source heat pumps or air conditioning units, they are also more expensive to install and will run upwards of $5,000 to $10,000 for the installed system.
Figure 30: Schematic of a Ground Source Heat Pump
There are three types of geothermal heat pumps, open loop, closed loop, and direct exchange (DX).
Open Loop
In an open loop system, ground water is used as the heat exchange fluid between the ground and the refrigerant loop. Water is drawn out of the ground and circulated through a heat exchange tank containing the refrigerant line. This is not a very common system due to the potential problems with dirt and debris and general water quality issues that may be encountered from using the natural ground water.
Closed Loop
The closed loop system is the most common system used with geothermal heat pumps. This system uses plastic tubing that is run through either vertical or horizontal wells to transfer the energy to the ground. While similar to an open loop system in that the water is circulated through a heat exchange tank containing the refrigerant line of the heat pump, in this system the water used is not connected to the ground water, but instead run in the plastic tubing.
This controls the water quality used in the system, and therefore reduces the potential for problems and maintenance. In heating climates, there is a concern for freezing of the system and therefore some form of anti-freeze will need to be added to the ground loop system.
Direct Exchange
Direct Exchange systems run the refrigerant line directly into the ground, eliminating the heat exchange tank. Because this extra heat transfer step is eliminated from the design, the system should be more efficient. In this system, copper lines are installed into the ground and the refrigerant of the system is circulated through them. Copper, due to its higher thermal conductivity is better able to exchange the heat with the ground when compared to a water circulated system with plastic pipe. While more efficient, there are some considerations that need to be made.
The cost of the system is higher due to the use of copper tubing instead of plastic tubing. Depending on the number and depth of the wells required, this can create a significant cost to the system. The system also has to be site charged with refrigerant, so unlike factory built closed loop GSHP systems, the efficiency is based on the quality of the installation.
Design Considerations
In order for the system to perform properly there must be adequate heat exchange with the ground. The heat exchange is through either vertical or horizontal wells in which the heat exchange fluid is circulated. A general rule of thumb is that a 200 ft ground well is required for each ton of cooling needed. Therefore, for a 3 ton cooling load, three 200 ft wells, would be needed.
From the heat pump side, there are generally two systems currently being used on the market, a packaged system and a split system.
In the packaged system, the compressor and heat exchange tank to the interior of the house and integrated into the air handler. The benefit of this system is that the charging of the refrigerant line is all done in the factory under controlled conditions and it is a fairly simple installation and connection to the ground loops at the site. On the other hand, the compressor is now inside the house, and issues with noise can sometimes occur.
Split systems place the compressor and heat exchange tank on the exterior and the refrigerant line is run to the air handler as in more conventional air source heat pumps. This reduces the noise inside the house; however the refrigerant charge must now be determined on site by a mechanical contractor.
Energy Model Results
The system used in the energy model is based on the specifications of a ClimateMaster Genesis Packaged Unit. The efficiency of the system is based on the entering water temperature. Therefore the performance of the system used in the energy model was based on the expected entering (returning) water temperature in the both the heating and cooling seasons. This entering water temperature is a function of the average ground temperature and the heat transfer efficiency of the ground. This resulted in a 17 EER for cooling and a 4.0 COP for heating. The resultant incremental whole house energy consumption reduction was 3.3%.
Solar Hot Water
The incorporation of domestic solar hot water system into residential homes has become increasingly popular over the last several years. The basic concept of all solar hot water systems is to use the sun’s energy to heat or preheat water, thereby reducing the gas or electric requirements to produce hot water.
In general all solar hot water systems have a solar collector (to collect the sun’s energy), and a storage tank (to store the hot water). From this however, the systems can be separated into two different categories, active and passive systems.
Active systems rely on pumps and valves to circulate the water or heat exchange fluid through the solar collector, while passive systems rely on the natural tendency of water to rise when heated, and thereby circulate through the system.
Figure 31: Schematic of a Closed Loop Solar Hot Water System
While active systems are slightly more complicated than passive systems, they can be more flexible in terms of the placement of the components since the location of the storage tank is not dependent on the physics of hot water buoyancy. On the other hand, passive systems, because of the lack of pumps have been argued to be more durable and less prone to problems.
Active Systems
There are three main types of active systems, direct, indirect, and drain back.
With direct systems, the domestic potable water is circulated directly through the solar collector. The pump circulates the water from the storage tank through the solar collector when the temperature of the solar collector is greater than that of the tank. Direct systems are generally not recommended for climates where the exterior temperature drops below freezing or for areas that have hard or acidic water.
For areas where freeze protection of the system is important, the recommended systems would either be an indirect (closed loop) or drain back system. The indirect (closed loop) systems use a propylene glycol heat exchange fluid in the solar collector. The low freezing temperature of the propylene glycol provides the freeze protection for the system allowing the solar systems to be used in climates prone to longer freezing times. These indirect systems require a check valve to prevent reverse thermosiphoning at night, since the hot water in the tank could convect heat back up to the typically roof mounted solar panels.
The drain back system uses water as the heat exchange fluid. In order to provide for freeze protection, the pump shuts off when the temperature of the collector cools down below that of the tank, and the water in the system “drains back” into storage reservoirs. The panel then fills with air protecting the system from freezing when the pump is turned off.
For both indirect and drain back systems, the solar collection loop is run to a heat exchange coil around a water storage tank. In that way, the systems are decoupled from the potable water delivered to the house.
Passive Systems
There are generally two types of passive systems; thermo-siphon, and integral collector storage.
A thermo-siphon system uses the tendency of water to rise as it is heated. In this system a storage tank is installed at elevation above the collector. As the water is heated, it becomes lighter, and naturally flows up and into the top of the storage tank. The cooler water from bottom of the tank flows down pipes to the bottom of the collector, creating the circulation through the system. As the temperature in the panel drops below the temperature of the storage tank, the circulation through the system stops as well. This prevents the cooler night time temperatures from removing heat from the system.
Thermo-siphon systems can also be designed with a closed loop and heat exchange fluid as well, in areas where freeze protection is required.
In the integral collector storage system, the storage tank is integrated into the solar collector. The cold water supply is connected directly to the collector. As water enters into the panel it is heated up by the sun. However, unlike other systems, the water remains in the panel until there is a call for hot water, and then the water is drawn directly from the panel to fulfill the demand. Since the hot water is stored in the panel, integrated systems require larger storage tubes in the collector (to increase collection ability) than a normal direct system, which also helps prevent freezing. This is likely the simplest solar hot water system available.
Design Considerations
The solar collectors should be placed on the South side of the building with the optimum tilt for the collector to be set to the azimuth angle for the location of the house. This is to provide the best year round performance of the system.
Due to the potential for high temperature water leaving the solar hot water system, a mixing valve must be installed on all systems to regulate the water temperature delivered to the house, and prevent any concerns about scalding. In addition, it is generally required to install some means of providing back up heat with any solar hot water systems to ensure that hot water demands can be met all year round. The simplest way to provide the back up heat is with a small electric heating coil inside the storage tank. Alternatively, instantaneous water heaters can also be used. If instantaneous water heaters are used for a back up, they must be designed to handle the potentially elevated water temperatures from the solar panel.
Finally, in a hot humid climate zone, the potential for freezing of a collector is relatively small, and therefore the simpler passive ICS style collector can be used. Not only do these units tend to be less expensive, they save interior floor or storage space. An ICS DHW system coupled with a tankless electric hot water heater basically avoids taking any floor space for water heating with solar back-up.
Energy Model
The system used in the energy model is based on an integrated collector storage (ICS) system, similar to CopperSun by Sun Systems. The collector is oriented to the South and the angle was set to the angle of the roof slope in order to approximate the most realistic installation of the panel on the roof. The resultant energy savings was a 7.9% decrease in the overall whole house energy consumption.
Photovoltaic Panels
Photovoltaic (PV) Panels are used as a means to generate on site energy. The panels are relatively easy to integrate into the design of the house and power system, and are a means to reduce source energy consumption. One of the draw backs are that at this point in time is that the cost of PV panels, while lower than a few years ago, still does not make them cost effective from a payback point of view. The amount of energy generated takes many years to pay off the initial cost of the panels. However, as the use and demand for PV technology increases and further advances in the technology increase the performance of the panels, the costs will continue to drop, making the technology more viable financially.
Figure 32: Schematic of a Photovoltaic System Design Considerations
Photovoltaic systems require a collector panel and an inverter in order to produce electricity that is able to be used by the home. Photovoltaic systems are either connected to a battery storage system located on site, or connected into the power grid of the community. For locations where connection to a power grid is not available or impractical, then a battery storage system is desirable. Battery storage systems however, do require maintenance to ensure that they continue to function adequately. Tying into the local power grid is generally recommended over battery storage when possible, due to the simplicity and costs. This removes the concerns with maintenance of the battery systems.
Design Considerations
There are several aspects of the design of photovoltaic systems that can significantly affect the performance of the system. The location and angle of the collector, internal losses, shading, and temperature should all be considered in the design of the system.
The collector plate should be installed on the South side of the building. Variations within 15 degrees of true South will create relatively little change in the performance of the panels, however, beyond 15 degrees the performance starts to drop off significantly. Also, setting the tilt of the panel to maximize the summer time solar incident angle can increase the energy production of the panel over the course of the year. This can be more difficult than it seems as aesthetic issues often come into play. It may not always be desirable to have the panel in a location of high visibility, and architectural design may limit the options for the collector tilt angle. If PV technologies are going to be incorporated into the design, it should be considered early on in the conceptual design stage, so that systems could be properly integrated into the aesthetic design of the building.
Most systems will experience some internal losses in the system, and only reach approximately 80% to 90% of the rated output of the panel at a maximum. The losses are from dirt, dust, the resistance in the wiring, elevated temperature of the panels, and losses through the inverter. This is common for most systems and should be accounted for in the design of the system.
Even the least bit of shading of the panels can dramatically decrease the performance and close attention to keeping the panels in direct sunlight is very important. This is due to the way the photosensitive cells are linked in the array. Therefore it is very important that the panels are placed in a location such that surrounding elements (such as trees and chimneys) do not cast a shadow over even a portion of the panel. Ideally, the panels would also be cleaned with some regularity of dust, leaves, snow, or any other matter that might get deposited on the solar collector.
The performance of the panels is also affected by temperature. As the temperature of the panel increases, the output of the panels is reduced. Therefore it is important to try to keep the panels as cool as possible. One strategy is to install the panels slightly off the surface of the roof, to allow for some ventilation behind the panel.
Energy Model
The system used in the energy model is based on a 1.9 kW photovoltaic system (Similar to SunWize Packaged PV system including a Sanyo 190BA3 Solar Module and a Fronius Grid-Tie Inverter). The area of panels required for this system was equivalent to 127 square feet or 10 panels. The amount of site generated energy was able to make up 13.9% of the whole house energy consumption.
Towards Zero Energy
With the advanced technologies described above, the Hot-Humid Case Study House reaches an impressive 64.2% reduction in energy use when compared to the Building America Benchmark. However, as uncertainty grows around our dependency on fossil fuel-based energy, even greater steps to reduce residential energy use are a priority. In response, the Building America program has established the goal of creating houses that generate as much energy as they use.
A Zero Energy Home (ZEH) is designed to balance energy consumption with site energy collection and conversion so that there is no net energy usage during normal operation of the house. In practical terms this means that over the course of the year, the homeowner’s energy consumption from the utility will be zero.
On the other hand, a Zero Cost Home (ZCH) would be a home that had no utility bills, and would need it’s own battery back-up systems, etc. to avoid utility service fees, and not have to worry about net metering being yearly or monthly, etc.
Design Considerations
The Advanced Technologies section above gives the first steps in making use of the available energy on the site to meet the remaining demand. The geothermal system, the solar hot water system and the photovoltaic panels have been chosen in that order, because they provide the most rational payback period for the energy collected. The final step to reach zero energy is to add significantly to the photovoltaic array.
With the previous sections of this report, the design strategy of looking first for ways to reduce the energy used by the house and then providing power generating capacity to meet the remaining demand. Having maximized the conservation aspects with this house design, reaching for Zero Energy is now left up to sizing the PV collection array based on reasonable assumptions of conservative usage. Therefore, the first and most important steps the design of a ZEH involve decisions that are made by (or for) the homeowner. To start with, the future occupant needs to be made aware of the energy conservation strategy. Experience with utility studies of energy efficient homes has demonstrated that the energy intensity of the homeowner’s lifestyle can make a significant difference in the overall utility use, by a factor of 3:1.
The energy reduction plan will include the choice of building site and the orientation of the house on the property (as discussed previously), as well as attention to energy-saving practices such as using the thermostat to control indoor conditions (as opposed to windows), using reasonably conservative set points for the heating and cooling systems and turning electrical devices off when not in use (rather than leaving them on the standby setting). These lifestyle-related changes made by, , the homeowner should be considered in concert with the energy load reduction by the building enclosure and mechanical system design described in Section 2 of this package.
Figure 33: Arrangement of Photovoltaic Array on ZEH
Energy Model Results
*Note that the energy cost of $0 doesn’t include monthly fees, etc.
In Conclusion
A house must be able to provide satisfactory service in its particular location on a number of different fronts, including occupant comfort, functional program needs, moisture and thermal performance, and durability.
In the preceding document we've shown you the results of a design process that takes into consideration aspects of building science as they relate to a hot-humid climate, as well as energy conservation measures that can be implemented today. We've presented strategies that can bring further reductions in energy use through the use of higher efficiency mechanical and solar collection equipment. And finally, we have discussed the strategy and sizing changes necessary to reach a Zero Energy Home.
With the plans available in this document, you can decide on the level of energy conservation versus cost that makes sense to you, and proceed with building a high-performance home appropriate for a hot-humid climate.
