The core of the house heating system is its "Solar Heat Pump" which uses a heat pump water heater (HPWH) to provide domestic hot water, space heating and supplemental space cooling. The solar heat pump uses waste heat harvested from beneath the photovoltaic array to enhance the heat pump water heaters heating efficiency. Removing the warm air from beneath the PV array may also increase the efficiency of the PV array. To optimize daily energy collected, the heat pump operates when the air under the solar collector is at its warmest. The energy collected is stored as hot water and as heat in non-passive solar heated portions of the concrete slab floor.
The system draws warmed air from under a solar electric array and moves it across the evaporator coil of an air-to-water heat pump. This enhances the performance of both the heat pump and the solar electric array. Hot water is then stored and used for both domestic and space conditioning. We explored three heat pump configurations, three solar array configurations, and two control strategies. All system configurations have the following common elements:
- A solar plenum
- An air-to-water heat pump
- Energy storage
Figure 1 shows a simplified air and water flow for the system. Airflow to the heat pump evaporator can come from under the solar array and be exhausted, or it can be drawn from occupied space and returned to occupied space as cool air. None of the designs considered would bring air from the solar plenum into the occupied space.
During the cooling season, the heat pump can be used to provide cooling to the space while heating water for domestic use. The warmest air inside the home can be drawn through the heat pump evaporator, cooled and delivered back into the space. This will provide roughly 48,000 Btu of cooling per day, which is sufficient to meet the design cooling loads in Oregon, while at the same time heating water for domestic use.
Solar photovoltaic electric power generation is a proven and reliable power source with great reductions in cost presently occurring. Photovoltaic power systems currently provide power for over a million structures worldwide. The application of photovoltaic power has traditionally not been applied to heating and cooling systems because of the intense electrical loads required by the equipment. Historically photovoltaics have been used for lighting loads in residences or commercial buildings. This project is an effort to demonstrate the potential of photovoltaics for heating and cooling equipment.
The potential for heating and cooling photovoltaic applications has been greatly enhanced by the integration of the ground source heat pump and the use of thermal storage. These two elements of mechanical system integration, ground source heat pump and thermal storage, are required to bring the economics of a photovoltaic powered heating and cooling system into feasibility.
The solar assisted heat pump provides an operating coefficient of performance from 2 to 8 times that of direct electrical power. A 100 watt photovoltaic panel can only power a 100 watt light bulb whereas a 100 watt photovoltaic panel can provide 400 watts of heat from a heat pump with a coefficient of performance of four.
The thermal storage system allows energy to be stored in water. In this system an existing four ton gas/electric heating and cooling system is being replaced with a 1 ton heat pump because the energy is being stored in water tanks. The water is then circulated across a fan-coil to provide the load. The reduction in peak system size makes the concept of photovoltaic power appropriate for heating and cooling system. Not only is the mechanical system size reduced in size to 1 tons but the required capacity of photovoltaic panels is reduced by the same amount.
The system is a grid tied system to take advantage of the electrical storage capacity of the grid. During start-up of motors and fans in the system a starting current of as much as four times the run current is required. If the system were not grid tied, the photovoltaic panels would be required to be sized that much larger to provide starting capacity.
The purpose of the system grid-tie capability is not for selling power back to the utility. The power created by the photovoltaic panels must be used for heating and cooling to take advantage of the COP.
Thermal Storage Water Tank
The concept of storing energy (thermal storage) as hot or cold water or as ice is already commonly used in many industrial and commercial building systems. Many commercial and industrial buildings use thermal storage as a way to offset utility rates off a peak period and allow the equipment to run more efficiently. Typically in these systems water surrounding refrigerant tubing in large tanks is frozen to store the cold thermal energy over night. This cold ice water is then circulated through the building or cooling process during the high load peaks of the day.
Energy is saved in two ways in these types of systems. Electrical power costs required to operate the system is lower at off-peak times. Mechanical equipment runs more efficiently at lower temperatures such as the night when ambient temperatures have dropped greatly.
In this mechanical system, one 120 gallon and one 80 gallon hot water tank is used for storage. The 120 gallon maximum size was driven by the code requirement that any tank over 120 gallons must be ASME rated greatly increasing the cost.
The design solution for thermal storage emerged around the use of common electric water heater tanks. The most cost-effective configuration would use either 80-gallon or 120-gallon sizes. The 80-gallon electric water storage tanks with 12-year warranties have a retail cost of around $350. The 120-gallon tanks cost nearly twice as much. Tank sizes large than this required additional testing. In addition, because they are manufactured in much smaller quantities, they are consequently more expensive. To build a storage system larger than about 240 gallons would result in substantially increased floor space and standby losses.
At 150 F, our two tank storage system (~200 gallons of water) is capable of providing 100,000 Btu of useful energy for space heating. Thus, if our heat pump is capable of making 150 F during the daytime, we could avoid running the heat pump water heater during the night, when its COP is lowest.
Figure 4 shows final storage configuration for heat pump Option B. A tempering valve is shown on the output of the backup tank to prevent scalding. A small electric backup tank (20 to 30 gallons) is needed because the space heating system may draw the water temperature down below 100 F, which is too cool for domestic hot water.
How much storage is sufficient to meet 80 to 95 percent of the demand and to prevent the heat pump from running during the coldest part of the day? The answer depends largely on the homes domestic hot water and space heating loads, the maximum temperature the heat pump can heat to, and how important it is to only collect heat during the daytime when solar pre-heated air can be fed to the heat pump.
To simplify the design, we have assumed that such a system would be installed in very well-insulated home with a net effective UA value of around 260 Btuh/ft2-F. The heat pump operating characteristics will determine the minimum operating temperature and the maximum stored water temperature.
If energy efficiency was our only consideration, water should be heated to the lowest possible temperature storage, thereby reducing losses, and keeping the heat pump from having to make water at higher temperatures where its COP is low. This is impractical, however, as it would limit the space heating system to low-temperature radiant floor systems or require the use of very large storage systems.
To get a general sense of how much storage is needed, consider how much energy is needed on a 90 percent design condition day in a 5000 HDD climate. Domestic hot water heating amounts to about 40,000 Btu per day. Daytime space heating loads are about 70,000 Btu and nighttime space heating loads are about 150,000 Btu. Thus, we would need approximately 260,000 Btu/day for this well-insulated light frame home. During the daytime, the heat pump will need to provide all space heating and water heating needs and still be able to charge the water storage tank to full temperature.
Space-heating energy can be delivered to the home in a variety of different ways. Energy stored as hot water may be delivered to rooms through the use of radiant floor circuits, individual fan-coils, hydronic baseboard units, a central air handler and hydronic coil, or any combination of these. Because of the net energy goal, however, the amount of electricity used by and losses in the delivery system(s) have to be carefully considered.
The Rose House uses two modes of heat delivery:
- An "energy recovery ventilator
- A radiant floor heating system
An energy recovery ventilator is a high efficiency heat transfer device that allows the transfer of the warm air exhausted from the house to the cool air coming from outside the house. The Stirling unit is so efficient that it can transfer nearly 90% of this heat.
The house is so tightly constructed against air leakage that it is necessary to draw outside air in to the house. This outside air is then preheated by the ERV. A hot water heating coil heats the air further on the way to the house when it is required.
After careful review of the current ERV models, a Stirling 200DX was chosen for this system. The 200DX model can use variable speed fan motors to deliver between 70 and 230 cfm of air. This ERV has several features that make it the best choice for this work:
- Energy recovery efficiency is above 90 percent at low speed and above 70 percent at high fan speed;
- Three ECM motors are used for fans and the heat exchange wheel allowing it to draw only 43 watts to deliver 70 cfm in low speed;
- A variable air volume can be easily set up by using the 0-10 vdc inputs to the fan speed motor control;
- The unit can be placed in "night-flush" mode. This is a night-time cooling mode where built-in sensors and software manage the economizer function.
The ERVs "night-flush" is unique among HRVs (Heat Recovery Ventilation) and ERVs cooling requirements. This is accomplished by stopping the heat transfer wheel from spinning and allowing air to be drawn into the house and exhausted without heat transfer. The ERV automatically monitors indoor and outdoor temperatures to prevent air from being drawn into the house unless it is cooler than indoor air. The night-flush mode stops once the interior temperature reaches 62 F.
To space heat with the ERV, a hydronic coil is added downstream of the ERV supply fan and a pump to respond to thermostat calls for space heating (or cooling). Several different companies are capable of providing small area three or four-row coils for less than $200. These specify coils that deliver more than 10,000 Btu/hour at 200 cfm and 120F entering water temperature at 5 gpm, while imposing an external static pressure penalty of less than 0.1 inch w.g. on the air side.
A limiting factor with this ERV/coil approach is ensuring that the delivered air temperature is warm enough for occupant comfort yet use the lowest possible water temperature. The big concern for this system design is in the morning when heat is being drawn for space heating, that the water is warm enough to meet shower needs.
Backup Heating System
Because of the significant downsizing of house loads and the systems used to meet them, 100 percent design temperature conditions, especially in colder climates, are a much larger challenge to system capabilities. For now, we generally see some rather conventional backup heating strategies to meet the last increment of space heating demand as being essential.
At a minimum, the electric resistance elements in the hot water storage tanks will serve this function. Some households will use strategically placed zonal electric units, wood heat, or a natural gas-fired appliance for backup heat.