Dual-fuel Combisystem: The Benefits of Geothermal
The appeal of geothermal heat pumps continues to grow. These days, you can check out them in just about every concern of every HVAC trade publication. They provide a way to supply inexpensive heating along with cooling.
An increasing variety of state energy offices are officially recognizing geothermal heat pump as renewable energy sources. This allows them to get various monetary reward programs. Some electric utilities likewise are providing incentive programs to motivate their use. The federal government recognizes geothermal heat pump that meet Energy Star requirements as eligible for a 30 percent Residential Renewable Energy Tax Credit through 2016.
Heat pumps also dovetail nicely with solar photovoltaic systems in buildings going for net-zero status. In areas where net metering is readily available, surplus electrical energy generated by the solar PV system can be returned to the electric energy grid. The utility meter records this reverse energy circulation. In impact, the surplus electrical energy is offered back to the energy at full retail cost. It can then be redeemed, when needed.
This permits surplus solar-derived electrical energy to be kept on the electrical grid, maybe for several months, and later on drawn back through the building’s electrical meter without financial penalty. Therefore, it’s possible for surplus solar-derived electrical energy, created during times when there is little if any operation of the heat pump, to be utilized by the heat pump during the cold and dark of winter. This is an extremely significant and synergistic advantage, one that will end up being progressively common as the need for net-zero buildings increases.
A brilliant future for electrically driven heat pumps is supported by enhancing utility-scale power generation by large wind turbine farms and multi-megawatt selections of solar PV panels.
It’s possible to size a geothermal heat pump to offer the full design heating load of a house, there are other alternatives. One option that provides itself to modern-day hydronics technology is to integrate the heat pump with a boiler. This is frequently called a dual-fuel approach, and numerous advantages are connected with it.
For example, in some locations, energies provide steeply discounted time-of-use electrical energy rates during durations of low demand. These rates can substantially minimize the operating cost of a heat pump throughout off-peak hours while enabling a gas-fired boiler to meet need throughout peak periods, when electrical rates are substantially greater.
A dual-fuel system also provides the security that a person heat source can cover some or all the heating load when the other heat source is down for upkeep.
A dual-fuel approach allows for the possibility of sizing the water-to-water geothermal heat pump to less than the design heating load of the structure. This may be necessary due to restricted land area for installation of the earth loop. It likewise may be essential in circumstances where earth-loop setup costs are high.
The schematic in Figure 2 reveals one method to combine a water-to-water geothermal heat pump with a propane-fired mod/con boiler. This combisystem provides heating, cooling, and domestic hot water.
A valve manifold station works as the beginning and ending point for all parallel earth loop branches. The earth-loop circuit has an expansion tank in addition to a combined air and dirt separator. Flow through the earth loop is provided by a single digitally commutated motor (ECM) - based circulator, which runs whenever the heatpump’s compressor is on. This circulator can operate in fixed-speed mode or in a variable-speed mode, if a suitable controller is supplied that can enhance earth-loop circulation rate for a given operating condition.
The buffer tank gets heat from the heat pump as well as the propane-fired boiler. It permits both heat sources to run concurrently, if required, under high-demand situations.
Domestic water is heated up on demand using an external stainless-steel, brazed-plate heat exchanger. Whenever there is a need for domestic hot water of 0.6 gpm or higher, the flow switch inside the tankless electric water heater closes. This closure is used in mix with a relay to turn on circulator (P6), which moves water from the upper portion of the buffer tank through the primary side of this heat exchanger.
Closure of the circulation switch stimulates a contactor within the tankless heating system that supplies 240 vac to the heating components. The electrical existing provided to these aspects is controlled through solid-state gadgets called triacs, which are under the control of a microprocessor within the tankless heating unit. The power delivered is restricted to that required to offer any essential temperature boost between the preheated water leaving the brazed-plate heat exchanger and the preferred domestic hot water delivery temperature level. All heated water leaving the tankless heater flows into a thermostatic mixing valve to guarantee a safe delivery temperature to the fixtures.
The desuperheater heat exchanger within the heat pump likewise adds heat to the buffer tank whenever the compressor is running. This heat can be used for space heating or by the on-demand domestic water heating subsystem. When the heat pump is operating in cooling mode, the heat transferred to the buffer tank by the desuperheater is really free heat that otherwise would be dissipated to the earth loop. This allows the earth loop to run at somewhat lower fluid temperatures, which enhances the EER of the heatpump.
When the heat pump operates in heating mode, heat is moved from the refrigerant to system water through the condenser along with the desuperheater. This a little enhances the heat pump’s co efficiency of performance when compared with its typical modus operandi in which just the condenser is getting rid of heat from the refrigerant.
The space-heating distribution system uses panel radiators that are sized to operate at supply water temperature levels of 120 F at design load conditions. Each panel radiator is equipped with a thermostatic radiator valve that enables room-by-room temperature level change. All panel radiators are provided through a homerun distribution system utilizing- inch PEX or PEX-AL-PEX tubing from a common manifold station.
Circulation is produced by a variable-speed, pressure-regulated circulator set for continuous differential pressure operation. For simplicity, just 4 radiators are shown in Figure 2. However, a bigger manifold could be used to expand the distribution system to supply several more radiators.
Under partial-load conditions, the water temperature level required for space heating reduces based upon outdoor reset control. At 50 percent load, the water temperature level provided to the panel radiators just needs to be about 95. This considerably improves the coefficient of efficiency (COP) of the heat pump and the mod/con boiler, if it is utilized.
Operation of the space-heating distribution circulator (P5) is managed by the master thermostat (T1). If this thermostat is satisfied, circulator (P5) is shut off and no heat streams to the circulation system. This enables the entire building to be taken into a reduced temperature level problem mode from a single thermostat. During the heating season, the setting of the master thermostat should be 2 or 3 above the typical wanted air temperature level. This preserves circulator (P5) in operation and permits the specific thermostatic radiator valves to fine tune the convenience level in their respective areas.
Cooling is supplied by a single chilled-water air handler matched to the cooling capacity of the heat pump. All portions of the piping system that deal with chilled water are insulated and vapor-sealed to avoid condensation.
Throughout heating mode operation, a two-stage outdoor reset controller provides the logic that favors use of the geothermal heat pump when possible, but likewise invokes the boiler, when required. If the output from the heat pump suffices to keep the buffer tank temperature level at or close to the target supply water temperature level, the boiler will not run. However, if heat output from the heat pump cannot keep pace with heat removal from the buffer tank, the boiler will be turned on as the second-stage heat source. In this scenario, both the heat pump and boiler can at the same time include heat to the buffer tank.
It also can operate in heating mode if the fan switch on thermostat (T1) is set to on. This allows 24 vac to stimulate the coil of relay (RO), when thermostat (T1) calls for heat. If switch (SW1) is left open, the blower will operate whenever thermostat (T1) calls for heating.
This system shows how modern-day hydronics technology can be utilized to balance the operation of 2 high-efficiency heat sources one renewable and the other standard. It uses state-of-the-art hardware to take full advantage of the efficiency of both heat sources, provide room-by-room comfort control in heating, whole-house cooling, and provide on-demand domestic warm water. This system is sophisticated, however not excessively made complex. Perhaps you can find a building where to use it.