Secondary Outside Air Path for Low-Ambient Heat Pump Heating
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Solution Overview
Problem
Heat pumps are limited in operating at low ambient conditions and cannot efficiently utilize waste heat for heating in cold climates due to compressor window limitations and reduced heat transfer efficiency in heat reclaim coils.
Innovation Solution
A heat pump design with a direct expansion (DX) coil and an additional outside air airflow path that bypasses the DX coil, allowing operation in both heating and cooling modes, and optionally includes a heat reclaim coil with the second outside air airflow path upstream of it to enhance heat reclaim efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the heat pump operates in heating mode with high outside air volumes required in cold climates, then the heating capacity is improved, but the compressor operating window limitation prevents operation below 40°F to 50°F DX coil entering air temperature
Solution Approach 1:
The outside air inlet is divided into two separate inlets: a first outside air inlet that delivers air over the DX coil for heating, and a second outside air inlet that delivers air bypassing the DX coil. This segmentation allows independent control of air streams to maintain DX coil entering temperature above the 40°F-50°F limitation while still providing high total outside air volumes for heating capacity.
Solution Approach 2:
Different quality air streams are created by directing outside air through different paths: one path heats air to appropriate temperatures for DX coil operation, while another path provides unheated outside air for ventilation. This local quality differentiation enables the system to operate reliably in cold climates while maintaining heating capacity.
2Adaptability or versatility
If the heat reclaim coil is located downstream of the DX coil to provide supplemental heating and reheat during cooling mode, then the dehumidification function is improved, but the heat transfer efficiency of the heat reclaim coil is significantly reduced during heating mode due to higher entering air temperature
Solution Approach 1:
The air handling system is segmented into separate paths: outside air from the second inlet bypasses the DX coil and flows directly over the heat reclaim coil, while return air flows over the DX coil. This segmentation allows the heat reclaim coil to receive cooler outside air for more efficient heat transfer during heating mode, while still providing dehumidification capability when needed.
Solution Approach 2:
Outside air is pre-cooled by bypassing the hot DX coil and flowing directly over the heat reclaim coil during heating mode. This preliminary action of delivering cooler air to the heat reclaim coil maximizes the temperature differential and heat transfer efficiency for waste heat recovery.
3Reliability
If the second outside air airflow path bypasses the DX coil, then the heat pump can operate at low ambient conditions, but the system complexity increases with additional airflow paths and inlets
Solution Approach 1:
The system uses two separate outside air inlets to create distinct airflow paths: one path for heating air over the DX coil and another path for bypassing the DX coil. This segmentation enables low ambient operation while keeping each individual path relatively simple and manageable.
Solution Approach 2:
The dual outside air inlet configuration serves multiple functions: it enables low ambient operation by providing a bypass path, maintains heating capacity by allowing high outside air volumes, and optimizes heat reclaim efficiency by delivering cooler air to the heat reclaim coil. This multi-functionality justifies the increased complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables heat pump operation at low ambient conditions, prolongs heating season operation, and maximizes waste heat utilization, maintaining high heating efficiency and extending the lifespan of the DX coil by maintaining high entering temperatures for the heat reclaim coil.
Implementation Method 1
a DX coil that operates as a condenser coil for heating air passing over the DX coil when the heat pump is operated in the heating mode
Implementation Method 2
the second outside air airflow path is upstream of the heat reclaim coil for delivering outside air over the heat reclaim coil after bypassing the DX coil
Data Source
AI summary
A heat pump is provided that is alternately operable in a heating mode for heating an interior space under cold or cool ambient conditions and in a cooling or dehumidification mode for cooling the interior space under warm or hot ambient conditions. The heat pump has a direct expansion (DX) coil that operates as a condenser coil for heating air passing over said DX coil when the heat pump is operated in the heating mode. The heat pump has a first outside air path that delivers outside air over the DX coil. The heat pump additionally has a second outside air airflow path that bypasses the DX coil. The heat pump optionally includes heat reclaim coil. When the heat pump has a heat reclaim coil, the second outside air airflow path is upstream of the heat reclaim coil for delivering outside air over the heat reclaim coil.