Question

10.5D A heat pump is under consideration for heating and cooling a 3600 -ft \(^{2}\) camp lodge in rural Wisconsin. The lodge is used continuously in the summer and on weekends in the winter. The system must provide adequate heating for winter temperatures as low as \(-23^{\circ} \mathrm{C}\), and an associated heating load of \(30 \mathrm{~kW}\). In the summer, the maximum outside temperature is \(38^{\circ} \mathrm{C}\), and the associated cooling load is \(44 \mathrm{~kW}\). The local water table is \(30 \mathrm{M}\), and the ground water temperature is \(14^{\circ} \mathrm{C}\). Compare the initial, operating, and maintenance costs of an air-source heat pump to a vertical well ground-source heat pump for this application, and make a recommendation as to which is the best option.

Short answer

Compare both systems' costs: Air-source heat pumps are cheaper initially but less efficient, while ground-source heat pumps have higher initial costs but lower long-term expenses. Recommendation: Ground-source heat pump is generally more cost-effective over time.

Step-by-step solution

- Determine Heating and Cooling Loads

Identify the heating requirement in winter and the cooling requirement in summer. Specifically, the lodge requires a heating load of 30 kW at -23°C and a cooling load of 44 kW at 38°C.

- Evaluate Air-Source Heat Pump (ASHP)

Assess the capabilities of an air-source heat pump. These typically have lower efficiency with very cold winter temperatures and very warm summer temperatures. Calculate efficiency or Coefficient of Performance (COP) for both conditions. For example, if COP is 2.0 in winter and 3.0 in summer, energy usage is derived by dividing load by COP.

- Evaluate Ground-Source Heat Pump (GSHP)

Assess the capabilities of a ground-source heat pump. These tend to have more stable temperatures and higher COPs due to consistent ground temperatures (e.g., water table at 14°C). Assuming a COP of 4.0 in both winter and summer, determine the energy usage similarly to Step 2.

- Calculate Operating Costs

Calculate the annual operating cost for each system. Consider energy consumption (kWh) times electricity rate. Summarize operating costs for both winter and summer usage for both ASHP and GSHP.

- Calculate Initial and Maintenance Costs

Research the initial installation costs for both ASHP (generally lower) and GSHP (often higher due to drilling costs). Additionally, account for annual maintenance costs. Summarize these figures for comparison.

- Comparison and Recommendation

Compare the summarized initial, operating, and maintenance costs of ASHP and GSHP. Determine the most cost-effective solution considering energy efficiency, long-term costs, and facility requirements. Generally, despite higher initial costs, GSHP may be more cost-effective over time due to lower operating costs.

Key concepts

Heating Load

The heating load represents the amount of heat required to maintain the interior of the camp lodge at a comfortable temperature during the cold months. This load depends on factors such as the outdoor temperature, insulation quality, and the size of the lodge. For this camp lodge in rural Wisconsin, the heating load is determined to be 30 kW at an outdoor temperature of -23°C. This is a crucial factor when selecting a heat pump, as the system must be capable of meeting this demand to ensure the lodge remains warm and habitable.

Cooling Load

Cooling load is the amount of cooling required to keep the lodge at a comfortable temperature during hot summer months. Factors influencing the cooling load include outdoor temperature, solar radiation through windows, and internal heat sources such as appliances and occupants. For the camp lodge, the cooling load is 44 kW at a maximum summer temperature of 38°C. Understanding the cooling load helps in sizing the appropriate heat pump system to effectively manage the high temperatures and ensure the lodge stays cool.

Coefficient of Performance (COP)

The Coefficient of Performance (COP) is a measure of a heat pump's efficiency. It is the ratio of heating or cooling provided to the electrical energy consumed. A higher COP indicates a more efficient heat pump. For reference:

• Air-source heat pumps (ASHP) typically have a lower COP in extreme temperatures, such as 2.0 in winter and 3.0 in summer.
• Ground-source heat pumps (GSHP) benefit from stable ground temperatures, maintaining a COP of around 4.0 year-round.

These values help to assess the energy consumption and operating costs of each type of heat pump.

Energy Efficiency

Energy efficiency of a heat pump is crucial for cost savings and environmental impact. Heat pumps with higher COP values are more energy-efficient. Ground-source heat pumps generally outperform air-source heat pumps in terms of energy efficiency due to stable underground temperatures.

• GSHPs are often more efficient in extreme climates, which leads to lower energy consumption.
• ASHPs might struggle in harsh weather, using more electricity to achieve the same heating or cooling effect.

Selecting a heat pump with higher energy efficiency can significantly reduce long-term operating costs and carbon footprint.

Operating Costs

Operating costs encompass the energy expenses incurred by running the heat pump. Calculating these costs involves multiplying the energy consumption (in kWh) by the local electricity rate.

• A typical ASHP with a COP of 2.0 in winter and 3.0 in summer will use more energy compared to a GSHP, which maintains a COP of 4.0.
• Lower operating costs of GSHPs over time can offset their higher initial installation costs.

Considering both the heating and cooling seasons is essential to estimate the total annual operating costs accurately.

Ground-Source Heat Pump

Ground-source heat pumps (GSHPs), also known as geothermal heat pumps, utilize stable underground temperatures to provide heating and cooling. These systems involve drilling to access the ground's consistent temperature, making them more efficient but also more expensive to install. Key benefits include:

• Higher COP, which results in lower energy consumption.
• Reduced operating costs over the long term.
• Stable performance in extreme temperatures.

However, the higher initial installation costs, mainly due to drilling, can be a barrier for some applications.

Air-Source Heat Pump

Air-source heat pumps (ASHPs) utilize ambient air to provide heating and cooling. These systems are generally easier and less expensive to install compared to GSHPs. However, they tend to be less efficient, especially in extreme temperatures, leading to higher energy consumption. Key points include:

• Lower initial installation costs.
• Fluctuation in COP based on outdoor temperatures.
• Potential for higher operating costs due to lower efficiency in severe weather.

ASHPs can be a viable option if installation budget is a primary consideration, but their higher operating costs in the long run should be carefully evaluated.