Hot water: heat pumps vs solar

Technical Comparative Analysis of Water Heat Pump Efficiency and Photovoltaic Solar Water Heating for Residential Applications in New Zealand

Abstract

Water heat pumps are widely promoted as energy-efficient solutions for domestic hot water heating. However, their performance is highly sensitive to ambient temperature and mechanical degradation over time. This paper evaluates the real-world efficiency of water heat pumps under low-temperature conditions and contrasts their operational limitations with photovoltaic (PV) solar water heating systems. A case study of OneEnergy.nz’s solar thermal storage solution demonstrates superior long-term performance, reliability, and affordability for Kiwi households.

1. Introduction

Domestic hot water heating accounts for a significant portion of residential energy consumption in New Zealand. While hot water heat pumps have gained popularity for their perceived efficiency, their mechanical complexity and temperature-dependent performance raise concerns about long-term viability. In contrast, PV-based solar water heating - particularly the system developed by OneEnergy.nz - offers a passive, durable, and cost effective alternative.

2. Water Heat Pump Efficiency Under Low Ambient Temperatures

Water heat pumps operate by extracting heat from ambient air and transferring it to water via a refrigerant cycle. Their efficiency is measured by the Coefficient of Performance (COP), which typically ranges from 2.5 to 3.5 under optimal conditions (Often claimed to be as high as 4 with ideal laboratory stable test conditions). However, in colder climates or during winter months, COP values can drop below 2.0 due to reduced thermal gradients and increased compressor workload.

2.1 Temperature Sensitivity

• At ambient temperatures below 10°C, heat pump efficiency declines sharply.
• Defrost cycles and auxiliary heating elements further reduce net energy savings.
• Night time operation exacerbates inefficiency due to lower air temperatures.

2.2 Mechanical Wear and Lifespan

• Heat pumps contain compressors, fans, and refrigerant lines subject to mechanical wear and corrosion.
• Typical lifespan is 8–12 years, with maintenance requirements increasing over time.
• Noise pollution from fans and compressors is a common residential complaint.

3. Photovoltaic Solar Water Heating: A Passive Alternative

PV solar water heating systems convert sunlight directly into electricity, which is used to power an immersion heating element in the hot water cylinder. OneEnergy.nz’s system utilizes 2.7kW of solar panels to heat water during daylight hours, storing thermal energy for use in the evening.

3.1 System Design and Operation

• Solar panels generate DC electricity, routed directly to the immersion heating element.
• The existing hot water cylinder serves as a thermal battery for heat and energy storage.
• Energy is stored passively and silently.

3.2 Longevity and Reliability

• Solar PV panels carry a 25-year manufacturers performance warranty and are designed for >25 years of operation.
• No mechanical wear, no refrigerants, no compressors or fans and no noise.
• Maintenance-free operation with consistent performance across seasons.

4. Comparative Analysis

5. Conclusion

Water heat pumps, while theoretically efficient, suffer from significant performance degradation when operated outside of optimum temperature environments and require ongoing maintenance. Their mechanical nature limits lifespan and introduces operational noise and grid reliance.

In contrast, OneEnergy.nz’s photovoltaic solar water heating system offers a passive, silent, and long-lasting solution tailored to New Zealand’s residential needs. By leveraging existing infrastructure -namely the hot water cylinder - as a thermal battery, this system delivers reliable hot water without the drawbacks of mechanical heating technologies.

For households seeking sustainable, low-maintenance energy solutions, Solar PV water heating represents a superior alternative to conventional heat pumps.

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