Read the following passage and answer the accompanying questions - Leaving Cert Physics - Question 11 - 2009

The bottom of a flat-plate collector is blackened to enhance its ability to absorb heat. A black surface is a good absorber of heat and radiation, increasing the efficiency of heat capture.

To calculate the energy required, we use the formula:
$Q = mc riangle T$
where:

• $Q$ is the heat energy in joules,
• $m$ is the mass of water (in kg),
• $c$ is the specific heat capacity of water (4200 J kg⁻¹ K⁻¹),
• $\Delta T$ is the change in temperature (50 °C - 20 °C = 30 °C).

First, convert 500 litres to kg (since 1 litre of water = 1 kg):
$m = 500 ext{ kg}$
Then, substituting the values:
$Q = (500)(4200)(30)$
Calculating gives:
$Q = 63000000 J$ or 63 MJ.

The liquid has a large specific latent heat of vaporisation because it allows for a significant amount of energy to be absorbed without a temperature change. This enables efficient heat transfer in the heat exchanger as the liquid transitions from liquid to vapor during the solar heating process.

The three ways heat could be lost are:

1. Conduction
2. Convection
3. Radiation

In a vacuum-tube solar collector, the silvered walls prevent radiation from escaping and keep the heat inside. The evacuated walls also minimize conduction and convection, enhancing heat retention.

A heat pump operates by transferring energy taken from one body to another. It extracts heat from a cooler area (making it colder) and moves that energy to a hotter area (making it hotter), usually involving a refrigerant that absorbs and releases heat during phase changes.

A geothermal heating system functions all the time, providing consistent energy regardless of the time of day or weather conditions, while a solar heating system only works during daylight hours and can be variable based on atmospheric conditions.