What is renewable energy?
Renewable energy is energy which is obtained from natural resources that are replenished as fast as they are used. Around 40% of New Zealand’s primary energy and 75% of New Zealand’s electricity is supplied from renewable energy sources. The following sections describe the different types of renewable energy and their potential costs and benefits.
Hydroelectricity is a major source of energy in New Zealand. Over 50% of New Zealand’s electricity is generated from hydrogeneration, mostly through large hydro dams such as Benmore, Manapouri and Clyde.
Hydroelectricity uses gravity as its energy source. As water flows downstream it creates kinetic energy which can be converted into electricity by directing the water through a turbine that drives a generator. The water is returned to the river or stream below the hydro plant.
- Transformation of the upstream environment from a riverine ecosystem to a reservoir
- Loss of natural variation in flows
- Sediment is trapped, transforming the downstream environment (including the coastline)
- Fish migration is blocked, with consequential effects on reproductive success
Most of these impacts can be mitigated to varying degrees of success.
New Zealand straddles the roaring forties and both main islands lie across the prevailing westerly winds. As a result, New Zealand has one of the best wind resources in the world. Wind energy can be used to turn the blades of a turbine, which spin a shaft, which connects to a generator and makes electricity.
Wind energy produces no air pollution or greenhouse gas emissions during operation. The key environmental impacts associated with wind farms are visual and noise impacts.
Geothermal energy is derived from the heat at the core of the Earth. New Zealand’s location on an active plate boundary means it has an abundant supply of geothermal resources. New Zealand was one of the first countries in the world to develop large-scale geothermal electricity generation in the 1950s.
- Geothermal fluid (pressurised water and steam heated to between 200oC and 300oC and containing dissolved minerals and gases) is drawn from a geothermal field by production wells at depths of 1 – 3 kilometres.
- The steam is used to spin a turbine which generates electricity.
- The remaining geothermal fluid is used to heat pentane (a hydrocarbon with a boiling point of 34oC) to produce gas that is used to spin a secondary turbine to produce electricity.
- The waste fluids may be injected back into the geothermal field or disposed of in rivers.
- Geothermal fluids contain dissolved gases, including carbon dioxide a greenhouse gas. However, utilisation of geothermal resources releases less greenhouse gases per unit of energy produced than fossil fuels.
- The extraction of geothermal fluids can reduce the pressure in underground reservoirs causing subsidence.
- Geothermal fluids contain minerals, including arsenic, mercury, lithium and boron. If waste is released into rivers or lakes instead being injected into the geothermal field these minerals can degrade the water quality.
- The extraction of geothermal steam and water also needs to be managed to avoid using heat from the ground at a rate higher than it is naturally replenished.
Biological material (biomass), such as wood, wood waste, manure, and straw, captures energy from the sun during photosynthesis and stores this energy as the plant grows. Biomass can be used to produce heat, electricity or transport fuel. New Zealand currently utilises the following forms of bioenergy:
- Biogas from landfill sites, farms and sewage treatment plants
- Biofuel from milk by-products, Brazilian sugarcane and rapeseed
Bioenergy can be derived from plants specifically grown for their energy content as well as waste products of industrial, commercial, agricultural and domestic activities. The production of energy from waste products is environmentally and economically efficient. The production of bioenergy from virgin sources may have significant effects as a result of land use change, including the diversion of land from food crop production to bioenergy feedstock production.
Bioenergy is greenhouse gas neutral, provided the carbon dioxide released during combustion is absorbed by the growth of the next crop of biomass.
Solar energy utilises the thermal energy from sunlight. Annual sunshine hours in New Zealand range from about 1,600 in Invercargill to over 2,400 in Blenheim, with the main centres receiving about 2,000 hours. This sunlight can be utilised through solar water heating systems or photovoltaic panels which generate electricity.
Technological improvements have reduced the cost of photovoltaic panels. Solar is now cost competitive with fossil fuels in many markets.
Solar energy produces no air pollution or greenhouse gas emissions during operation. However, like electronics, solar energy systems contain hazardous materials, including heavy metals. Recycling programs are being established which will reduce the environmental impact of solar energy systems at end-of-life.
Wave and tidal energy is an emerging renewable energy source. Waves and tides create moving water which can be used to spin a turbine which generates electricity. The technology for utilising these resources is in the research and development phase. To date, only one wave and tidal power device has been developed in New Zealand.
Marine energy produces no air pollution or greenhouse gas emissions during operation. The effects of marine energy production on the marine ecosystem are currently uncertain, including interactions between marine mammals and fish and turbines (collision and noise effects), although they are predicted to be manageable in most locations.
Last updated at 9:55AM on February 25, 2015